Zwitterionic vs. charge-solvated structures in the binding of arginine to alkali metal ions in the gas phase
The relative alkali metal ion (M+) affinities between a side-chain functionalized amino acid in its canonical form (free acid) and the corresponding methyl ester (M = Li, Na, K, Cs), and between a zwitterionic amino acid and isomeric alkyl esters (M = Na), are determined in the gas phase based on the dissociation of A1–M+–A2 heterodimers, in which A1 and A2 represent ester and/or amino acid ligands (kinetic method). With all dimers studied, the affinities increase in the order free acid < ester < zwitterion. With this information at hand, the M+ affinities of arginine, which may bind M+ as a free acid or as a zwitterion, are compared with those of arginine methyl ester, which cannot form a zwitterion, and with those of the amino acid betaine, a permanent zwitterion. These experiments indicate that the Li+ and Na+ complexes contain the free acid form of arginine (charge solvation), whereas the complexes with the larger K+ and Cs+ ions contain zwitterionic arginine (salt bridge).