Computational studies of the photophysics of neutral and zwitterionic glycine in an aqueous environment: The glycine–(H2O)2 cluster

The glycine–(H2O)2 cluster has been considered as a model for the study of the photochemistry of neutral and zwitterionic glycine in an aqueous environment. It has been found that the detachment of neutral NH3 in the S1 state of zwitterionic glycine leads in an essentially barrierless manner to a low-lying conical intersection of the S1 and S0 energy surfaces. This conical intersection can provide the mechanism for efficient radiationless deactivation of the excited state, leading to internal conversion or deamination (loss of ammonia). These results provide a mechanistic explanation of the high quantum yield of ammonia in the UV photolysis of non-aromatic amino acids in aqueous solution.