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 function alized amino acid in its canonical form (free acid) and the corresponding m ethyl ester(M = Li, Na, K, Cs), and between a zwitterionic amino acid and i someric alkyl esters (M = Na), are determined in the gas phase based on the dissociation of A(1)-M+-A(2) heterodimers, in which A(1) and A(2) represen t eater 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 met hyl ester, which cannot form a zwitterion, and with those of the amino acid betaine, a permanent zwitterion. These experiments indicate that the Li+ a nd 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).