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 3 6 + 4H] 4 + , ∼35% of hydrogen-bonding interactions are associated with i → i + 3 contacts, while ∼65% correspond to i → i + 4 associations. In the larger [Ala 4 8 + 4H] 4 + polymer, ∼85% of residues are involved in i → i + 4 hydrogen-bonding interactions.