The transition from unregulated, prebiotic chemistry to metabolic-like systems capable of supporting an evolving protocell has remained difficult to explain. One hypothesis is that early catalysts beg Show more
The transition from unregulated, prebiotic chemistry to metabolic-like systems capable of supporting an evolving protocell has remained difficult to explain. One hypothesis is that early catalysts began to prune the chemical landscape in a manner that facilitated the emergence of modern-day enzymes. As enzymes frequently rely on the intrinsic reactivity of metal ions, it follows that these early catalysts may have been metal ions coordinated to prebiotic peptides that have remained as core structures within extant proteins. Here, we demonstrate that UV light directly selects for the types of metal-binding peptide motifs found in biology. This is because bare cysteine is much more susceptible to photolysis than cysteine bound by a metal ion. Therefore, peptides with greater affinity for environmentally available metal ions, such as Fe2+ or Zn2+, are more stable. Our results are supported by mass spectrometry, calorimetry, X-ray absorption, NMR spectroscopy, transient absorption pump probe spectroscopy, and excited-state quantum-chemical calculations. Photostability arises from the ability of the metal ion to engage transiently generated reactive radical centers in a manner that prevents subsequent degradative processes. The data are consistent with the enrichment of a restricted set of high affinity, extant-like metallopeptides in surficial environments on the early Earth. Show less
Conditions that led to the synthesis of iron-sulfur clusters coordinated to tripeptides with a single thiolate ligand were investigated by UV-vis, NMR, EPR, and Mössbauer spectroscopies and by electro Show more
Conditions that led to the synthesis of iron-sulfur clusters coordinated to tripeptides with a single thiolate ligand were investigated by UV-vis, NMR, EPR, and Mössbauer spectroscopies and by electrochemistry. Increasing concentrations of hydrosulfide correlated with the formation of higher nuclearity iron-sulfur clusters from mononuclear to [2Fe-2S] to [4Fe-4S] and finally to a putative, nitrogenase-like [6Fe-9S] complex. Increased nuclearity was also associated with decreased dynamics and increased stability. The synthesis of higher nuclearity iron-sulfur clusters is compatible with shallow, alkaline bodies of water on the surface of the early Earth, although other niche environments are possible. Because of the plasticity of such complexes, the type of iron-sulfur cluster formed on the prebiotic Earth would have been greatly influenced by the chemical environment and the thiolate containing scaffold. The discovery that all the major classes of iron-sulfur clusters easily form under prebiotically reasonable conditions broadens the chemistry accessible to protometabolic systems. Show less