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 (b2+H2O, c2, and z1 peaks) and compare these predictions to our tandem mass spectra and infrared spectroscopy experiments. Formation of the b2+H2O 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 b2+H2O anion having structures that are essentially identical to C-terminally deprotonated arginylglycine, [RG-H]-. Formation of the c2 anion is predicted to require concerted expulsion of CO2 from the aspartyl side chain carboxylate and cleavage of the N-Calpha bond to produce a proton-bound dimer of arginylglycinamide and acrylate. Proton transfers within the dimer then enable predominant detection of a c2 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-1. Alternate means of cleaving the same N-Calpha bond produce deprotonated cis-1,4-dibut-2-enoic acid z1 anion structures. These lowest energy processes involve C-H proton mobilization from the aspartyl side chain prior to N-Calpha bond cleavage consistent with proposals from the literature.