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 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 III and IIII 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 IIII 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 (III and IIIII) 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.Contribution to the Jacopo Tomasi Honorary Issue