Conformations of Gly(n)H⁺ and Ala(n)H⁺ peptides in the gas phase

High-resolution ion mobility measurements and molecular dynamics simulations have been used to probe the conformations of protonated polyglycine and polyalanine (Gly(n)H⁺ and Ala(n)H⁺, n = 3-20) in the gas phase. The measured collision integrals for both the polyglycine and the polyalanine peptides are consistent with a self-solvated globule conformation, where the peptide chain wraps around and solvates the charge located on the terminal amine. The conformations of the small peptides are governed entirely by self-solvation, whereas the larger ones have additional backbone hydrogen bonds. Helical conformations, which are stable for neutral Ala(n) peptides, were not observed in the experiments. Molecular dynamics simulations for Ala(n)H⁺ peptides suggest that the charge destabilizes the helix, although several of the low energy conformations found in the simulations for the larger Ala(n)H⁺ peptides have small helical regions.