SCANNING tunnelling microscopy (STM) has been used to map the surface topography of inorganic materials at the atomic level, and is potentially one of the most powerful techniques for probing biomolec Show more
SCANNING tunnelling microscopy (STM) has been used to map the surface topography of inorganic materials at the atomic level, and is potentially one of the most powerful techniques for probing biomolecular structure1–3. Recent STM studies of calf thymus DNA4,5and poly(rA) · poly(rU)5 have shown that the helical pitch and periodic alternation of major and minor grooves can be visualized and reliably measured. Here we present the first STM images of poly(dG-me5d) · poly(dG-me5dC) in the Z-form. Both the general appearance of the fibres and measurements of helical parameters are in good agreement with models derived from X-ray diffraction6–4. Show less
In the equilibrium between B-DNA and Z-DNA in poly(dC-dG), the [Co(NH3)6]3+ ion stabilizes the Z form 4 orders of magnitude more effectively than the Mg2+ ion. The structural basis of this difference Show more
In the equilibrium between B-DNA and Z-DNA in poly(dC-dG), the [Co(NH3)6]3+ ion stabilizes the Z form 4 orders of magnitude more effectively than the Mg2+ ion. The structural basis of this difference is revealed in Z-DNA crystal structures of d(CpGpCpGpCpG) stabilized by either Na+/Mg2+ or Na+/Mg2+ plus [Co(NH3)6]3+. The crystals diffract X-rays to high resolution, and the structures were refined at 1.25 A. The [Co(NH3)6]3+ ion forms five hydrogen bonds onto the surface of Z-DNA, bonding to a guanine O6 and N7 as well as to a phosphate group in the ZII conformation. The Mg2+ ion binds through its hydration shell with up to three hydrogen bonds to guanine N7 and O6. Higher charge, specific fitting of more hydrogen bonds, and a more stable complex all contribute to the great effectiveness of [Co(NH3)6]3+ in stabilizing Z-DNA. Show less