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Tuning the hydrolytic aqueous chemistry of osmium arene complexes with N,O-chelating ligands to achieve cancer cell cytotoxicity.
PublishedonWeb02/24/2007
Tuning the Hydrolytic Aqueous Chemistry of Osmium Arene
Complexes with N,O-Chelating Ligands to Achieve Cancer
Cell Cytotoxicity
Anna F. A. Peacock, Simon Parsons, and Peter J. Sadler*
ContributionfromtheSchoolofChemistry,UniVersityofEdinburgh,WestMainsRoad,
EdinburghEH93JJ,U.K.
ReceivedNovember21,2006; E-mail:p.j.sadler@ed.ac.uk
Abstract:Potentialbiologicalandmedicalapplicationsoforganometalliccomplexesarehamperedbya
lackofknowledgeoftheiraqueoussolutionchemistry.Weshowthatthehydrolyticandaqueoussolution
chemistryofhalf-sandwichOsIIarenecomplexesofthetype[(Ł6-arene)Os(XY)Cl]canbetunedwithXY
chelatingligandstoachievecancercellcytoxicitycomparabletocarboplatin.Complexescontainingarene
)p-cymene, XY ) N,O-chelating ligands glycinate (1), L-alaninate (2), R-aminobutyrate (3), (cid:226)-alaninate
(4),picolinate(5),or8-hydroxyquinolinate(7)weresynthesized.Although,1-4and7hydrolyzedrapidly
(<min),complexeswith(cid:240)-acceptorpyridineasN-donorandcarboxylateasO-donor(5and6)hydrolyzed
muchmoreslowly(t )0.20and0.52h,298K).Theaquapicolinatecomplexesweremoreacidic(pK*
1/2 a
)6.67,6.33)thantheotheraquaadducts(pK*)7.17-7.71).Atbiologicaltestconcentrations(micromolar),
a
thechelatingligandsdissociatedfromcomplexes1-4togivetheinerthydroxo-bridgeddinuclearspecies
[(Ł6-arene)Os((cid:237)-OH) Os(Ł6-arene)]+(8),andthesecomplexeswereinactivetowardhumanlungA549and
3
ovarianA2780cancercells.Incontrast,5-7werecytotoxic,especially6(IC valuesof8and4.2(cid:237)M).
50
TheX-raystructuresof9-ethylguanine,[(Ł6-p-cym)Os(pico)(9EtG-N7)]PF,and9-ethyladenine,[(Ł6-p-cym)-
6
Os(pico)(9EtA-N7)]PF ,adductsof5arereported(thefirstreportedforGorAadductsofOsII).Crystalsof
6
the9EtAcomplexcontainhomoadeninebasepairing.The9EtGadductinparticularexhibitsremarkable
aqueouskineticstability.Thisworkshowshowtherationalcontrolofchemicalreactivity(hydrolysis,acidity,
formation of hydroxo-bridged dinuclear species) can allow the design of cytotoxic anticancer OsII arene
complexes.
Introduction transdiam(m)inePtIIcomplexeshasbeenaidedbythediscovery
that trans carboxylato ligands can decrease kinetic activity by
Transition metal complexes offer enormous scope for the
about an order of magnitude compared to their chloro ana-
designoftherapeuticagentswithnovelmechanismsofaction.1
logues.4 The choice of the types of coordinated ligands and
Advances in this field depend upon demonstrations that the
coordination geometry provides an ability to “fine-tune” the
concept of rational design can be applied to metal complexes.
chemical reactivity of complexes, potentially allowing control
Forthis,knowledgeofhowtocontrolthethermodynamicsand
ofpharmacologicalproperties,includingcelluptake,distribution,
kinetics of ligand substitution and redox reactions under
DNA binding, metabolism, and toxic side effects.
biologically relevant conditions is essential, in particular, the
Organometallic chemistry offers a potentially rich field for
hydrolytic chemistry. This is well illustrated for the platinum
biological and medical applications,5 but, as pointed out
anticancerfield.Itwasapparentearlyon2thatanunderstanding
recently,6alackofunderstandingoftheaqueouschemistryof
of the aqueous chemistry of cisplatin and related complexes,
organometallic complexes is currently a major obstacle for
includingaquationrates,hydrolysisequilibria,acidityofaqua
further developments. This is particularly true for osmium(II)
adducts,formationofhydroxo-bridgedoligomers,aidsnotonly
arenecomplexes.7-9Hereweattempttodesignactiveosmium
the design of new generations of platinum agents but also an
half-sandwichcomplexesbycontrollingtheiraqueouschemistry.
understandingofthemechanismofcytotoxicity.Theearlyuse
ofcisplatin,whichhydrolyzesreadily,hasbeensupplemented
(3) ReedijkJ.Chem.Commun.1996,801-806.
by more stable complexes such as carboplatin and oxaliplatin (4) Bulluss, G. H.; Knott, K. M.; Ma, E. S. F.; Aris, S. M.; Alvarado, E.;
which have less side effects.3 Recently, the design of active
Farrell,N.Inorg.Chem.2006,45,5733-5735.
(5) (a) Halpern, J. Pure Appl. Chem. 2001, 73, 209-220. (b) Fish, R. H.;
Jaouen,G.Organometallics2003,22,2166-2177.
(1) (a)Guo,Z.;Sadler,P.J.AdV.Inorg.Chem.2000,49,183-306.(b)Yan, (6) Jaouen, G.; Beck, W.; McGlinchey, M. J. In Bioorganometallics: Bio-
Y.K.;Melchart,M.;Habtemariam,A.;Sadler,P.J.Chem.Commun.2005, molecules,Labeling,Medicine;Jaouen,G.,Ed.;Wiley-VCH: Weinheim,
38, 4764-4776. (c) Melchart, M.; Sadler, P. J. In Bioorganometallics; Germany,2006;pp1-37.
Jaouen,G.,Ed.;Wiley-VCH: Paris,2005;Vol.1,pp39-64.(d)Allardyce, (7) Hung,Y.;Kung,W.;Taube,H.Inorg.Chem.1981,20,457-463.
C.S.;Dorcier,A.;Scolaro,C.;Dyson,P.J.Appl.Organomet.Chem.2005, (8) Stebler-Ro¨thlisberger,M.;Hummel,W.;Pittet,P.A.;Bu¨rgi,H.B.;Ludi,
19,1-10. A.;Merbach,A.E.Inorg.Chem.1988,27,1358-1363.
(2) (a)Howe-Grant,M.E.;Lippard,S.J.MetalIonsBiol.Syst.1980,11,63- (9) Mui,H.D.;Brumaghim,J.L.;Gross,C.L.;Girolami,G.S.Organome-
196.(b)Martin,R.B.ACSSymp.Ser.1983,209,231-244. tallics1999,18,3264-3272.
3348 9 J.AM.CHEM.SOC. 2007,129,3348-3357 10.1021/ja068335pCCC:$37.00 ©2007AmericanChemicalSociety
TuningtheHydrolyticAqueousChemistryofOsAreneComplexes ARTICLES
Osmiumcomplexesareusuallyconsideredtoberelativelyinert Calcd for C ClH NOOs (447.99): C, 34.85; H, 4.50; N, 3.13%.
13 20 2
in keeping with the normal behavior of a third row transition Found: C,34.38;H,4.01;N,3.00%.1HNMR(MeOD-d 4 ): (cid:228))7.51
metal.However,ourrecentworkonrutheniumarenecomplexes (br,NH),6.40(br,NH),6.02(d,2H),5.99(d,1H),5.95(d,1H),5.81
has shown that aqueous reactivity of [(Ł6-arene)Ru(XY)Z]n+ (m,3H),5.74(d,1H),5.29(br,NH),3.98(br,NH),3.40(q,1H),3.25
(q,1H),2.73(sept,H),2.72(sept,H),2.23(s,3H),2.22(s,3H),1.42
complexes is highly dependent on the nature of the chelating
(d, 3H), 1.34 (d, 3H), 1.32 (d, 6H), 1.31 (d, 6H). Diastereoisomers
ligand XY and monodentate ligand Z (the leaving group), as
werepresentina1.5:1ratio.
well as the arene.10,11 A similar pattern of reactivity may
[(Ł6-p-cym)Os(aiba)Cl](3).AsolutionofR-aminoisobutyricacid
thereforeapplytoosmium,theheaviercongenerofruthenium.
(13.9 mg, 0.13 mmol), sodium methoxide (7.3 mg, 0.14 mmol), and
AtheoreticalanalogybetweencisplatinandRuIIareneanticancer
[(Ł6-p-cym)OsCl] (50.8mg,0.06mmol)inMeOH(5mL)wasstirred
22
complexes has recently been proposed by Deubel et al.12 atambienttemperaturefor22handfilteredthroughaglasswoolplug.
Hereweinvestigatethetuningofthereactivityofosmium- The solvent was removed on a rotary evaporator, and the residue
(II) arene complexes [(Ł6-arene)Os(XY)Cl] containing XY ) extractedwithdichloromethane(15mL)andfilteredthroughacotton
an anionic N,O-chelating ligand with a primary amine or wool plug. The solvent volume was reduced to ca. 3 mL, and the
pyridine as the N-donor and carboxylate or aryloxide as the productprecipitatedafteradditionofdiethyletherandstorageat253
O-donor.Wehavestudiedtherateofhydrolysis,acidityofthe K for 1 h. The mustard yellow powder was recovered by filtration,
washed with diethyl ether (10 mL), and air-dried. Yield: 24.8 mg
aqua adducts, dynamic chelate ring opening, interactions with
(42%).Anal.CalcdforC ClH NOOs(462.01): C,36.40;H,4.80;
nucleobases, and relationship to cancer cell cytotoxicity. Our 14 22 2
N,3.03%.Found: C,36.44;H,4.68;N,2.88%.1HNMR(MeOD-d):
datasuggestthattheaqueoussolutionchemistryoforganome- 4
(cid:228))6.04(d,1H,J)5.3Hz),5.99(d,1H,J)5.0Hz),5.87(d,1H,
tallicosmiumarenecomplexescanbecontrolledfortherational J)5.3Hz),5.84(d,1H,J)5.3Hz),2.72(sept,1H,J)7.0Hz),
design of biologically active agents. 2.21(s,3H),1.43(s,3H),1.34(s,3H),1.33(d,3H,J)6.9Hz),1.30
ExperimentalSection
(d,3H,J)6.8Hz).
