Unfolding of α-helical 20-residue poly-glutamic acid analyzed by multiple runs of canonical molecular dynamics simulations

Elucidating the molecular mechanism of helix–coil transitions of short peptides is a long-standing conundrum in physical chemistry. Although the helix–coil transitions of poly-glutamic acid (PGA) have been extensively studied, the molecular details of its unfolding process still remain unclear. We performed all-atom canonical molecular dynamics simulations for a 20-residue PGA, over a total of 19 μs, in order to investigate its helix-unfolding processes in atomic resolution. Among the 28 simulations, starting with the α-helical conformation, all showed an unfolding process triggered by the unwinding of terminal residues, rather than by kinking and unwinding of the middle region of the chain. The helix–coil–helix conformation which is speculated by the previous experiments was not observed. Upon comparison between the N- and C-termini, the latter tended to be unstable and easily unfolded. While the probabilities of helix elongation were almost the same among the N-terminal, middle, and C-terminal regions of the chain, unwinding of the helix was enriched at the C-terminal region. The turn and 3(10)-helix conformations were kinetic intermediates in the formation and deformation of α-helix, consistent with the previous computational studies for Ala-based peptides.