Large anhydrous polyalanine ions: evidence for extended helices and onset of a more compact state.

Ion mobility measurements and molecular modeling calculations have been used to examine the conformations of large multiply charged polyalanine peptides. Two series of [Ala(n)+3H](3+) conformations which do not interconvert during the 10 to 30 ms experimental timescales are observed: a family of elongated structures for n = 18 to 39 and a series of more compact conformations for n = 24 to 41. The more compact state becomes the dominant conformer type for n > 32. Molecular modeling studies and comparisons of calculated collision cross sections with experiment indicate that the elongated ions have extended helical conformations. We suggest that the more compact state corresponds to a new conformer type: a folded hinged helix-coil state in which helical and coil regions have similar physical dimensions. The competition between extended and compact states is rationalized by considering differences in charge stabilization and entropy.