Materials. OsCl(cid:226)nHO was purchased from Alfa Aesar, sodium
[(Ł6-p-cym)Os((cid:226)-ala)Cl](4).Asolutionof(cid:226)-alanine(12.0mg,0.13
methoxide,R-amino 3 isob 2 utyricacid,2-picolinicacid,silverhexafluo- mmol),sodiummethoxide(7.3mg,0.14mmol),and[(Ł6-p-cym)OsCl 2 ] 2
(50.7 mg, 0.06 mmol) in MeOH (4 mL) was stirred at ambient
rophosphate,9-ethylguanine,9-ethyladenine,adenosine,cytidine,and
temperature for 19 h. The solvent volume was reduced to ca. 3 mL,
thymidinewerefromSigma,andsodiumglycinate,L-alanine,(cid:226)-alanine,
andtheproductprecipitatedafteradditionofdiethyletherandstorage
8-hydroxyquinoline,anddeuteratedsolventswerefromAldrich.The
at 253 K overnight. The yellow powder was recovered by filtration,
dimers [(Ł6-p-cym)OsCl] and [(Ł6-bip)OsCl] were prepared by
22 22 washed with diethyl ether (10 mL), and air-dried. Yield: 15.1 mg
previouslyreportedprocedures.13,14Methanolwasdistilledovermag-
(26.3%).Anal.Calcdfor(4(cid:226)HO)C ClH NOOs(466.00): C,33.51;
nesium/iodinepriortouse. 2 13 22 3
Complexes1-7weresynthesizedfromthedimericprecursor[(Ł6- H, 4.76; N, 3.01%. Found: C, 33.86; H, 4.33: N, 3.06%. 1H NMR
(MeOD-d): (cid:228))7.13(b,1H),6.04(d,1H,J)5.3Hz),5.96(d,1H,
arene)OsCl], using procedures similar to those reported previously 4
22 J)5.3Hz),5.86(d,1H,J)5.3Hz),5.72(d,1H,J)5.3Hz),4.14
forotherhalf-sandwichRuIIarenecomplexes.15,16
(b,1H),3.04(m,1H),2.75(sept,1H,J)6.8Hz),2.64(m,1H),2.46
PreparationofComplexes:[(Ł6-p-cym)Os(gly)Cl](1).Asolution
(ddd,1H,J)17.1,11.1,and2.7Hz),2.29(dd,1H,J)17.0and6.3
ofsodiumglycinate(12.3mg,0.13mmol)and[(Ł6-p-cym)OsCl] (47.3
22 Hz),2.19(s,3H),1.31(d,3H,J)6.9Hz),1.30(d,3H,J)7.2Hz).
mg,0.06mmol)inMeOH(5mL)wasstirredatambienttemperature
[(Ł6-p-cym)Os(pico)Cl](5).Asolutionofpicolinicacid(18.6mg,
for5.5h,thesolventremovedonarotaryevaporator,andtheresidue
0.15mmol)andsodiummethoxide(8.3mg,0.15mmol)inMeOH(3
extractedwithdichloromethaneandfilteredthroughacottonwoolplug.
mL) was stirred at ambient temperature for 45 min and added to a
Thesolventvolumewasreducedtoca.3mL,andthesolutionstored
solutionof[(Ł6-p-cym)OsCl] (50.8mg,0.06mmol)inMeOH(5mL)
at 253 K overnight. The pale yellow powder which formed was 22
underargon.Theresultingmixturewasstirredatambienttemperature
recovered by filtration, washed with diethyl ether (10 mL), and air-
dried.Yield: 33.1mg(59%).Anal.Calcdfor1+HOC ClH NO- underargonfor20h,thesolventwasremovedonarotaryevaporator,
2 12 20 3
andtheresidueextractedwithdichloromethaneandfilteredthrougha
Os(451.98): C,31.89;H,4.46;N,3.10%.Found: C,31.98;H,3.92;
N,3.05%.1HNMR(MeOD-d): (cid:228))6.02(d,1H,J)5.3Hz),5.98 cottonwoolplug.Thesolventvolumewasreducedtoca.3mL,and
4
(d,1H,J)5.3Hz),5.80(d,1H,J)5.3Hz),5.76(d,1H,J)5.3 the product precipitated after addition of diethyl ether. The yellow
Hz),3.20(m,2H),2.73(sept,1H,J)6.8Hz),2.25(s,3H),1.32(d, powderwasrecoveredbyfiltration,washedwithdiethylether(10mL),
6H,J)7.0Hz). andair-dried.Yield: 55.8mg(90%).Anal.CalcdforC 16 ClH 18 NO 2 Os
(482.00): C,39.87;H,3.76;N,2.91%.Found: C,39.87;H,3.67;N,
[(Ł6-p-cym)Os(L-ala)Cl](2).AsolutionofL-alanine(11.6mg,0.13
2.80%.1HNMR(CDCl): (cid:228))8.78(d,1H,J)5.7Hz),8.12(d,1H,
mmol),sodiummethoxide(7.7mg,0.14mmol),and[(Ł6-p-cym)OsCl] 3
22 J)7.6Hz),7.93(td,1H,J)7.6Hz),7.50(td,1H,J)6.4Hz),5.94
(47.2 mg, 0.06 mmol) in MeOH (5 mL) was stirred at ambient
(d,1H,J)5.7Hz),5.89(d,1H,J)5.7Hz),5.72(d,1H,J)5.7
temperaturefor22h.Thesolventwasremovedonarotaryevaporator,
Hz),5.67(d,1H,J)5.7Hz),2.74(sept,1H,J)6.8Hz),2.35(s,
andtheresidueextractedwithdichloromethaneandfilteredthrougha
3H),1.22(d,3H,J)5.3Hz),1.21(d,3H,J)5.3Hz).Crystalsof
cotton wool plug. The solvent was again removed and the product
5 suitable for X-ray diffraction were obtained by evaporation of a
recoveredbytitratingwithdiethylether.Yield: 46mg(86%).Anal.
chloroform/diethylethersolutionatambienttemperatureinthedark.
(10) Wang,F.;Chen,H.;Parsons,S.;Oswald,I.D.H.;Davidson,J.E.;Sadler, [(Ł6-bip)Os(pico)Cl](6).Asolutionof[(Ł6-bip)OsCl] (51.3mg,
P.J.Chem.sEur.J.2003,9,5810-5820. 22
(11) Chen,H.;Parkinson,J.A.;Novakova,O.;Bella,J.;Wang,F.;Dawson, 0.06mmol)inMeOH(5mL)wasrefluxedunderargonfor1hbefore
A.;Gould,R.;Parsons,S.;Brabec,V.;Sadler,P.J.Proc.Natl.Acad.Sci. addingasolutionofpicolinicacid(16.8mg,0.14mmol)andsodium
U.S.A.2003,100,14623-14628.
(12) Deubel,D.V.;Lau,J.K.-C.Chem.Commun.2006,2451-2453. methoxide (7.3 mg, 0.14 mmol) in MeOH (2 mL), which had been
(13) Stahl,S.;Werner,H.Organometallics1990,9,1876-1881. stirredatambienttemperaturefor30min.Theresultingmixturewas
(14) Peacock,A.F.A.;Habtemariam,A.;Ferna´ndez,R.;Walland,V.;Fabbiani, stirredatambienttemperaturefor20h,filteredthroughacottonwool
F.P.A.;Parsons,S.;Aird,R.E.;Jodrell,D.I.;Sadler,P.J.J.Am.Chem.
Soc.2006,128,1739-1748. plug,andthesolventremovedonarotaryevaporator.Theresiduewas
(15) Carter,L.;Davies,D.L.;Fawcett,J.;Russell,D.R.Polyhedron1993,12, extractedwithdichloromethane,filtered,andthesolventremovedagain.
1599-1602.
Theresultingsolidwasredissolvedinmethanolandthesolventvolume
(16) Gemel,C.;John,R.;Slugovc,C.;Mereiter,K.;Schmid,R.;Kirchner,K.
J.Chem.Soc.,DaltonTrans.2000,2607-2612. reduced until a precipitate began to form. The vessel was stored at
J.AM.CHEM.SOC.9VOL.129,NO.11,2007 3349
ARTICLES Peacocketal.
