Gas phase polyalanine:: Assessment of i→i+3 and i→i+4 helical turns in [Alan+4H]4+ (n=29-49) ion

The structures of a series of quadruply protonated polyalanine peptides ([Ala(n) + 4H](4+), n = 29-49) have been examined in the gas phase using high-pressure drift tube/mass spectrometry and molecular modeling techniques. The results indicate that [Ala(n) + 4H](4+) ions favor stretched helices; i --> i + 3 and i --> i + 4 hydrogen-bonding interactions are significant in stabilizing the extended structures. Experiment and theory indicate that the fraction of i --> i + 4 interactions increases with increasing polymer length. It appears that i --> i + 3 interactions stabilize smaller polymers, where repulsive Coulombic interactions are greater. For [Ala(36) + 4H](4+), similar to35% of hydrogen-bonding interactions are associated with i --> i + 3 contacts, while similar to65% correspond to i --> i + 4 associations. In the larger [Ala(48) + 4H](4+) polymer, similar to85% of residues are involved in i --> i + 4 hydrogen-bonding interactions.