Large anhydrous polyalanine ions: substitution of Na + for H + destabilizes folded states

Abstract

The conformations of a series of anhydrous sodiated polyalanine ions ([Ala n + 3Na] 3+, where n = 18–36) have been examined in the gas phase by ion mobility measurements and molecular modeling simulations. The experimental results indicate that these ions exist as highly extended conformations. There is no strong evidence for a folded state, observed previously for a series of analogous protonated polyalanines ([Ala n + 3H] 3+, where n = 24–41) (A.E. Counterman, D.E. Clemmer, J. Am. Chem. Soc., submitted). Molecular dynamics simulations for the [Ala n + 3Na] 3+ ions also indicate that extended structures are favored. The simulations show that extensive helical regions are present; however, near the sites where Na + ions are attached, helical regions appear to be substantially disrupted by intramolecular charge solvation of the Na +. Simulations of some [Ala n + 3Na] 3+ charge site assignments show evidence for structures that are similar to folded structures observed for analogous [Ala n + 3H] 3+ ions; however, the calculated energy gap between the folded and unfolded states for the triply sodiated system is slightly greater than the gap in analogous protonated polyalanines. We propose this as a possible explanation for the absence of experimental evidence for a folded state in the sodiated system. Keywords: Polyalanine; Ion mobility; Sodium; Stability; Electrospray