Salt bridge stabilization of charged zwitterionic arginine aggregates in the gas phase

The discovery of several new unusually stable aggregates of arginine that a re intermolecularly bound by salt bridges is reported. Quadrupole ion-trap mass spectrometry provides evidence for the stability of arginine in the zw itterionic state, where the protonated guanidinium group of one arginine in teracts strongly with the carboxylate of another to form stable noncovalent complexes, coordinated to either a cation or anion. Clusters of arginine w ith itself, sodium, potassium, lithium, magnesium, chloride, fluoride, brom ide, iodide, and nitrate are observed. DFT calculations at the B3LYP/6-31G* * level are used to assess the structures and energetics of particularly pr ominent clusters. An examination of mixtures of D-arginine with isotopicall y labeled L-arginine indicates that the stability of these clusters does no t depend on arginine enantiomeric purity. The cyclic trimers of arginine, c apped with either Cl- or NO3-, possess exceptional stability.