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

Ion mobility measurements and molecular modeling calculations have been use d to examine the conformations of large multiply charged polyalanine peptid es. Two series of [Ala(n)+3H](3+) conformations which do not interconvert d uring the 10 to 30 ms experimental timescales are observed: a family of elo ngated structures for n = 18 to 39 and a series of more compact conformatio ns for n = 24 to 41. The more compact state becomes the dominant conformer type for n > 32. Molecular modeling studies and comparisons of calculated c ollision cross sections with experiment indicate that the elongated ions ha ve extended helical conformations. We suggest that the more compact state c orresponds to a new conformer type: a folded hinged helix-coil state in whi ch helical and coil regions have similar physical dimensions. The competiti on between extended and compact states is rationalized by considering diffe rences in charge stabilization and entropy.