278 K, and the yellow powder was recovered by filtration, washed MethodsandInstrumentation:X-rayCrystallography.Alldif-
withdiethylether(10mL),andair-dried.Yield: 23.9mg(39%).Anal. fractiondatawerecollectedusingaBruker(Siemens)SmartApexCCD
Calcd for 6+HO C ClH NOOs (520.01): C, 41.57; H, 3.10; N, diffractometerequippedwithanOxfordCryosystemslow-temperature
2 18 16 3
2.69%.Found: C,41.85;H,2.50;N,3.17%.1HNMR(CDCl): (cid:228)) deviceoperatingat150K.Absorptioncorrectionsforalldatasetswere
3
8.30(d,1H,J)5.4Hz),8.12(d,1H,J)7.9Hz),7.86(t,1H,J) performed with the multiscan procedure SADABS;17 structures were
7.7Hz),7.51(m,2H),7.40(m,2H),7.28(7,1H,J)6.6Hz),6.40(d, solved using either Patterson or direct methods (SHELXL18 or
1H,J)5.3Hz),6.36(d,1H,J)4.9Hz),6.23(t,1H,J)5.0Hz), DIRDIF19);complexeswererefinedagainstF2usingSHELXTL,and
6.21(t,1H,J)5.0Hz),6.15(t,1H,J)5.0Hz). H-atoms were placed in geometrically calculated positions. The
[(Ł6-p-cym)Os(oxine)Cl](7).Asolutionof8-hydroxyquinoline(18.6 modelingprogramDiamond3.020wasusedforproductionofgraphics.
mg,0.13mmol)andsodiummethoxide(6.9mg,0.13mmol)inMeOH X-ray crystallographic data for complexes 5, 7, 9PF 6 , and 11PF 6 (cid:226)
(2mL)wasaddedtoasolutionof[(Ł6-p-cym)OsCl 2 ] 2 (48.3mg,0.06 0.5Et 2 O are available as Supporting Information and have been
mmol) in MeOH (4 mL), and the resulting mixture was stirred at deposited in the Cambridge Crystallographic Data Centre under the
ambient temperature for 5 h. The solvent was removed on a rotary accession numbers CCDC 626657, 626658, 626655, and 626656,
evaporator, the residue extracted with acetone (ca. 10 mL), and the respectively.
solventvolumereduceduntilayellowprecipitatestartedtoform,and NMRSpectroscopy.1HNMRspectrawereacquiredonaBruker
wasstoredat253Kovernight.Theyellowmicrocrystallinesolidwas AVA600(1H)600MHz)spectrometerandforD 2 Osolutionswith
recoveredbyfiltration,washedwithdiethylether(5mL),andair-dried. water suppression by Shaka21 or presaturation methods. 1H NMR
Yield: 43.5mg(71%).Anal.CalcdforC ClH NOOs(504.05): C, chemicalshiftswereinternallyreferencedto1,4-dioxane(3.75ppm)
19 20
45.27; H, 4.00; N, 2.78%. Found: C, 45.35; H, 4.03; N, 2.68%. 1H foraqueoussolutions,CHD 2 OD(3.34ppm)formethanol-d 4 ,andCHCl 3
NMR(CDCl 3 ): (cid:228))8.77(d,1H,J)4.9Hz),8.05(d,1H,J)8.3 (7.26ppm)forchloroform-d 1 solutions.Alldataprocessingwascarried
Hz),7.37(t,1H,J)7.9Hz),7.28(dd,1H,J)8.3and5.0Hz),7.03 outusingXWIN-NMRversion3.6(BrukerU.K.Ltd.).
(d,1H,J)7.9Hz),6.84(d,1H,J)7.9Hz),5.93(d,1H,J)5.3 MassSpectrometry.Electrosprayionizationmassspectra(ESI-MS)
Hz),5.82(d,1H,J)5.3Hz),5.70(d,1H,J)5.3Hz),5.63(d,1H, wereobtainedonaMicromassPlatformIImassspectrometer,andD
2
O/
J)5.3Hz),2.62(sept,1H,J)6.8Hz),2.36(s,3H),1.12(dd,6H, H
2
Osolutionswereinfuseddirectly.Thecapillaryvoltagewas3.5V,
J)8.9and7.6Hz).Crystalsof7suitableforX-raydiffractionwere andtheconevoltagewasvariedbetween20and45Vdependingon
obtainedbyevaporationofanacetone/diethylethersolutionatambient sensitivity. The source temperature was 353 K. Mass spectra were
temperatureinthedark. recordedwithascanrangeofm/z300-1000forpositiveions.
[(Ł6-p-cym)Os(pico)(9EtG)]PF (9PF).Asolutionof[(Ł6-p-cym)- pH*Measurement.pH*values(pHmeterreadingwithoutcorrec-
6 6
Os(pico)Cl] (32.6 mg, 0.07 mmol) and AgPF 6 (18.5 mg, 1.1 molar tionforeffectsofDonglasselectrode)ofNMRsamplesinD 2 Owere
equiv)inMeOH(3mL)wasstirredatambienttemperaturefor5.5h. measured at ca. 298 K directly in the NMR tube, before and after
TheAgClprecipitatewasremovedbyfiltrationthroughaglasswool recordingNMRspectra,usingaCorning240pHmeterequippedwith
plug,and9-ethylguanine(12.2mg,1.1molarequiv)wasaddedtothe amicrocombinationelectrodecalibratedwithAldrichbuffersolutions
resulting solution. The reaction mixture was stirred at ambient atpH4,7,and10.
temperature under argon for ca. 42 h. The resulting pale yellow Hydrolysis.Solutionsof1-7(2mM)in5%MeOD-d 4 /95%D 2 O
precipitatewasrecoveredbyfiltration,washedwithmethanol(3mL) (v/v)werepreparedbydissolutionofthecomplexinMeOD-d 4 followed
anddiethylether(10mL),andair-dried.Yield: 28.0mg(54%).Anal. by rapid dilution with D 2 O. Solutions of 50 (cid:237)M were prepared by
CalcdforC 23 H 27 N 6 O 3 OsPF 6 (770.69): C,35.84;H,3.53;N,10.90%. subsequentdilutionofthese2mMstocksolutionswithD 2 O(finalratio
Found: C,35.74;H,3.21;N,11.23%.1HNMR(MeOD-d 4 ): (cid:228))9.64 0.125%MeOD-d 4 /99.875%D 2 O(v/v)).The1HNMRspectraforthe
(d,1H,J)5.3Hz),8.12(td,1H,J)7.8and1.3Hz),7.95(d,1H, 2mMand50(cid:237)Msolutionswererecordedwithinthefirst10minof
J)7.9Hz),7.85(s,1H),7.74(td,1H,J)6.8and1.5Hz),6.50(d, sample preparation and after incubation at 310 K for 24 h. The
1H,J)5.7Hz),6.24(d,1H,J)5.7Hz),6.12(d,1H,J)5.7Hz), speciationof1-7in100mMNaClwasinvestigatedbyadding2.5M
6.04 (d, 1H, J ) 5.7 Hz), 4.13 (dq, 2H, J ) 7.3 and 2.6 Hz), 2.53 NaCl(25(cid:237)L)tothe2mMNMRsampleinD 2 O.Theeffectsofvarying
(sept,1H,J)6.8Hz),2.04(s,3H),1.39(t,3H,J)7.2Hz),1.17(d, concentrations of chloride were investigated by preparing aqueous
3H,J)6.8Hz),1.03(d,3H,J)6.8Hz).Crystalsof9PF suitable solutions of 5 (1 mM and 50 (cid:237)M) in 100, 22.7, and 4 mM NaCl in
6
forX-raydiffractionwereobtainedbydiffusionofdiethyletherintoa D 2 O, recording 1H NMR spectra within the first 10 min of sample
methanolsolutionat278K. preparation,andafterincubationat310Kfor24h.
[(Ł6-p-cym)Os(pico)(9EtA)]PF (11PF).Asolutionof[(Ł6-p-cym)- Thelongertermstabilityofcomplex5wasinvestigatedbypreparing
6 6
1mMsolutionsin5%MeOD-d/95%isotonicsalinesolution(150mM
Os(pico)Cl] (24.9 mg, 0.05 mmol) and AgPF (13.3 mg, 1.0 molar 4
6
NaCl)(v/v)bydissolutionof5inMeOD-d followedbyrapiddilution
equiv) in MeOH (3 mL) was stirred at ambient temperature for 2 h. 4
withtheisotonicsalinesolution.1HNMRspectrawererecordedafter
TheAgClprecipitatewasremovedbyfiltrationthroughaglasswool
varioustimeintervals(10min,1week,2weeks,and2months)during
plug, and 9-ethyladenine (8.6 mg, 1.0 molar equiv) was added. The
whichtimethesamplewasstoredatambienttemperatureinthedark.
reaction mixture was stirred at ambient temperature under argon for
DeterminationofpK*Values.FordeterminationsofpK*values
ca. 78 h. A yellow product precipitated out after addition of diethyl a a
(pK valuesforsolutionsinDO),thepH*valuesoftheaquacomplexes
etherandwasrecoveredbyfiltration,washedwithmethanol(2mL) a 2
of 1-7 in DO (formed in situ by dissolution of the parent chloro
anddiethylether(10mL),andair-dried.Yield: 13.3mg(34%).Anal. 2
complexes)werevariedfromca.pH*3to10bytheadditionofdilute
CalcdforC H NOOsPF (754.69): C,36.60;H,3.61;N,11.14%.
23 27 6 2 6
Found: C,37.04;H,3.57;N,10.95%.1HNMR(MeOD-d): (cid:228))9.59 NaODandHNO 3 ,and1HNMRspectrawererecorded.pH*titrations
(d,1H,J)5.3Hz),8.34(s,1H),8.30(s,1H),8.30(overl 4 appedt,1H, of complexes 9 and 11 were carried out from pH* 3-12 and 1-8,
J)ca.7Hz),8.06(d,1H,J)7.5Hz),8.01(t,1H,J)6.3Hz),6.64
(d,1H,J)5.6Hz),6.52(d,1H,J)5.4Hz),6.16(d,1H,J)5.3 (17)
2
S
0
h
0
e
1
ld
-
ri
2
c
0
k
0
,
4
G
.
.M.SADABS;UniversityofGo¨ttingen: Go¨ttingen,Germany,
Hz),6.11(d,1H,J)5.6Hz),4.63(brs,2H),4.31(sept,2H,J)7.0 (18) Sheldrick, G. M. SHELXL-97, Program for the refinement of crystal
Hz),4.25(sept,2H,J)7.0Hz),2.64(sept,1H,J)6.8Hz),1.85(s, structures;UniversityofGo¨ttingen: Go¨ttingen,Germany,1997.
