Quasidegeneracy of zwitterionic and canonical tautomers of arginine solvated by an excess electron.

The zwitterionic tautomers of isolated amino acids have not been identified in the gas-phase despite serious experimental efforts.1,2 The main reason is that a typical OH bond is stronger than a NH bond by ca. 17 kcal/mol,3 and thus the canonical tautomer is lower in energy. Indeed, numerous ab initio calculations have confirmed the energy preference for canonical tautomers H2N-CHR-COOH in the gas phase.4-6 Because zwitterion formation is important for the structure and function of peptides and proteins, and zwitterions are encountered in the structure of turns and bends of RNA,7 it is important to know when such species can be more stable than their canonical tautomers. In the present communication, we suggest a new means of preferentially stabilizing zwitterion structures, and we illustrate it for arginine. New experimental efforts have concentrated on complexes of amino acids with neutral or ionic species.8-11 Recently we suggested an alternative pathway to stabilize a zwitterion structure through “solvation” of an amino acid by an excess electron.12 For example, we demonstrated that the instability of the zwitterion structure of glycine relative to its canonical structure is reduced by 8 kcal/mol upon attachment of an excess electron, and we found that a local minimum develops on the anionic potential energy surface. However, the energy at this minimum was still higher than that of the anion derived from the canonical isomer of glycine. We also studied electron attachment to betaine, which is a permanent zwitterion, and we found that the excess electron provides an extra stabilization of 6 kcal/mol relative to its nonionic structure.13