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 ∼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.