(19) Beurskens,P.T.;Beurskens,G.;Bosman,W.P.;deGelder,R.;Garcia-
3H),1.41(t,3H,J)7.3Hz),1.17(d,3H,J)6.8Hz),1.01(d,3H,
Granda,S.;Gould,R.O.;Israel,R.;Smits,J.M.M.DIRDIF;Crystal-
J)7.0Hz).Crystalsof11PF(cid:226)0.5EtOsuitableforX-raydiffraction lographyLaboratory,UniversityofNijmegen: TheNetherlands,1996.
6 2
(20) Brandenburg, K.; Putz, H. Diamond. Crystal and Molecular Structure
wereobtainedbydiffusionofdiethyletherintoamethanolsolutionat
VisualizationCrystalImpactGbR;Bonn,Germany.
278K. (21) Hwang,T.L.;Shaka,A.J.J.Magn.Reson.,Ser.A1995,112,275-279.
3350 J.AM.CHEM.SOC.9VOL.129,NO.11,2007
TuningtheHydrolyticAqueousChemistryofOsAreneComplexes ARTICLES
respectively.Thechemicalshiftsofthearenering(1-7),purineH8
(9and11), and H2 (11) protons were plotted against pH*. The pH*
titration curves were fitted to the Henderson-Hasselbalch equation,
with the assumption that the observed chemical shifts are weighted
averagesaccordingtothepopulationsoftheprotonatedanddeproto-
natedspecies.TheexperimentalandfittingerrorsinpK*valuesare
a
estimatedtobeca.(0.04units.ThesepK*valuescanbeconverted
a
to pK values by use of the equation pK ) 0.929pK* + 0.42 as
a a a
suggestedbyKrezelandBal,22forcomparisonwithrelatedvaluesin
theliterature.
KineticsforHydrolysis.Althoughcomplexes1-4and7hydrolyzed
toorapidlytomonitorby1HNMR,thekineticsfor5and6couldbe
followedatvarioustemperatures(278-298K).Forthis,thecomplex
was dissolved in methanol-d, and aliquots were added to DO
4 2
(equilibratedattherequiredtemperature)togiveafinalconcentration
ofca.0.8mMcomplexin95%DO/5%methanol-d (pH*ca.3,so
2 4
that the aqua ligand is not deprotonated). Samples were filtered and
spectrarecordedatvarioustimeintervals.Data,basedonpeakintegrals,
were fitted to the appropriate equation for first-order kinetics. The
Arrhenius activation energies (E), activation enthalpies (¢Hq), and
a
activationentropies(¢Sq)weredeterminedfromArrheniusandEyring
plots.23
VariableTemperatureNMR.The1HNMRspectraofcomplexes
4, 5, and 7 in DO (1 mM, 5% MeOD-d to assist dissolution) were
2 4
recordedafterequilibrationatvarioustemperatures.Kineticdatawere
obtained using the equations k ) ((cid:240)¢V)/x2, where k is the rate
c c
constantatcoalescence,t )1/k,wheret isthelifetimeofseparate Figure1. Osmiumarenecomplexesstudiedinthiswork.
c c c
isomersatcoalescence,and¢Gq)19.143T(10.318-logk/T),where
c c c
T isthetemperatureofcoalescence.24Similarly,10mMsolutionsof acidates, picolinate, or 8-hydroxyquinolinate as anionic N,O-
c
5 and 7 in methanol-d 4 were recorded after equilibration at various chelating ligands (Figure 1) in good yields via the Cl-bridged
temperatures. dimer,[(Ł6-arene)OsCl ] ,arene)p-cymene(p-cym,1-5,7)
2 2
Cancer Cell Cytotoxicity. After plating, human ovarian A2780 orbiphenyl(bip,6).WedeterminedtheX-raycrystalstructures
cancer cells were treated with OsII complexes on day 3, and human of [(Ł6-p-cym)Os(pico)Cl] 5 and [(Ł6-p-cym)Os(oxine)Cl] 7
lungA549cancercellsonday2,atconcentrationsrangingfrom0.1to
(Figure2).Thecomplexesadoptthefamiliarpseudo-octahedral
100 (cid:237)M. Solutions of the OsII complexes were made up in 0.125%
“three-leg piano stool” geometry with the osmium (cid:240)-bonded
DMSOtoassistdissolution.Cellswereexposedtothecomplexesfor
to the arene ligand (Os ring centroid distances of 1.652(2) Å
24h,washed,suppliedwithfreshmedium,allowedtogrowforthree
for5and1.6557(12)Åfor7),(cid:243)-bondedtoachloride(2.4048-
doubling times (72 h), and then the protein content measured
(proportionaltocellsurvival)usingthesulforhodamineB(SRB)assay.25 (13)and2.4235(7)Å),apyridinenitrogen(2.090(4)and2.098-
Thestandarderrorsarebasedonthreereplicates. (2)Å),andadeprotonatedoxygenatom(2.080(3)and2.081(2)
InteractionswithNucleobases.Thereactionof5withnucleobases Å) of the chelating ligand, which constitute the three legs of
typicallyinvolvedadditionofasolutioncontaining1molarequivof the piano stool. Crystallographic data, selected bond lengths,
nucleobase in D 2 O (or 1 molar equiv of 9-ethylguanine and 1 molar and angles are given in Tables S1 and S2.
equiv 9-ethyladenine in a competition reaction), to an equilibrium
Independent molecules in the crystal structures of 5 and 7
solutionof5inDO(>90%aqua).ThepH*valueofthesamplewas
2 arelinkedbyshort-rangeinteractionsbetweenthecoordinated
adjustedifnecessarysoastoremaincloseto7.4(physiological).1H
chloride and an aromatic ring proton of the chelating ligand
NMRspectraofthesesolutionswererecordedat298Kaftervarious
(Cl(1)(cid:226)(cid:226)(cid:226)H(321)2.81Å)orp-cymenearene(Cl(1)(cid:226)(cid:226)(cid:226)H(211)2.69
timeintervals.
Å), respectively. Further short-range interactions are present
NMR spectra of aqueous solutions (DO) of the 9EtG and 9EtA
2
complexes9and11,respectively,wererecordedatvariousconcentra- betweenaromaticringprotonsandoxygenatomsofthechelate
tions (20 (cid:237)M to 2 mM) at 298 K and after incubation at 310 K for (Figure S1 and Table S3).
varioustimeintervals.Theequilibriumconstantfordissociationof9EtA Aqueous Solution Chemistry and pK * Determination.
a
from11wasobtainedfromtheslopeoftheplotof[boundcomplex]/ Aqueoussolutionsofthechlorocomplexes1-6,directlyafter
[free9EtA]versus[5A],usingconcentrationsbasedon1HNMRpeak samplepreparation(<10min),gaverisetoonemajorandone
integrals. minor set of 1H NMR peaks, with resolved peaks for each of
the four p-cymene ring protons, consistent with the presence
Results
of stereogenic osmium centers (the L-alanine complex 2 was
Synthesis and Characterization of Complexes. We syn-
presentasaca.1:1mixtureofdiastereoisomers).WhenthepH*
thesized seven new OsII arene complexes containing amino
values of the solutions were increased from ca. 3 to 10, the
majorsetofNMRpeaksgraduallyshiftedtohighfield(Figure
(22) Krezel,A.;Bal,W.J.Inorg.Biochem.2004,98,161-166.
(23) Atkins,P.W.PhysicalChemistry,6thed.;OxfordUniversityPress: Oxford, 3A), consistent with assignment to aqua adducts. Plots of the
1998. chemicalshiftsagainstpH*(Figures3AandS2)werefittedto
(24) Delpuech,J.-J.DynamicsofSolutionsandFluidMixturesbyNMR;John
Wiley&Sons: England,1995. the Henderson-Hasselbalch equation and gave rise to pK a *
(25) Skehan,P.;Storeng,R.;Scudiero,D.;Monks,A.;McMahon,J.;Vistica, values between 7.27 and 7.55 for complexes containing both
D.;Warren,J.T.;Bokesch,H.;Kenney,S.;Boyd,M.R.J.Natl.Cancer
Inst.1990,82,1107-1112. primary amines and carboxylate groups as donors, but lower
J.AM.CHEM.SOC.9VOL.129,NO.11,2007 3351
ARTICLES Peacocketal.
Table1. pKa*ValuesfortheDeprotonationoftheCoordinated
D2OinComplexes1A-7AandtheN1-HofN7-Coordinated
Nucleobasein9and11
complex pKa*
[(Ł6-p-cym)Os(gly)(OD2)]+ 1A 7.27
[(Ł6-p-cym)Os(L-ala)(OD2)]+ 2A 7.14
[(Ł6-p-cym)Os(aiba)(OD2)]+ 3A 7.17
[(Ł6-p-cym)Os((cid:226)-ala)(OD2)]+ 4A 7.55
[(Ł6-p-cym)Os(pico)(OD2)]+ 5A 6.67
[(Ł6-bip)Os(pico)(OD2)]+ 6A 6.33
[(Ł6-p-cym)Os(oxine)(OD2)]+ 7A 7.71
[(Ł6-p-cym)Os(pico)(9-EtG-N7)]+ 9 8.97
[(Ł6-p-cym)Os(pico)(9-EtA-N7)]+ 11 2.06
primary amine donor (Table 1). The minor sets of peaks for
coordinated p-cym increased in intensity on addition of NaCl
andareassignedtotheintactchlorocomplex(TableS4).During
the pH* titration of the (cid:226)-alaninate complex 4, new p-cym
doublets appeared at (cid:228) 6.04 and 5.82, with increase in pH*.
