Salt Bridge Stabilization of Charged Zwitterionic Arginine Aggregates in the Gas Phase

The discovery of several new unusually stable aggregates of arginine that are intermolecularly bound by salt bridges is reported. Quadrupole ion-trap mass spectrometry provides evidence for the stability of arginine in the zwitterionic state, where the protonated guanidinium group of one arginine interacts strongly with the carboxylate of another to form stable noncovalent complexes, coordinated to either a cation or anion. Clusters of arginine with itself, sodium, potassium, lithium, magnesium, chloride, fluoride, bromide, iodide, and nitrate are observed. DFT calculations at the B3LYP/6-31G** level are used to assess the structures and energetics of particularly prominent clusters. An examination of mixtures of d-arginine with isotopically labeled l-arginine indicates that the stability of these clusters does not depend on arginine enantiomeric purity. The cyclic trimers of arginine, capped with either Cl- or NO3 -, possess exceptional stability.