Site-Specific Photochemistry along a Protonated Peptide Scaffold

We present a detailed study of the time-dependent photophysics and photochemistry of a known conformation of the two protonated pentapeptides Leu-enkephalin (Tyrosine-Glycine-Glycine-Phenylalanine-Leucine, YGGFL) and its chromophore-swapped analogue FGGYL, carried out under cryo-cooled conditions in the gas phase. Using ultraviolet-infrared (UV-IR) double resonance, we record excited state IR spectra as a function of time delay between UV and IR pulses. We identify unique Tyr OH stretch transitions due to the S₁ state and the vibrationally excited triplet state(s) formed by intersystem crossing, T_n(v). Photofragment mass spectra are recorded out of the S₁ origin and following UV-IR double resonance. Several competing site-specific fragmentation pathways are discovered involving peptide backbone cleavage, Tyr side chain loss, and N-terminal NH₃ loss mediated by electron transfer. In YGGFL, IR excitation in the S₁ state promotes electron transfer (ET) from the aromatic ring to the N-terminal R-NH₃⁺ group leading to loss of neutral NH₃. This product channel is missing in FGGYL due to the larger distance for ET from Y(4) to NH₃⁺. Selective loss of the Tyr side chain occurs out of an excited state process following UV excitation and is further enhanced by IR excitation in S₁ and T_n(v) states of both YGGFL and FGGYL. Finally, IR excitation in the S₁ or T_n(v) states fragments the peptide backbone exclusively at amide(4), producing the b₄ cation. We postulate that this selective fragmentation results from intersystem crossing to produce vibrationally excited triplets with enough energy to launch the proton along a proton conduit present in the known starting structure.