These peaks have previously been assigned26 to the hydroxo-
bridged dinuclear species, [(Ł6-p-cym)Os((cid:237)-OD) Os(Ł6-p-
3
cym)]+,8.Afterincubationofaqueoussolutionsof1-4(2mM)
at 310 K for 24 h (pH* ca. 6.2-6.7), the only changes to the
spectrawerenewpeaksfor8(accountingfor10%of1,6%of
2,3%of3,and37%of4).NMRspectraofaqueoussolutions
of the hydroxyquinolate complex 7 contained two doublets
assignable to the p-cym ring protons of the aqua adduct 7A,
which shifted to high field with increase in pH* (Figure 3B)
withanassociatedpK*of7.71.WhenthepH*wasincreased
a
to 8.55, these doublets broadened, and at pH* 10.15, they
resolvedintofoursharpdoublets((cid:228)6.07,6.01,5.73,and5.68;
Figure2. X-raycrystalstructuresandatomnumberingschemesfor(A)
Figure3B).TheNMRspectrumofasolutionof7in100mM
[(Ł6-p-cym)Os(pico)Cl] (5) and (B) [(Ł6-p-cym)Os(oxine)Cl] (7) (50%
probabilityellipsoids).Hatomshavebeenomittedforclarity. NaClcontainedfourdoubletsforp-cymeneringprotons(Table
S4) assignable to the intact chloro complex 7.
Freshaqueoussolutions(5%MeOD-d /95%D O)containing
4 2
complexes1-4atlowconcentration(50(cid:237)M)at298Kallgave
risetopeaksforaquaadductsandfor2and4peaksforhydroxo-
bridgeddinuclearspecies8aswell.Afterincubationat310K
for24h,compound8waspresentinallthesolutions,andfor
4,itwastheonlyspeciespresent(FigureS3).Similarsolutions
ofcomplexes5and6(50(cid:237)M)initiallycontainedamixtureof
theintactchloroandaquacomplexes,butafterincubation,only
peaks for the aqua complex were present. Complex 7 was
presentastheaquacomplexbothinitiallyandafterincubation.
It was notable therefore that neither complexes 5, 6, nor 7
formedthehydroxo-bridgeddinuclearspecies8whenincubated
at biologically relevant concentrations (50 (cid:237)M).
1H NMR spectra of 5 in isotonic saline solution (150 mM
NaCl), ca. 10 min after sample preparation, contained peaks
predominantlyfortheintactchlorospecies,togetherwithsmall
peaks assignable to the aqua adducts (<20%). No new peaks
appeared in the spectrum after storage at ambient temperature
Figure3. Thedependenceoftheareneregionof1HNMRspectraof(A) in the dark for 2 months (Figure S4).
[(Ł6-p-cym)Os(pico)Cl] (5) and (B) [(Ł6-p-cym)Os(oxine)Cl] (7) in D2O Theeffectsofchlorideconcentrationstypicalofbloodplasma
onpH*.TheplotsshowfitsgivingpKa*valuesof6.67and7.71forthe
(100mM),cellcytoplasm(22.7mM),andcellnucleus(4mM)27
aquacomplexes5Aand7A,respectively.Thefourmagneticallyinequivalent
arene ring protons give rise to separate peaks in the spectra of 5A and on the speciation of 5 in aqueous solution were investigated.
exhibit the same pH* dependence. However, the four peaks for 7A are 1HNMRspectraof5(50(cid:237)M)wererecordedwithin10minof
resolvedonlyatbasicpH*andbroadenandsharpenintotwodoubletsat
acidic pH*, indicative of involvement in a dynamic chemical exchange
(26) Peacock,A.F.A.;Melchart,M.;Deeth,R.J.;Habtemariam,A.;Parsons,
process. S.;Sadler,P.J.Chem.sEur.J.2006,10.1002/chem.200601152.
(27) (a)Martin,R.B.InCisplatinChemistryandBiochemistryofaLeading
valuesof6.33and6.67forthepicolinatecomplexes(5and6) AnticancerDrug;Lippert,B.,Ed.;Wiley-VCH: Zurich;1999;pp183-
205.(b)Jennerwein,M.;Andrews,P.A.DrugMetab.Dispos.1995,23,
which contain pyridine as a tertiary amine donor instead of a 178-184.
3352 J.AM.CHEM.SOC.9VOL.129,NO.11,2007
TuningtheHydrolyticAqueousChemistryofOsAreneComplexes ARTICLES
Table2. RateDatafortheAquationofComplexes5and6atVaryingTemperatures
compound T/K k/h-1 t1/2/h Ea/kJmol-1 ¢Hq/kJmol-1 ¢Sq/JK-1mol-1
5 278 0.243(0.003 2.85(0.03 91.5(3.0 89.1(3.0 -64.8(10.5
285 0.697(0.018 0.99(0.03
288 1.063(0.035 0.65(0.02
298 3.491(0.027 0.20(0.01
6 278 0.092(0.001 7.51(0.10 93.1(4.4 90.7(4.3 -61.6(15.1
285 0.211(0.003 3.29(0.04
288 0.339(0.008 2.04(0.05
298 1.346(0.053 0.52(0.02
Figure5. Determinationofcancercellcytotoxicity.Effectofvariationin
theconcentrationsof[(Ł6-p-cym)Os(pico)Cl](5),[(Ł6-bip)Os(pico)Cl](6),
and[(Ł6-p-cym)Os(oxine)Cl](7)onthesurvivalof(A)humanA549lung
Figure4. Timedependenceforformationoftheaquacomplex5A(based
on1HNMRpeakintegrals)duringhydrolysisof[(Ł6-p-cym)Os(pico)Cl]
cancer cells and (B) human A2780 ovarian cancer cells, giving the IC50
(5)inacidic(pH*2)D2Oat298K(1),288K(b),285K(2),and278K valuesinTable3.
(9). The inset shows an Arrhenius plot the slope of which gives the
ArrheniusactivationenergyEaof91.5kJmol-1. T
A
a
2
b
7
l
8
e
0
3
O
.
v
I
a
n
ri
V
an
itr
C
o
a
G
n
r
c
o
e
w
r
th
Ce
In
ll
h
s
ibitionofHumanA549Lungand
samplepreparationandafterincubationat310Kfor24h.On compound IC50/(cid:237)MA549 IC50/(cid:237)MA2780
the basis of 1H NMR peak integrals, complex 5 was found to 1 >100 >100
be present only as the intact chloro complex at 100 mM [Cl] 2 >100 >100
3 >100 >100
(pH* 7.0), as 45% hydrolyzed complex 5A at 22.7 mM [Cl]
4 >100 >100
(pH*7.1)andas72%5Aat4mM[Cl](pH*6.8);seeFigure
5 17 4.5
S7 and Table S5. 6 8 4.2
Kinetics of Hydrolysis. The kinetics of hydrolysis for 7 60 15.2
complexes 1-4 and 7 were too fast to measure by NMR. Ru-7a >100
However, we were able to follow the formation of the aqua aRuanalogueof7,[(Ł6-p-cym)Ru(oxine)Cl];seeref43.
complexes of 5 and 6 based on integration of 1H NMR peaks
inspectrarecordedatvarioustimeintervalsandtemperatures. and 5.98 at 293 K which merged into broad peaks at (cid:228) 6.27
These experiments were carried out at acidic pH* (ca. 2) so and 6.00 at 353 K. 1H NMR spectra of the aqua complex 7A
thatdeprotonationoftheaquacomplexasasecondaryreaction contained two p-cymene doublets ((cid:228) 6.31 and 6.09) at 298 K,
did not occur. The hydrolysis data were fitted to pseudo-first- one of which broadened on cooling to 283 K (broad peak (cid:228)
order kinetics (Figures 4 and S5). At 298 K, the half-life for 6.31, doublet (cid:228) 6.08) and resolved into broad peaks at (cid:228) 6.35
hydrolysis of the p-cymene complex 5 was 0.2 h, about 2.5 and 6.28, on cooling to 275 K (Figure S8). The rates of the
times shorter than that for the biphenyl complex 6 (Table 2).
dynamic chemical exchange processes (k) which gave rise to
Arrheniusactivationenergies(E),activationenthalpies(¢Hq), c
a these line shape changes, lifetimes of the contributing species
and activation entropies (¢Sq) (Figure S6) are listed in Table (t), and the Gibbs free energies (¢Gq) at the coalescence
2. The large negative ¢Sq values are notable. te c mperature (T) were calculated. These range from k ) 47
Variable Temperature Dynamic NMR. 1H NMR spectra s-1 at T ) 353 c K for 5A, 57 s-1 at 323 K for 4A, to 1 c 02 s-1
c
of aqueous solutions (1 mM, 5% MeOD-d /95% D O) of
4 2 at 283 K for 7A (Table S6). 1H NMR spectra of solutions of
complexes 4, 5, and 7 (containing predominantly the aqua
theintactchloroadducts5and7inmethanol-d weretemper-
complexes 4A, 5A, and 7A) at pH* ca. 4 (so as to prevent 4
aturedependentwiththefourp-cymenepeaksresolvingoutat
deprotonationofthecoordinatedwater)wererecordedoverthe
lower temperatures (Figure S8D and E).
temperaturerangeof275-353K.Atlowertemperatures(285
K), four peaks for the p-cymene arene protons of the aqua Cancer Cell Cytotoxicity. The cytotoxicity of complexes
complex4Awereresolved(doubletsat(cid:228)6.27and6.22,(cid:228)5.98
1-7towardhumanovarianA2780andlungA549cancercell
and 5.96). These peaks broadened at 323 K (broad peak at (cid:228) lines was investigated. Complexes 1-4 were nontoxic up the
6.26,doubletat(cid:228)6.01)butsharpenedintotwodoublets((cid:228)6.27 highest test concentration of 50 (cid:237)M. The IC 50 (50% growth
and 6.03) at 343 K. The aqua complex 5A gave four sharp
inhibitoryactivity)valuesarethereforelikelytobe>100(cid:237)M,
doublets((cid:228)6.41,6.38,6.17,and6.10)at293K,whichmerged andwiththiscutoffvalue,thecomplexesaredeemedinactive.
intotwobroadpeaks((cid:228)6.40and6.17)at353K.Peaksforthe However, complexes 5-7 showed moderate to high activity
intactchlorocomplex5(accountingforca.13%ofthespecies (Figure5andTable3)withIC valuesof4-60(cid:237)M.Notable
50
insolution)showedasimilarbehavior: doubletsat(cid:228)6.27,6.00, isthehighactivityofthebiphenyl/picolinatecomplex6against
J.AM.CHEM.SOC.9VOL.129,NO.11,2007 3353
ARTICLES Peacocketal.
