Tyrosine-(H2O)(2) and tryptophan-(H2O)(2) clusters have been considered as models for the study of the photochemistry of neutral and zwitterionic tyrosine and tryptophan in an aqueous environment. It has been found that the detachment of neutral NH3 in the S-1 state of the zwitterionic clusters leads to a low-lying conical intersection of the S-1 and S-0 energy surfaces. This conical intersection can provide the mechanism for efficient radiationless deactivation of the excited state back to the ground state or, alternatively, deamination (loss of ammonia). These results provide a mechanistic explanation of the efficient fluorescence quenching and the high quantum yield of ammonia in the UV photolysis of tyrosine and tryptophan in aqueous solution.