Gas-Phase Dissociation Chemistry of Deprotonated RGD.

We investigate the structure and dissociation pathways of the deprotonated amphoteric peptide arginylglycylasparic acid, [RGD-H]. We model the pertinent gas-phase structures and fragmentation chemistry of the precursor anions and predominant sequence-informative bond cleavages (, , and peaks) and compare these predictions to our tandem mass spectra and infrared spectroscopy experiments. Formation of the anions requires rate-limiting intramolecular back biting to cleave the second amide bond and generate an anhydride structure. Facile cleavage of the newly formed ester bond with concerted expulsion of a cyclic anhydride neutral generates the product structure. IR spectroscopy supports this anion having structures that are essentially identical to C-terminally deprotonated arginylglycine, [RG-H]. Formation of the anion is predicted to require concerted expulsion of CO from the aspartyl side chain carboxylate and cleavage of the N-C bond to produce a proton-bound dimer of arginylglycinamide and acrylate. Proton transfers within the dimer then enable predominant detection of a anion with the negative charge nominally on the central, glycine nitrogen (amidate structure) as the proton affinity of this structure is predicted to be lower than acrylate by ∼27 kJ mol. Alternate means of cleaving the same N-C bond produce deprotonated -1,4-dibut-2-enoic acid anion structures. These lowest energy processes involve C-H proton mobilization from the aspartyl side chain prior to N-C bond cleavage consistent with proposals from the literature.