Water-catalyzed isomerization of the glycine radical cation. From hydrogen-atom transfer to proton-transport catalysis

Abstract

The isomerization reactions of the glycine radical cation, from [NH2CH2COOH]+, I, to [NH3CHCOOH]+, II, or [NH2CHC(OH)2]+, III, in the presence of a water molecule have been studied theoretically. The water molecule reduces dramatically the energy barriers of the I→II and I→III tautomerizations owing to a change in the nature of the process. However, the role of the water molecule depends on the kind of isomerization, the catalytic effect being more important for the I→III reaction. As a consequence, the preferred mechanism for the interconversion of glycine radical cation I to the stablest isomer, III, is the direct one-step mechanism instead of the two step (I→II and II→III) process found for isolated [NH2CH2COOH]+. When using ammonia as a solvent molecule, a spontaneous proton-transfer process from [NH2CH2COOH]+ to NH3 is observed and so no tautomerization reactions take place. This behavior is the same as that observed in aqueous solution, as has been confirmed by continuum model calculations.