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 sequenceinformative bond cleavages (b(2)+H2O, C-2, and z(1) 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 b(2)+H2O anion having structures that are essentially identical to C-terminally deprotonated arginylglycine, [RG-H](-). Formation of the c(2) anion is predicted to require concerted expulsion of CO2 from the aspartyl side chain carboxylate and cleavage of the N-C-alpha bond to produce a proton-bound dimer of arginylglycinamide and acrylate. Proton transfers within the dimer then enable predominant detection of a c(2) 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 similar to 27 kJ mol(-1). Alternate means of cleaving the same N-C-alpha bond produce deprotonated cis-1,4-dibut-2enoic acid z1 anion structures. These lowest energy processes involve C-H proton mobilization from the aspartyl side chain prior to N-C-alpha bond cleavage consistent with proposals from the literature. [Graphics]