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

We utilized gas phase hydrogen/deuterium (H/D) exchange reactions and ab initio calculations to investigate the complexation between a model peptide (Arg-Gly-AspRGD) with various alkali metal ions. The peptide conformation is drastically altered upon alkali metal ion complexation. The associated conformational changes depend on both the number and type of complexing alkali metal ions. Sodium has a smaller ionic diameter and prefers a multidentate interaction that involves all three amino acids of the peptide. Conversely, potassium and cesium form different types of complexes with the RGD. The RGD + 2Cs ? H]⁺ species exhibit the slowest H/D exchange reactivity (reaction rate constant of 6 × 10^?13 cm³molecule^?1s^?1 for the fastest exchanging labile hydrogen with ND₃). The reaction rate constant of the protonated RGD is two orders of magnitude faster than that of the RGD + 2Cs ? H]⁺. Addition of the first cesium to the RGD reduces the H/D exchange reaction rate constant (i.e., D₀) by a factor of seven whereas sodium reduces this value by a factor of thirty. Conversely, addition of the second alkali metal ions has the opposite effect; the rate of D₀ disappearance for all RGD + 2Met ? H]⁺ species (MetNa, K, and Cs) decreases with the alkali metal ion size.