Conformation of polyalanine and polyglycine dications in the gas phase: insight from ion mobility spectrometry and replica-exchange molecular dynamics.

The conformation of model [Arg(Ala)(4)X(Ala)(4)Lys+2H](2+) and [Arg(Gly)(4)X(Gly)(4)Lys+2H](2+) peptides has been systematically investigated as a function of the central amino acid X through a combined experimental and theoretical approach. Mass spectrometry-based ion mobility measurements have been performed together with conformational sampling using replica-exchange molecular dynamics to probe the influence of each amino acid on the stable peptide conformation. Satisfactory agreement is obtained between measured and calculated diffusion cross section distributions. The results confirm the propensity of alanine-based peptides to form alpha-helices in the gas phase, differences between peptides arising from the local arrangement of the central side chain with respect to the charged ends. More generally, we find that charge solvation plays a major role in secondary structure stabilization, especially in the case of glycine-based peptides. The rich variety of conformations exhibited by the latter is qualitatively captured by the simulations. This work illustrates the potentiality of such combined experimental/theoretical strategy to determine peptide secondary structures. The present polyalanine and polyglycine peptides also offer a series of benchmark systems for future conformation-resolved studies.