Excited-state deactivation mechanisms of protonated and neutral phenylalanine: a theoretical study

The potential energy (PE) profiles of neutral and protonated phenylalanine, as the simplest aromatic amino acid, at different electronic states have been investigated extensively using RI-MP2 and RI-CC2 methods. The PE profiles have been determined, considering the C-alpha-C-beta and C-alpha-C-(COOH) bond stretching following proton transfer to the aromatic ring and CO group, respectively, as well as the hydrogen detachment reaction coordinate. The calculated results reveal that a low-barrier proton transfer process from ammonia to the aromatic chromophore, leading the excited system to C-alpha-C-beta bond cleavage, plays the most prominent role in the deactivation mechanism of excited PheH(+) at the origin of the S-1-S-0 electronic transition. On the contrary, for excited neutral phenylalanine at the band origin of the S-1-S-0 transition, a large barrier in the S-1 profile along the C-alpha-C-beta bond-stretching hinders the excited system from approaching the dissociative part of PE curve. This barrier may explain the large lifetime of the S-1 excited phenylalanine (nanosecond range), while a low barrier in the S-1 PE profile of the protonated species along the PT process explains the short-range lifetime of the protonated species (in the picosecond range).