Vibrational Characterization of Simple Peptides Using Cryogenic Infrared Photodissociation of H₂-Tagged, Mass-Selected Ions

Abstract

We present infrared photodissociation spectra of two protonated peptides that are cooled in a ~10 K quadrupole ion trap and “tagged” with weakly bound H2 molecules. Spectra are recorded over the range 600 – 4300 cm−1 using a table-top laser source, and are shown to result from one-photon absorption events. This arrangement is demonstrated to recover sharp (Δν~6 cm−1) transitions throughout the fingerprint region, despite the very high density of vibrational states in this energy range. The fundamentals associated with all of the signature N-H and C=O stretching bands are completely resolved. To address the site-specificity of the C=O stretches near 1800 cm−1, we incorporated one 13C into the tripeptide. The labeling affects only one line in the complex spectrum, indicating that each C=O oscillator contributes a single distinct band, effectively “reporting” its local chemical environment. For both peptides, analysis of the resulting band patterns indicates that only one isomeric form is generated upon cooling the ions initially at room temperature into the H2 tagging regime.

Infrared Spectroscopy of Mobility-Selected H+-Gly-Pro-Gly-Gly (GPGG)

“…The other two vibrations fall outside our observation region, at 3125 and 3017 cm -1 for conformer A and at 3147 and 3106 cm -1 for conformer B. A strong red shift in the bands associated with the protonated amine is consistent with a number of previous experimental studies [51,52,64].…”

“…The H 2 -tagged molecules are then extracted from the trap and intersected with a tunable infrared OPO 2 μs after extraction. Resonant absorption with subsequent intramolecular vibrational redistribution leads to photo-evaporation of H 2, which has a typical binding energy of 370-600 cm -1 [50][51][52]. This is detected as a dip in the H + GPGG ·H 2 signal in the time-of-flight analysis.…”

Characterizing the Intramolecular H-bond and Secondary Structure in Methylated GlyGlyH+ with H2 Predissociation Spectroscopy

“…This spectroscopic data is most often available through the integration of tunable infrared laser photodissociation with mass spectrometry and, in most cases, spectra of the mass-selected ions are obtained under ambient temperature conditions using infrared multiple photon dissociation (IRMPD) [28][29][30][31][32][33]. A recent variation on this scheme [34], yielding a resolution enhancement of 5 to 10 times that which is typically reported, involves cooling the ions close to 10 K [35][36][37] and "tagging" them with a weakly bound mass "messenger" using cryogenic ion trapping techniques [38][39][40]. The resulting spectra are obtained in a linear regime that is readily compared with calculated band patterns for ions frozen into their zero-point energy levels.…”

“…The protonated diglycine ion has been investigated previously using FTMS [43][44][45], IRMPD [18], and H 2 tagging approaches [34], and there is a consensus that the protonation site is located at the N-terminus. Although several isomers were invoked to explain the room temperature IRMPD spectrum, the sharper bands recovered in the 10 K H 2 tagged spectrum could be assigned in the context of a dominant, fully extended "all trans" conformer, which was identified as the lowest energy isomer by Wu et al [18].…”

Equatorial coordination of uranyl: Correlating ligand charge donation with the Oyl-U-Oyl asymmetric stretch frequency

“…Infrared spectra obtained using IRMPD are comparable to those collected using linear absorption techniques. 14,[16][17][18] For these experiments, the FEL wavelength was tuned in 5 cm ). The range was always sufficient to clearly identify the uranyl asymmetric stretch frequency of interest.…”