Gas phase hydrogen deuterium exchange reactions of a model peptide: FT-ICRand computational analyses of metal induced conformational mutations

We utilized gas phase hydrogen/deuterium (H/D) exchange reactions and ab in itio calculations to investigate the complexation between a model peptide ( Arg-Gly-Aspa drop RGD) with various alkali metal ions. The peptide conforma tion is drastically altered upon alkali metal ion complexation. The associa ted conformational changes depend on both the number and type of complexing alkali metal ions. Sodium has a smaller ionic diameter and prefers a multi dentate interaction that involves all three amino acids of the peptide. Con versely, potassium and cesium form different types of complexes with the RG D. The [RGD + 2Cs - H](+) species exhibit the slowest H/D exchange reactivi ty (reaction rate constant of similar to6 X 10(-13) cm(3) molecule(-1)s-(1) for the fastest exchanging labile hydrogen with ND3). The reaction rate co nstant of the protonated. RGD is two orders of magnitude faster than that o f the [RGD + 2Cs - H](+). Addition of the first cesium to the RGD reduces t he H/D exchange reaction rate constant (i.e., D-0) by a factor of seven whe reas sodium reduces this value by a factor of thirty. Conversely, addition of the second alkali metal ions has the opposite effect; the rate of Do dis appearance for all [RGD + 2Met - H](+) species (Met drop Na, K, and Cs) dec reases with the alkali metal ion size. (C) 2001 American Society for Mass S pectrometry.