UV and IR spectroscopy of cold protonated leucine enkephalin

Abstract

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. [N. C. Polfer et al. J. Am. Chem. Soc. 129 (2007) 5887]. 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 Nterminal 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 COO-H group to the F-L peptide bond. The resulting structure suggests some relevance to the fragmentation pathways of protonated YGGFL.

Initial Steps of Amyloidogenic Peptide Assembly Revealed by Cold‐Ion Spectroscopy

“…[21] Moreover,t he positions and relative intensities of several peaks in the spectrum of the monomer reproduce those in the spectrum of PheH + strikingly accurately,b ut blue-shifted by 4.7 cm À1 .T his resemblance allows us to assign the two reddest sharp peaks to the UV band origin of the Phe-105 residue,although we cannot identify the band origin of the second chromophore.D rawing from the literature on the UV spectroscopy of cold ionic and neutral gas-phase molecules,wecan postulate that the position of the Phe band origin is little influenced (compared, for instance, with Ty r) by its local environment and, typically,r ed-shifts substantially because of either the proton···p or p···p coupling of aromatic rings. [20,22,23] Our observation of no redshift for the monomer thus indicates that in [M+ +H] + the chromophores are remote from the proton sites and are not strongly coupled to each other.…”

Making Mass Spectrometry See the Light: The Promises and Challenges of Cryogenic Infrared Ion Spectroscopy as a Bioanalytical Technique

“…First and foremost, the methodology is applicable to any molecule, as IR modes of the molecule are probed. In contrast, for instance the IR-UV double resonance scheme [38, 39] or IR photofragment gain spectroscopy [40] requires the presence of a UV chromophore (with a suitably long lifetime of the excited state) [41], thereby constraining the types of molecules that can be probed. Another key advantage of the tagging scheme ensues from the known mass of the tagging gas, which, barring any mass interferences, allows for simultaneous tagging of multiple analyte molecules at different m/z .…”

Operation and Performance of a Mass-Selective Cryogenic Linear Ion Trap

“…This major advance in principle allows the coupling of any ionization technique with cryogenic spectroscopic interrogation. In terms of biomolecular structure identification, the combination with electrospray ionization (ESI) especially stands out, and correspondingly there has been a surge in papers in recent years to demonstrate bioanalytical applications of IR ion spectroscopy [3, 18, 27–32], as well as cryogenic UV spectroscopy [33, 34]. …”