The conformations of unsolvated glycine-based peptides have been probed using high-resolution ion mobility measurements and molecular dynamics simulations. Alanine-based Ac-Ala(n)-LysH(+) peptides have previously been shown to form helices in the gas phase. In contrast, the polyglycine analogues (Ac-Gly(n)-LysH(+)) do not form helices at room temperature; they adopt random globular conformations. Thus, the stabilities of alanine and glycine helices in vacuo are consistent with the helix propensities in aqueous solution, where alanine has the highest helix propensity and glycine has one of the lowest.