Collision-Induced Dissociation of Halide Ion–Arginine Complexes: Evidence for Anion-Induced Zwitterion Formation in Gas-Phase Arginine

Abstract

We report the first low-energy collisional-induced dissociation studies of the X^–·arginine (X^– = F^–, Cl^–, Br^–, I^–, NO₃^–, ClO₃^–) series of clusters to investigate the novel phenomenom of anion-induced zwitterion formation in a gas-phase amino acid. Fragmentation of the small halide ion clusters (F^–·arginine and Cl^–·arginine) is dominated by deprotonation of the arginine, whereas the major fragmentation channel for the largest ion clusters (I^–·arginine and ClO₃^–·arginine) corresponds to simple cluster fission into the ion and neutral molecule. However, the fragmentation profiles of Br^–·arginine and NO₃^–·arginine, display distinctive features that are consistent with the presence of the zwitterionic form of the amino acid in these clusters. The various dissociation pathways have been studied as a function of % collision energy and are discussed in comparison to the fragmentation profiles of protonated and deprotonated arginine. Electronic structure calculations are presented for Br^–·arginine to support the presence of the zwitterionic amino acid in this complex. The results obtained in this work provide important information on the low-energy potential energy surfaces of these anion–amino acid clusters and reveal the presence of several overlapping surfaces in the low-energy region for the Br^–·arginine and NO₃^–·arginine systems