Gas Phase Polyalanine: Assessment of i → i + 3 and i → i + 4 Helical Turns in [Alan + 4H]4+ (n = 29−49) Ion

Abstract

The structures of a series of quadruply protonated polyalanine peptides ([Alan + 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 [Alan + 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 [Ala36 + 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 [Ala48 + 4H]4+ polymer, ∼85% of residues are involved in i → i + 4 hydrogen-bonding interactions.