The hydrogen/deuterium (H/D) exchange of protonated and alkali-metal cationized Arg-Gly and Gly-Arg peptides with D(2)O in the gas phase was studied using electrospray ionization quadropole ion trap mass spectrometry. The Arg-Gly and Gly-Arg alkali metal complexes exchange significantly more hydrogens than protonated Arg-Gly and Gly-Arg. We propose a mechanism where the peptide shifts between a zwitterionic salt bridge and nonzwitterionic charge solvated conformations. The increased rate of H/D exchange of the alkali metal complexes is attributed to the peptide metal complexes' small energy difference between the salt-bridge conformation and the nonzwitterionic charge-solvated conformation. Implications for the applicability of this mechanism to other zwitterionic systems are discussed.
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