UV and IR spectroscopy of cold protonated leucine enkephalin

•UV spectrum of cold (<10 K) protonated leucine enkephalin via action spectroscopy.•IR spectrum of cold protonated leucine enkephalin via IR–UV double resonance.•DFT calculated structures and vibrational frequency analyses of cold protonated leucine enkephalin.•Assigned conformational family of structures for cold protonated leucine enkephalin.

We have applied ultraviolet and infrared–ultraviolet (IR–UV) double resonance photofragment spectroscopy in a tandem mass spectrometer for the spectroscopic characterization of cryogenically-cooled protonated leucine enkephalin (H+-YGGFL), for the purposes of elucidating its three-dimensional structure. The primary UV-induced photofragmentation pathway following excitation of the tyrosine chromophore is loss of the tyrosine side chain (107 Da). IR-enhanced photofragmentation via this channel makes IR–UV depletion spectroscopy difficult, and IR photofragment gain spectroscopy is used instead to record the infrared spectrum in the hydride stretch and amide I/II regions. By comparing the experimental spectrum with the predictions of DFT M05-2X/6-31+G(d) calculations, a single backbone structure was assigned that is similar to, but distinct from, that assigned in the recent work of Polfer et al. [15]. Additionally, the assigned structure’s theoretical cross-section is comparable to previous ion mobility results. The structure is characterized by a compact hydrogen-bonding architecture in which the peptide backbone self-solvates the N-terminal ammonium group carrying the charge. In addition to H-bonds to the tyrosine π cloud and the second glycine carbonyl oxygen, the ammonium group is involved in a series of cooperatively strengthened H-bonds between the N and C termini, linking the COOH group to the FL peptide bond. The resulting structure suggests some relevance to the fragmentation pathways of protonated YGGFL.

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