Ultrafast deactivation mechanisms of protonated aromatic amino acids following UV excitation

Abstract

Deactivation pathways of electronically excited states have been investigated in three protonated aromatic amino acids: tryptophan (Trp), tyrosine (Tyr) and phenylalanine (Phe). The protonated amino acids were generated by electrospray and excited with a 266 nm femtosecond laser, the subsequent decay of the excited states being monitored through fragmentation of the ions induced and/or enhanced by another femtosecond pulse at 800 nm. The excited state of TrpH⁺ decays in 380 fs and gives rise to two channels: hydrogen atom dissociation or internal conversion (IC). In TyrH⁺, the decay is slowed down to 22.3 ps and the fragmentation efficiency of PheH⁺ is so low that the decay cannot be measured with the available laser. The variation of the excited state lifetime between TrpH⁺ and TyrH⁺ can be ascribed to energy differences between the dissociative πσ* state and the initially excited ππ* state.