Photodissociation of leucine-enkephalin protonated peptide: an experimental and theoretical perspective

Understanding the competing processes that govern far ultraviolet photodissociation (FUV-PD) of biopolymers such as proteins is a challenge. Here, we report a combined experimental and theoretical investigation of FUV-PD of protonated leucine-enkephalin pentapeptide ([YGGFL + H]⁺) in the gas-phase. Time-dependent density functional theory (TD-DFT) calculations in combination with experiments and previous results for amino acids and shorter peptides help in rationalizing the evolution of the excited states. The results confirm that fragmentation of [YGGFL + H]⁺ results mainly from vibrationally excited species in the ground electronic state, populated after internal conversion. We also propose fragmentation mechanisms for specific photo-fragments such as tyrosine side chain loss (with an extra hydrogen) or hydrogen loss. In general, we observe the same mechanisms as for smaller peptides or protonated Tyr and Phe, that are not quenched by the presence of other amino acids. Nevertheless, we also found some differences, as for H loss, in part due to the fact that the charge is solvated by the peptide chain and not only by the COOH terminal group.