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

The isomerization reactions of the glycine radical cation, from [NH 2CH2COOH].+, 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 [NH2CH 2COOH].+ 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. © Springer-Verlag 2004.