Compact → extended helix transitions of polyalanine in vacuo

The relative stabilities of conformations of [Ala(n) + 3H](3+) peptides (where n = 27-39) have been examined in the gas phase using ion mobility/time-of-flight techniques. At room temperature, two series of conformations that do not interconvert during the 10-40-ms experimental time scales are observed: extended helical structures and more compact geometries assigned as hinged helix-coils (Counterman, A. E.; Clemmer, D. E. J. Am. Chem. Soc., 2001, 123, 1490). At elevated temperatures, a single structural transition is observed; the more compact state unfolds and adopts extended helical structures for all sizes. Relative activation energies for the compact --> extended helix transitions increase linearly by similar to 1 kcal.mol(-1).residue(-1) over the n = 27-39 size range. Mechanisms that consider important steps in the helix-coil --> helix transition are discussed. We propose that release of the hinge might be a difficult step in this transition that is instigated by intramolecular reorganization of protons.