Vibrational Signatures of S-​Nitrosoglutathione as Gaseous, Protonated Species

Gas-​phase ions of protonated L-​glutathione as native species, [GSH + H]​+, and S-​nitroso deriv., [GSNO + H]​+, were generated by electrospray ionization and probed via IR multiple photon dissocn. (IRMPD) action spectroscopy. Insight into the conformational landscape is gained from interpretation of the IR spectra aided by high-​level theor. calcns., which enables structural assignment disclosing both the site of protonation and the intramol. hydrogen-​bond network. Calcns. yield the low-​energy structures of [GSNO + H]​+. A admixt. of the four most stable ones (SN1, AN1, SN2, and AN2) is apt to account for the exptl. IRMPD spectra obtained in both the 1000-​2000 and the 3100-​3700 cm-​1 spectral ranges. The most stable form of [GSNO + H]​+, SN1, protonated at the amino group, presents a syn conformation at the S-​N (partial) double bond and all peptidic carbonyls involved in (strong) C=O···H-​N hydrogen bonds, so allowing closure of a C5 (β-​strand)​, two C7 (γ-​turn)​, and one C9-​membered rings. An appreciable barrier to rotation of 43 kJ mol-​1 about the S-​N bond is found to sep. SN1 from the analogous anti isomer AN1, which lies only 0.70 kJ mol-​1 higher in free energy. Conformers obtained for [GSH + H]​+ are very similar to the [GSNO + H]​+ counterparts, indicating that the S-​nitrosation motif does not affect significantly the geometry of the peptide. The obsd. ν(NO) signatures at 1622 and 1690 cm-​1, merged with other absorptions, are revealed by their sensitivity to 15NO isotope labeling and by comparison with the IRMPD spectrum of native [GSH + H]​+, providing a diagnostic probe for the S-​nitrosation feature in natural peptides.