Prebiotic Environments with Mg2+ and Thiophilic Metal Ions Increase the Thermal Stability of Cysteine and Non-cysteine Peptides.

Wet-dry cycles driven by heating to high temperatures are frequently invoked for the prebiotic synthesis of peptides. Similarly, iron-sulfur clusters are often cited as an example of an ancient catalyst that helped prune early chemical systems into metabolic-like pathways. Because extant iron-sulfur clusters are metallocofactors of protein enzymes and nearly ubiquitous across biology, a reasonable hypothesis is that prebiotic iron-sulfur peptides formed on the early Earth. However, iron-sulfur clusters are coordinated by multiple cysteine residues, and the stability of cysteines to the heat steps of wet-dry cycles has not been determined. It, therefore, has remained unclear if the peptides needed to stabilize the formation of iron-sulfur clusters could have formed. If not, then iron-sulfur-dependent activity may have emerged later, when milder, more biological-like peptide synthesis machinery took hold. Here, we report the thermal stability of cysteine-containing peptides. We show that temperatures of 150 °C lead to the rapid degradation of cysteinyl peptides. However, the presence of Mg2+ at environmentally reasonable concentrations leads to significant protection. Thiophilic metal ions also protect against degradation at 150 °C but require concentrations not frequently observed in the environment. Nevertheless, cysteine-containing peptides are stable at lower, prebiotically plausible temperatures in seawater, carbonate lake, and ferrous lake conditions. The data are consistent with the persistence of cysteine-containing peptides on the early Earth in environments rich in metal ions. High concentrations of Mg2+ are common intra- and extra-cellularly, suggesting that the protection afforded by Mg2+ may reflect conditions that were present on the prebiotic Earth.