Zwitterionic vs. charge-solvated structures in the binding of arginine to alkali metal ions in the gas phase

Abstract

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).