Figure7. 1H NMR spectra showing the arene CH region for 1:1:1
competitionreactionsoftheaquaadduct5Awith9EtGand9EtAinD2O
at1mMand50(cid:237)M.(A)and(B)showthespectraobtainedca.10min
aftersamplepreparation,and(C)and(D)thoseobtainedafterincubation
at310Kfor24h.5and5Acorrespondtotheintactchloroandaquaspecies,
respectively,9to[(Ł6-p-cym)Os(pico)(9EtG)]+and11to[(Ł6-p-cym)Os-
(pico)(9EtA)]+.
twonewsetsofpeaks(38%majorspecies10and21%minor
species10b).ESI-MSstudiesonthisNMRsamplegavepeaks
atm/z448.2and715.1,consistentwith{(Ł6-p-cym)Os(pico)}+
F
th
i
e
gu
X
r
-
e
ra
6
y
.
cry
X
s
-
t
r
a
a
l
y
str
s
u
tr
c
u
tu
ct
r
u
e
r
o
e
f
s
(
o
A
f
)
n
[(
u
Ł
c
6
l
-
e
p
o
-
b
c
a
y
s
m
e
)O
ad
s
d
(p
u
i
c
c
t
o
s.
)(
H
9E
-b
tG
on
)]
d
+
ed
(9)
c
,
a
b
ti
e
o
tw
ns
ee
in
n
({5-Cl}+,calcdm/z448.1)and[(Ł6-p-cym)Os(pico)(Ado)]+(10,
the oxygen O92/O82 of the chelated pico ligand and N1H (N73H7311) calcdm/z715.2),respectively.Complex11,the9-ethyladenine
andC2NH2(N113H1131)ofcoordinated9EtGfromanadjacentmolecule. (9EtA) analogue, was synthesized, and the X-ray crystal
(B) Metal-modified A:A homo base pairing between C2NH (N51H51A) structureofthePF saltshowedthepresenceofN7-bound9EtA
and N1 (N41) of an adjacent molecule for [(Ł6-p-cym)Os(pico)(9EtA)]+ 6
(FigureS10B).Hydrogenbondslinkindependentcoplanar9EtA
(11);50%probabilityellipsoids,remainingHatomshavebeenomittedfor
clarity. units in the crystal structure, N(51)H(51A)(cid:226)(cid:226)(cid:226)N(41) 2.11 Å
(Figure 6B). NH of 9EtA and the chelated oxygen of the
2
bothcelllines(IC 50 4.2and8.0(cid:237)MforA2780andA549cells, picolinateligandarealsoH-bonded,N(51)H(51B)(cid:226)(cid:226)(cid:226)O(82)2.18
respectively). Å (Table S3).
Binding to Nucleobases, Possible Target Sites on DNA.
ThechemicalshiftsoftheH8andH2singletsof9EtAbound
The reactions of the cytotoxic complex, [(Ł6-p-cym)Os(pico)-
to Os in aqueous solutions of 11 shifted to high field with
Cl]5,withmodelnucleobaseswereinvestigated,asbindingto increaseinpH*overtherangeof1-8,withanassociatedpK*
a
DNA is often associated with the cytotoxic action of metal of2.06(0.02(FigureS11B),assignabletoN1protonationof
anticancer drugs.12,28
N7-bound 9EtA.
Inthe1HNMRspectrumofasolutioncontaining5(1mM)
Nonewpeaksappearedinthe1HNMRspectrumof5after
and1molarequiv9-ethylguanine(D O,pH*7.68,298K),new
2 addition of thymidine (Thy), over a period of 24 h (310 K).
peaksassignabletothe9EtGadduct9appeared(FigureS9A),
However,forcytidine(Cyt),asmallsetofnewpeaksappeared
with an approximate half-time for reaction, based on peak
accountingfor<10%of{(Ł6-p-cym)Os(pico)}+present(Figure
integrals,of3.5h(FigureS9B).ESI-MSstudiesonthisNMR
S12).
sample gave a major peak at m/z 627.2, consistent with the
The addition of both 1 molar equiv of 9EtG and 1 molar
presenceof[(Ł6-p-cym)Os(pico)(9EtG)]+(9,calcdm/z627.2).
equiv of 9EtA to 1 mM or 50 (cid:237)M equilibrium solutions of 5
Complex9wassynthesized,andtheX-raycrystalstructureof
(81 and 90% aqua adduct 5A, respectively) did not result in
the PF salt confirmed the binding of 9EtG by N7 (Figure
6 any new peaks after ca. 10 min. However, after incubation at
S10A). An interesting feature of the structure is the hydrogen
310Kfor24h,newpeaksfor9and11hadappeared.Onthe
bonding between NH and NH of 9EtG and the two O atoms
2 basis of the peak integrals (Figure 7), the 1 mM solution of 5
of the chelated picolinate on an adjacent molecule (N(73)H-
contained6.8%aquaadduct5A,80.7%ofthe9EtGadduct9,
(731)(cid:226)(cid:226)(cid:226)O(92) 1.91 Å and N(113)H(1131)(cid:226)(cid:226)(cid:226)O(82) 2.19 Å)
and12.5%ofthe9EtAadduct11,aratioofca.0.5:6:1,whereas
giving rise to chains in the crystal (Figure 6A and Table S3).
the50(cid:237)Msolutionof5contained58.7%aquaadduct5A,32.9%
BindingtoN7of9EtGin9wasfurtherconfirmedbyapH*
9, and 8.4% 11, a ratio of ca. 7:4:1.
titration, monitored by 1H NMR spectroscopy, over the pH*
rangeof3-12.Peaksassignedtospecies9shiftedtohighfield
Solutionsofthe9EtGadduct9,[(Ł6-p-cym)Os(pico)(9EtG)]+,
with increasing pH*, with an associated pK* ) 8.97 ( 0.02 in D 2 O at varying concentrations (2 mM to 20 (cid:237)M) were
a
prepared,andtheir1HNMRspectrawererecordedca.10min
(FigureS11A),consistentwithN1deprotonationofN7-bound
after sample preparation and after incubation at 310 K for 24
9EtG.
h,6days,and13days.Nonewpeakswerepresentinthespectra
Additionof1molarequivofadenosine(Ado)toanaqueous
recordedafter10min,butafter24h,minornewpeaks(3%at
solutionof5(1mM,pH*7.38,298K)gaverisetolittleproduct
2mMand12%at20(cid:237)M)appeared,consistentwithformation
inthe1HNMRspectrumafter5min,butafter24h,therewere
of aqua complex 5A and free 9EtG; see Figure 8A,C. After
(28) Zhang,C.X.;Lippard,S.J.Curr.Opin.Chem.Biol.2003,7,481-489. incubation of the samples for 6 and 13 days, these peaks
3354 J.AM.CHEM.SOC.9VOL.129,NO.11,2007
TuningtheHydrolyticAqueousChemistryofOsAreneComplexes ARTICLES
chelated complexes studied here are racemates due to the
presence of a stereogenic osmium center.
AqueousSolutionChemistry.Aminoacidatecomplexes1-4
containingfive-memberedchelateringshydrolyzedrapidly(t
1/2
< min), in agreement with a report for [(Ł6-benzene)Os(gly)-
Cl](t )0.6minat298K),7acharacteristicsharedbyanionic
1/2
O,O-chelatedOsIIandRuIIarenecomplexes.14,26,29Substitution
Figure8. Low-field 1H NMR spectra of 1 mM, 100 (cid:237)M, and 20 (cid:237)M oftheprimaryamine-NH 2 donorbythe(cid:240)-acceptorpyridine
solutionsof(A)[(Ł6-p-cym)Os(pico)(9EtG)]+(9)and(B)[(Ł6-p-cym)Os- (complexes 5 and 6) in N,O-chelated complexes slows down
(pico)(9EtA)]+(11)directlyaftersamplepreparation,and(C)and(D)after
therateofhydrolysis,31andreplacementofthearenep-cymene
incubation at 310 K for 24 h. Peaks for the free aqua adduct 5A are
(complex 5) by the more electron-deficient arene biphenyl7,30
highlightedbyboxes.Complex9remainsmostlyintact,whereascomplex
11dissociatesalmostcompletelyafter24h(fordeterminationofstability (complex 6) slows down the rate of hydrolysis even further.
constant,seeFigureS13). The hydroxyquinolate complex (7) also hydrolyzes rapidly
despite the presence of the (cid:240)-acceptor pyridine N-donor. This
can be attributed to the higher partial charge on the aryloxide
increased in intensity. After incubation of the 20 (cid:237)M solution
O-donor compared to the carboxylate group in 5 where the
of9for13daysat310K,40%oftheintactcomplexwasstill
charge is delocalized over two oxygens. The large negative
present. Attempts to analyze the data as a simple dissociation
activationentropies(¢Sq)obtainedforthehydrolysisof5and
wereunsuccessful,suggestingthatequilibriumhadnotyetbeen
6suggestthatthemechanisminvolvesanassociativepathway.
reachedundertheseconditions.Similarlythestabilityof2mM
For all of the complexes studied, we observed slow exchange
to20(cid:237)Maqueoussolutionsofthe9EtAadduct11,[(Ł6-p-cym)-
Os(pico)(9EtA)]+, was investigated by 1H NMR spectroscopy betweenthechloroandaquaspeciesonthe1HNMRtimescale,
acharacteristicofN,N-chelatedcomplexes,butnotO,O-chelated
over a period of 24 h at 310 K. Again, no new peaks were
(cid:226)-diketonate complexes. Hence it appears that N,O-chelated
presentinthespectrarecordedafter10min;however,after24
complexes display aqueous chemistry behavior intermediate
hat310K,peaksassignabletofree9EtAandtheaquaosmium
betweenthatofcomplexescontainingneutralN,N-andanionic
complex 5A were present and increased in relative proportion
O,O-chelatingdonors,andthatwithinthegroupofN,O-chelates
withdecreaseinOsconcentration(Figure8B,D).Anequilibrium
thechoiceofN-andO-donorgroupcanhaveasignificanteffect.
