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

We report the first low-energy collisional-induced dissociation studies of the X-center dot 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-center dot arginine and Cl-center dot arginine) is dominated by deprotonation of the arginine, whereas the major fragmentation channel for the largest ion clusters (I-center dot arginine and ClO3-center dot arginine) corresponds to simple cluster fission into the ion and neutral molecule. However, the fragmentation profiles of Br-center dot arginine and NO3-center dot 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-center dot 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-center dot arginine and NO3-center dot arginine systems.