Gas phase H/D exchange of protonated arginine monomers and dimers

An electrospray ionization-fast flow technique has been employed to study the gas-phase H/D exchange reactions of protonated monomers and dimers of L-arginine with ND3 and CH3OD. Experimental results include mass spectra, semilogarithmic decay plots of reactant ions and relative abundances of the various cations undergoing consecutive H/D exchanges as a function of the flow rates (or concentrations) of the neutral deuterating reagents. Optimum apparent and site-specific rate constants are deduced by simulated fits based on solutions of simultaneous first-order differential equations. We find that the protonated monomer, which is known to be non-zwitterionic in the gas phase, exchanges efficiently with ND3 a maximum of 4 hydrogens out of the 8 labile ones. The bulk of the reactivity of ND3 with the protonated monomer is centered at a single site. The data are interpreted by ammonia stabilizing the zwitterion structure of the protonated arginine monomer by forming a salt-bridge complex, C(NH2)(2)(+)-NH-CH2-CH2-CH2-CH(NH2)-COO--NH4+. Ammonia/arginine.H+ complexes are indeed observed in the flow tube experiment upon increasing the flow tube pressure through an increased flow of the helium carrier gas. The protonated dimer has been calculated previously to be more stable in the salt-bridge or ion-zwitterion form than in the simple protonated or ion-molecule form. We find that the dimer exchanges with ND3 all of its 15 labile hydrogen atoms. A "relay" exchange mechanism can explain the results for the zwitterionic dimer since there is a carboxylate group onto which the proton can be transferred. This allows exchange of all the guanidino, hydrogens that are not exchanged in the monomer. Some experimental results are explained by assuming the coexistence of the two isomeric protonated dimer structures: the ion-zwitterion form and the ion-molecule form. These results include bimodal deuterium distributions in the mass spectra and two equal site-specific reactivities that are ascribed to two equivalent carboxyl groups of the ion-molecule isomer form of the dimer.