Deuterium exchange reactions as a probe of biomolecule structure. Fundamental studies of cas phase H/D exchange reactions of protonated glycine oligomers with D2O, CD3OD, CD3CO2D, and ND3

A Fourier transform ion cyclotron resonance mass spectrometer was used to examine the hydrogen/deuterium exchange reactions of protonated glycine oligomers (Gly(n), n = 1-5) with D2O CD3OD, CD3CO2D, and ND3. Exchange rates in this study were monitored over three orders of magnitude, from 10(-9) to 10(-12) cm(3) molecule(-1) s(-1). Reaction kinetics are highly dependent on peptide structure and the properties of the exchange reagents. The rate and extent of H/D exchange of the protonated oligomers increases with reagent gas basicity, D2O < CD3OD < CD3CO2D < ND3. ND3 is the most efficient reagent studied, as it exchanges every labile hydrogen in each of the oligomers. Several distinct mechanisms, supported by semiempirical AM1 and PM3 calculations, are proposed to explain the observed patterns of reactivity. An onium ion mechanism is proposed for the exchange of N-terminus hydrogens of Gly(n)H(+) oligomers with ND3, in which an endothermic proton transfer from the N-terminus is rendered energetically feasible by simultaneous solvation of the resultant ammonium ion by the neutral peptide. This mechanism is consistent with the observation of multiple exchanges in a single collision event with ND3. For those reagents whose proton affinities are too low to form solvated onium ion intermediates, a relay mechanism is proposed in which the reagent shuttles a proton from the N-terminus to a slightly less basic site in the molecule. For glycine oligomers, this sits is an amide oxygen. A tautomer mechanism is proposed for the exchange of the amide hydrogens with ND3. Exchange occurs by proton transfer from the N-terminus to the amide carbonyl in concert with transfer of the amide proton to ammonia, forming an ammonium ion solvated by a tautomerized peptide. Semiempirical calculations suggest that exchange of the C-terminus hydrogen proceeds via formtion of a salt bridge with the reagent gas, which deprotonates the C-terminus acid group, with the nearby protonated N-terminus stabilizing the resultant ion pair. Betaine, [(CH3N+-CH2CO2H], used in this study to determine the isotopic purity of the exchange reagents, serves as a model for salt bridge formation since it does not possess a labile proton and readily exchanges the carboxylic acid hydrogen. The effect of translational and vibrational excitation on H/D exchange rates was studied for several oligomers using off-resonance collisional activation. For those oligomers that undergo facile H/D exchange with the reagent gases, excitation decreases rates. For those oligomers which do not undergo facile H/D exchange, reactivity is not promoted by collisional activation.