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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