RNA Binding and Coacervation Promotes Preservation of Peptide Form and Function Across the Heterochiral-Homochiral Divide

Recent evidence suggests that peptide-RNA coacervates may have buffered the emergence of folded domains from flexible peptides. As primitive peptides were likely composed of both L- and D-amino acids, we hypothesized that coacervates may have also supported the emergence of chiral control. To test this hypothesis, we compared the coacervation propensities of an isotactic (homochiral) peptide and a syndiotactic (alternating chirality) peptide, both with an identical sequence derived from the ancient helix-hairpin-helix (HhH) motif. Using electron paramagnetic resonance (EPR) spectroscopy and molecular dynamics (MD) simulations, we found that the syndiotactic peptide does not form stable dimers with high α-helicity in solution, unlike the isotactic peptide. However, both peptides do coacervate with RNA, albeit with distinct reentrant phase behaviors. Coacervation in each case is facilitated by oligomer formation, likely dimerization, upon RNA binding that promotes RNA cross-linking. Additionally, RNA cross-linking and coacervation of the syndiotactic peptide seems to involve α-helical conformations. We attribute differences in reentrant phase behavior to differences in dimer flexibility and stability that alter the effectiveness of RNA cross-linking. These results illustrate how RNA-binding and/or coacervation by early protein forms could have promoted the transition of flexible, heterochiral peptides into folded, homochiral domains.