bindingconstantoflogK3.95wasobtainedfromtheslopeof
the plot of [11]/[free 9EtG] versus [5A], based on peak Theacidityoftheaqualigandintheaminoacidatecomplexes,
integration (Figure S13). 1-4 (pK a * 7.14-7.55), is intermediate between that of com-
plexes containing the neutral N,N-chelating ethylenediamine
Discussion (en), [(Ł6-bip)Os(en)(OD )]2+ (pK* 6.37), and anionic O,O-
2 a
chelates such as acetylacetonate (acac), [(Ł6-p-cym)Os(acac)-
Ouraiminthisworkwastoinvestigatewhethertheaqueous
(OD )]+(pK*7.84).14Introductionofthe(cid:240)-acceptorpyridine
chemistry of organometallic osmium(II) arene complexes can 2 a
as the N-donor reduces the electron density on the metal,
befine-tunedsoastoachievecancercellcytotoxicityanddesign
lowering the pK* of the coordinated water by 0.6 pK* units
potentialosmiumanticanceragents.Previously,wehadfound a a
(Table 1).32 Due to the unsymmetrical nature of the chelating
that osmium(II) arene complexes containing en as a N,N-
chelating ligand hydrolyze slowly (t ) 6.4 h at 298 K and ligands,theosmiumcenterischiralinallthecomplexesstudied,
1/2
asisevidentfromtheiraqueous1HNMRspectrainwhichall
pH* ca. 7), whereas those with (cid:226)-diketonate O,O-chelating
of the protons of the coordinated arene are magnetically
ligandshydrolyzerapidlybutaredeactivatedbychelateligand
inequivalent. This is in contrast to the unsymmetrical O,O-
lossunderbiologicaltestconditions.14Herewehaveinvestigated
chelated maltolate complexes, for which averaging of proton
complexeswithmixedN,O-chelatingligandsandshowthatthe
environments is observed due to a dynamic (ring-opening)
aqueousreactivitycanbefine-tunedevenwithinN,O-chelates
process which occurs at the metal center.26 Complex 5, for
by the choice of the types of N- and O-donor groups.
Wesynthesizedchloridecomplexes1-7containingprimary example, in aqueous solution gives four p-cymene 1H NMR
peaks, each showing the same pH* dependence (Figure 3A).
amineandtertiarypyridinenitrogensasN-donorsandcarbox-
However, complex 7 behaves differently during the pH*
ylato or aryloxides as O-donors. They were expected to have
titration. At acidic pH*, there are only two sharp doublets in
the familiar half-sandwich, piano-stool structure, and this was
thespectrum;thesebroadenasthepH*isincreasedandresolve
verifiedbyX-raycrystalstructuredeterminationfortwoofthem,
out into the expected four doublets, one for each arene ring
thep-cymene/picolinatecomplex5andthep-cymene/hydroxy-
proton, at basic pH* (Figure 3B). This suggests that in acidic
quinolate complex 7. The structures of OsII arene complexes
are often similar to those of their RuII analogues. Both 7 and
(29) Fernandez,R.;Melchart,M.;Habtemariam,A.;Parsons,S.;Sadler,P.J.
itsRuIIanalogue16formalmostidenticalshort-rangeinteractions Chem.sEur.J.2004,10,5173-5179.
in their crystal structures, and complex 5 is structurally very (30) Dougan,S.;Melchart,M.;Habtemariam,A.;Parsons,S.;Sadler,P.J.Inorg.
Chem.2006,45,10882-10894.
similar to the related RuII complex, [(Ł6-1,3,5-C Me H )Ru-
6 3 3 (31) Thepresenceofa(cid:240)-acceptorligandcanstabilizethetransitionstateinan
(pico)Cl].15 The Os-Cl bond length in 7 (2.4235(7) Å) is associativereactionandthereforeincreasetherate,asobservedforsquare-
planarPtIIcomplexes: Summa,N.;Schiessl,W.;Puchta,R.;vanEikema
significantly longer than that in 5 (2.4048(13) Å), consistent Hommes, N.; van Eldik, R. Inorg. Chem. 2006, 45, 2948-2959. If the
withtheincreasedchargeattheOsIIcenterin7andtheslower reaction mechanism for the pseudo-octahedral osmium complexes is
associative,H-bondingandstericeffectsmaycontributetotheslowerrates
hydrolysis rate of 5. A similar difference in bond length and ofhydrolysis.
hydrolysisratewasnotedforOsII/RuIIacetylacetonateandOsII/ (32) During the pH* titrations of complexes 1-7, we observed no new
independentpeaksinthe1HNMRspectrawhichmightbeassignableto
RuII ethylenediamine half-sandwich complexes.14,29 The N,O- ring-openedspecies.
J.AM.CHEM.SOC.9VOL.129,NO.11,2007 3355
ARTICLES Peacocketal.
solutions the dynamic process involves rapid chelate ring
opening via oxygen atom protonation, which slows down at
basic pH*. 8-Hydroxyquinolate is a stronger base (pK HQH
a
)5.0(0.1)33than2-picolinate(pK picoH)1.01)34andthe
a
othercarboxylateN,O-ligandsinvestigatedhere,suggestingthat
itwouldbemorereadilyprotonated.Thisdynamicprocesswas
rapid for the maltolate complex [(Ł6-p-cym)Os(mal)(OD )]+,
2
even at basic pH*, which is in keeping with the even higher
basicity of maltolate (pK malH ) 8.62).26,35 The dynamic
a
process was monitored for D O solutions of 4, 5, and 7 at
2
various temperatures. The exchange rates imply that ring
openingoccursmostreadilyfortheoxinecomplex7andleast
readilyforthepicolinatecomplex5,whichisagainconsistent
with the acidity of the chelated oxygen atom (pK of free
a
(cid:226)-alanineis3.43).36Therefore,themoreacidicligandsexhibit
enhancedstabilitywithrespecttochelateprotonationandring
opening, which may also contribute to the higher stability of
complexes with respect to formation of the hydroxo-bridged
dinuclear species (8).
It seems likely that the mechanism of formation of the
hydroxo-bridgeddinuclearspecies(8)initiallyinvolvesbreakage
oftheOs-Obond,assistedbyprotonation,followedbyOs-N
bond breakage. This does not occur when the chelated ligand
contains a pyridine N, despite the ready protonation of the
coordinated oxygen in complex 7. Evidently, the Os-N(pyri-
dine) bond does not break under the conditions studied,
preventingformationofthehydroxo-bridgeddinuclearspecies
(8).Clearlytheincorporationofa(cid:240)-acceptorsuchaspyridine
is crucial for the overall stability of the complex, due to the
strengtheningofthemetal-Nbondformed.Complexescontain-
ingsix-memberedaminoacidatechelaterings(complex4)show
less stability with respect to formation of 8 than their five-
memberedanalogues(complexes1-3),atrendsimilartothat
found previously for anionic O,O-chelates.26 Figure9. Bar charts illustrating the relationship between cytotoxicity
toward human cancer cells, stability with respect to formation of inert
Athighchlorideconcentrationstypicalofbloodplasma(100
hydroxo-bridgeddinuclearspecies,ratesofhydrolysis,andacidityofaqua
mM),complex5waspresentasthelessreactiveintactchloro adductsforosmiumarenecomplexes[(Ł6-arene)Os(XY)Cl]n+containing
species.Atthelowerchlorideconcentrationof4mM(atypical differentXY)N,N-,N,O-,andO,O-chelatingligands.Shadingindicates
nucleusconcentration),27thecomplexisactivatedbyhydrolysis, therangeofobservedvalues.
with 72% present as the reactive aqua species (5A). This
lines, respectively, are comparable to those of the anticancer
suggests a possible mode of activation toward DNA binding.
drug carboplatin (IC ) 10 and 6 (cid:237)M for A549 and A2780,
In addition, complex 5 was stable in isotonic saline solution 50
respectively).38,39
when stored in the dark at ambient temperature for 2 months,
Figure 9 provides an overview of the relationships between
making aqueous drug formulations feasible.
Complexes containing aminoacidate chelates (1-4) hydro- cancer cell cytotoxicity, complex stability with respect to
formation of hydroxo-bridged dinuclear species, rates of hy-
lyzed rapidly, were unstable with respect to formation of the
drolysis, and acidity of coordinated water. Our work demon-
hydroxo-bridgeddinuclearspecies(8),andwereinactivetoward
humanovarianandlungcancercells.However,complexes5-7 strateshowrationalchemicaldesigncanbeappliedtoosmium
arene complexes resulting in specific windows of reactivity,
containing a pyridine as the N-donor atom, which were stable
stability, and cancer cell cytotoxicity.
at micromolar concentrations and reacted with purine nucleo-
bases, exhibit cytotoxic activity (IC ) 4-60 (cid:237)M, Table 3). BindingtoNucleobases.NuclearDNAisoftenbelievedto
50
Activityincreaseswithadecreaseintherateofhydrolysisand be the major target of metal-based anticancer complexes.28
wasgreatestforcomplex6whichexhibitedtheslowestkinetics.
