Ion-Induced Dipole Interactions and Fragmentation Times: Cα-Cβ Chromophore Bond Dissociation Channel

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.

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, Tn(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 Tn(v) states of both YGGFL and FGGYL. Finally, IR excitation in the S1 or Tn(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.

Excited-state proton transfer in protonated adrenaline revealed by cryogenic UV photodissociation spectroscopy

We report a comprehensive study of the structures and deactivation processes of protonated adrenaline through cryogenic UV photodissociation spectroscopy. Single UV and double-resonance UV-UV hole burning spectroscopies have been performed and compared to coupled-cluster SCS-CC2 calculations performed on the ground and first electronic states. Three conformers were assigned, two lowest energy gauche conformers along with a higher energy conformer with an extended structure which is indeed the global minimum in solution. This demonstrates the kinetic trapping of this high energy gas phase conformer during the electrospray process. At the band origin of all conformers, the main fragmentation channel is the C_α-C_β bond cleavage, triggered by an excited state proton transfer to the catechol ring. Internal conversion leading to the water loss channel competes with the direct dissociation and tends to prevail with the increase of excess energy brought by the UV laser. Picosecond time-resolved pump-probe spectroscopy was performed to measure the excited state lifetimes of the three conformers of AdH⁺, which decay with the increase of excess energy in the ππ* state, from 2 ns at the band origin down to few hundreds of picoseconds 0.5 eV to the blue. Finally, about 0.8 eV above the band origin, the πσ* state is directly reached, leading to the opening of the H-loss channel.

Classical Dynamics Simulations of Dissociation of Protonated Tryptophan in the Gas Phase

Gas phase decomposition of protonated amino acids are of great interest due to their role in understanding protein and peptide chemistry. Several experimental and theoretical studies have been reported in the literature on this subject. In the present work, decomposition of the aromatic amino acid protonated tryptophan was studied by on-the-fly classical chemical dynamics simulations using density functional theory. Mass spectrometry and electronic structure theory studies have shown multiple dissociation pathways for this biologically relevant molecule. Unlike aliphatic amino acids, protonated tryptophan dissociates via NH₃ elimination rather than the usual iminium ion formation by combined removal of H₂O and CO molecules. Also, a major fragmentation pathway in the present work involves C_α-C_β bond fission. Results of the chemical dynamics simulations reported here are in overall agreement with experiments, and detailed atomic level mechanisms are presented.

Photofragmentation mechanisms in protonated chiral cinchona alkaloids

Photo-fragmentation of protonated alkaloids results in C₈–C₉ cleavage accompanied or not by hydrogen migration, with a stereochemistry-dependent branching ratio.