Disrupting helix formation in unsolvated peptides

Protonated polyalanine peptides form helices in the gas phase when their most basic protonation site (the N-terminus) is blocked by acetylation: Ac-A(a)+H+ (Ac acetyl and A = alanine). The glycine analogues, Ac-G(n)+H+ (G glycine), on the other hand, form random globules. The disruption of helix formation in unsolvated Ac-A(n)G(x)A(m)+H+ peptides has been examined as a function of n+m, and x using high resolution ion mobility measurements and molecular dynamics simulations. A surprisingly large block of glycine residues is required to disrupt helix formation in these peptides. For example, Ac-A(5)G(3)A(5)+H+ and AC-A(6)G(5)A(6)+H+ both remain helical at room temperature. According to molecular dynamics simulations, the glycines do not cause a localized disruption of the helices, as might be expected for a residue considered a helix breaker. This is consistent with helix disruption occurring through a global effect on the relative energies of the helix and globule rather than through a localized entropic effect.