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

Abstract

We report vibrational predissociation spectra of the four protonated dipeptides derived from glycine and sarcosine, GlyGlyH•(H 2 ) 2 , and SarSarH + •(D 2 ) 2 , generated in a cryogenic ion trap. Sharp bands were recovered by monitoring photoevaporation of the weakly bound H 2 (D 2 ) molecules in a linear action regime throughout the 700-4200 cm -1 range using a table-top laser system. The spectral patterns were analyzed in the context of the low energy structures obtained from electronic structure calculations. These results indicate that all four species are protonated on the N-terminus, and feature an intramolecular H-bond involving the amino group. The large blue-shift in the H-bonded N-H fundamental upon incorporation of a methyl group at the N-terminus indicates that this modification significantly lowers the strength of the intramolecular H-bond. Methylation at the amide nitrogen, on the other hand, induces a significant rotation (~110 o ) about the peptide backbone.

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.…”

Communication: He-tagged vibrational spectra of the SarGlyH+ and H+(H2O)2,3 ions: Quantifying tag effects in cryogenic ion vibrational predissociation (CIVP) spectroscopy

“…This approach has several advantages over excitation inside the trap including unambiguous identification of the species undergoing excitation and the acquisition of isomer-specific spectra using photochemical hole-burning (so-called IR 2 MS 3 ) in the vibrational manifold. 17,18 Here we quantify the bandshifts displayed by the more strongly bound H 2 , Ar, and Ne complexes of three archetypal ions reported earlier: the dipeptide 19 SarGlyH + and the protonated water clusters: H + (H 2 O) 2,3 . 20-22 Figure 1 of the supplementary material 23 presents a schematic illustration of the octopole ion trap central to this work.…”

“…Infrared predissociation spectra over the NH and OH stretching region of (a) SarGlyH + · He prepared in the cryogenic octopole trap described in this work and (b) SarGlyH + · H 2 ions prepared in a 3D Paul trap reported previously. 19 The minimum energy structure (MP2/6-311+G(d,p)) of the bare ion is inset in (a), highlighting the free NH that binds the messenger tag in red. The 12 cm −1 blueshift that selectively occurs in the NH a fundamental when H 2 is replaced by He is indicated with the dotted black line.…”

Cryogenic Methods for the Spectroscopy of Large, Biomolecular Ions

“…10) [139,140]. They use the Paul trap both to cool the molecular ions and to form clusters with deuterium.…”