Osmiumarenecomplexes,[(Ł6-arene)Os(XY)Cl]n+,containing
The rate of hydrolysis of 6 is comparable to the rate of a neutral N,N-chelate bind selectively to G-type nucleobases,
hydrolysisofactiverutheniumareneencomplexes.37IC values and those containing an anionic O,O-chelate have similar
50
for 6 of 8 and 4.2 (cid:237)M for the A549 and A2780 cancer cell affinitiesforbothG-andA-typenucleobases.14,26Thecompeti-
tionreactionscarriedoutwithcomplex5showbindingtoboth
(33) Irving,H.;Ewart,J.A.D.;Wilson,J.T.J.Chem.Soc.1949,2672-2674. GandA,butwithastrongpreferenceforG.Itisnotablethat,
(34) Green,R.W.;Tong,H.K.J.Am.Chem.Soc.1956,78,4896-4900.
(35) Dutt,N.K.;Rahut,S.J.Inorg.Nucl.Chem.1970,32,1035-1038.
(36) Vcˇela´kova´,K.;Zuskova´,I.;Kennaler,E.;Gasˇ,B.Electrophoresis2004, (38) Cui,K.;Wang,L.;Zhu,H.;Gou,S.;Liu,Y.Bioorg.Med.Chem.Lett.
25,309-317. 2006,16,2937-2942.
(37) Wang,F.;Chen,H.;Parsons,S.;Oswald,I.D.H.;Davidson,J.E.;Sadler, (39) Aird,R.E.;Cummings,J.;Ritchie,A.A.;Muir,M.;Morris,R.E.;Chen,
P.J.Chem.sEur.J.2003,9,5810-5820. H.;Sadler,P.J.;Jodrell,D.I.Br.J.Cancer2002,86,1652-1657.
3356 J.AM.CHEM.SOC.9VOL.129,NO.11,2007
TuningtheHydrolyticAqueousChemistryofOsAreneComplexes ARTICLES
even at micromolar concentrations, >40% of the osmium is CisplatinbindingtoDNAresultsin25%oftotallesionsbeing
boundtopurinenucleobases.Intriguingly,thebindingconstant ApGintrastrandadducts,andtheseare5timesmoremutagenic
for 9EtA is only moderate (log K 3.95), and equilibrium for thanthemorecommonGpGadducts.50Therefore,suchinterac-
dissociationof9EtAfrom[(Ł6-p-cym)Os(pico)(9EtA-N7)]+(11) tionsresultingfromAbindingonDNAcouldcontributetothe
is reached within 24 h of incubation (310 K). In contrast, anticancer activity of complex 5.
dissociation of the 9EtG adduct [(Ł6-p-cym)Os(pico)(9EtG-
N7)]+(9)atmicromolarconcentrationsoccursrelativelyslowly, Conclusions
and equilibrium is not reached even after incubation at 310 K The goal of the present study was to design osmium arene
for 13 days. Hence, once G adducts (on DNA or RNA) are complexeswhicharecytotoxictowardcancercellsandtherefore
formed, they are likely to persist. Such kinetic stability may potential novel anticancer drugs. This has been achieved by
also make G adducts less susceptible to repair compared to A employing chemical principles to tune the reactivity, aqueous
adducts. Binding to the N7 of 9EtG or 9EtA acidifies the N1 chemistry, and stability of this class of complexes.
proton,whichisconsistentwithmetalationattheN7site.11,40,41 We have found that half-sandwich, piano stool OsII arene
Littleandnobindingof[(Ł6-p-cym)Os(pico)Cl]tothepyrimi- complexesofgeneralformula[(Ł6-arene)Os(N,O)Cl],containing
dine bases, Cyt and Thy, was observed. The N3 position is ananionicN,O-chelate,displaychemicalreactivityintermediate
sterically crowded in both pyrimidine bases making it an between those of the neutral N,N- and anionic O,O-chelated
unfavorable binding site, and at physiological pH, N3 of Thy parent compounds. However, even within this group of N,O-
isprotonatedmakingitlessavailableforbinding(andinvolved chelates,thechoiceoftheN-andO-donorsiscrucial.Themore
in Watson-Crick H-bonding). acidicthechelatedoxygenthelessreadilyringopeningoccurs,
AsearchoftheCambridgeDatabaserevealednopreviously and the less readily hydroxo-bridged dinuclear species are
reportedX-raycrystalstructuresofosmiumcomplexescontain- formed. More critically, the introduction of a (cid:240)-acceptor such
ingcoordinatedguanineoradeninenucleobases.However,the aspyridine(asincomplex5[(Ł6-p-cym)Os(pico)Cl])minimizes
structureof[Os(9MeHyp)(NH ) ]Cl (cid:226)H O(9MeHyp)9-meth- chelate ring opening through strengthening the Os-N bond.
3 5 3 2
ylhypoxanthine),whichcontainsthepurinebasehypoxanthine Thesefactorsappeartobecrucialinmaintainingstabilitywith
(Hyp), has been reported.42 The X-ray structures of 9 and 11 respect to formation of inert (biologically inactive) hydroxo-
confirmedthebindingof{(Ł6-p-cym)Os(pico)}+tothesterically bridged dinuclear species which can deactivate osmium arene
non-hindered N7 site. In solution, a minor product (10b; ca. complexes even at micromolar concentrations and in the
20%) also formed with adenosine and can be assigned to an presence of saline (0.1 M).14,26
N1-bound species. The Os-N7(nucleobase) bond lengths Complex5showsastrongpreferenceforbindingtoGbases
comparewellwiththosereportedfor[Os(9MeHyp-N7)(NH 3 ) 5 ]- over A bases, with little or no reaction with pyrimidines. The
Cl 3 (cid:226)H 2 O (2.107 Å), [(Ł6-p-cym)Ru(gly)(9EtG-N7)]PF 6 (2.136 X-ray crystal structures of the G and A adducts of complex 5
Å),43 and [(Ł6-benzene)Ru(L-ala)(9EtG-N7)]Cl (2.115(6) and are,asfarasweareaware,thefirstosmiumGandAadducts
2.112(7)Å).44Intriguingly,thenucleobasefunctionalitylieson tobereported.ReactivitytowardDNAnucleobases,alongwith
oppositesidesofthechelateforGandAbases.Theexocyclic theaqueouschemistryandstability(atmicromolarconcentra-
oxygen of 9EtG is on the N-chelated side of the picolinate tions), makes [(Ł6-p-cym)Os(pico)Cl] 5, [(Ł6-bip)Os(pico)Cl]
ligand, and the NH
2
of 9EtA is on the O-chelated side (as is 6,and[(Ł6-p-cym)Os(oxine)Cl]7suitablecandidatesforfurther
alsothecaseforrelatedrutheniumcomplexes),43,44bothforming investigation as anticancer agents. These complexes exhibit
favorableshort-rangeinteractionswiththechelate(O(cid:226)(cid:226)(cid:226)CHand activity against human A549 lung and A2780 ovarian cancer
NH(cid:226)(cid:226)(cid:226)O, respectively). cells, comparable to that of carboplatin.
HomobasepairinginvolvingC6NH (cid:226)(cid:226)(cid:226)N1(C51NH (cid:226)(cid:226)(cid:226)N41)
2 2
Acknowledgment. WethanktheEPSRCandTheUniversity
H-bonding is observed between the metal-modified adenine
of Edinburgh (studentship for A.F.A.P.), Rhona E. Aird and
fragmentsintheX-raycrystalstructureof11(Figure6B).This
Professor Duncan Jodrell (Western General Hospital, Cancer
base pairing is of the “extended pairing” type45 as the N7
positionisblockedbythecoordinatedosmium.Non-Watson- ResearchUKCentre)foradviceandassistancewithcellculture,
andDr.AbrahaHabtemariam(Edinburgh)andmembersofEC
Crick adenine homo base pairing is not unusual and is also
present in the X-ray crystal structures of 2¢,3¢-O-anhydroad- COST groups D20 and D39 for stimulating discussions.
enosine46 and tetraaqua-(9-methyladenine) copper(II) sulfate Supporting Information Available: Details of the crystal-
monohydrate.47,48Evidenceforsuchpairinghasevenbeenfound lographic data (Tables S1-S3, Figures S1 and S10), aqueous
incytosine-richDNAatlowpH.49SuchbindingtoDNAmight chemistry (Tables S4-S6, Figures S2-S8), and nucleobase
therefore result in mismatches in the base pairing of adenine. studies (Figure S9-S13); X-ray crystallographic data in CIF
(40) Sigel,H.J.Am.Chem.Soc.1975,97,3209-3214. format.ThismaterialisavailablefreeofchargeviatheInternet
(41) Inagaki,K.;Kidani,Y.J.Inorg.Biochem.1979,11,39-47. at http://pubs.acs.org.
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E.;Jodrell,D.I.;Sadler,P.J.J.Med.Chem.2006,49,6858-6868. (48) RelatedPtII-G-(cid:226)(cid:226)(cid:226)G-andPtII-G-(cid:226)(cid:226)(cid:226)GhomobasepairingbetweenN1(de)-
(44) Sheldrick,W.S.;Heeb,S.Inorg.Chim.Acta1990,168,93-100. protonated9-ethylguaninebaseshasbeenobservedintheX-raycrystal
(45) B R i o o b c i h n e s m on is , tr H y .; 19 v 9 an 2, d 3 e 1 r , M 10 a 5 r 1 e 0 l, - G 1 . 05 A 1 . 7 ; . van Boom, J. H.; Wang, A. H.-J. s H tr 9 u E c t t G ur (cid:226) e 7H o 2 f O, ci r s e - s [ p P e t( c N tiv H e 3 l ) y 2 : (9E S t c G hr - o¨ ) d 2] e (cid:226) r 4 , H G 2O .;L a i n p d per c t i , s- B [P .; t( S N a H ba 3 t ) , 2( M 9E .; tG L - o ) c 2 k ](cid:226) ,
(46) Koole,L.H.;Neidle,S.;Crawford,M.D.;Krayevski,A.A.;Gurskaya, C.J.L.;Faggiani,R.;Song,B.;Sigel,H.J.Chem.Soc.,DaltonTrans.
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J.AM.CHEM.SOC.9VOL.129,NO.11,2007 3357