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Research is presented describing the study of protonated triglycine to determine its tautomerism and migration of protons.

Despite reports to the contrary, doubly charged lead monohydrate is a stable species against both proton and charge transfers. [Pb(H2)O)]2+ has been observed as a minor product in the ligand-exchange reaction of [Pb(CH3)CN]2+ with H2O after collisional activation. Density functional theory has been used to examine reaction profiles of [Pb(H2)On]2+ where n = 1, 2, and 3. [ABSTRACT FROM AUTHOR]

Investigates the proton migration and tautromerism in protonated triglycine. Neutral structures; Energy hypersurface adjusted by the addition of entropy to provide relative free energies; Calculated proton affinities and gas-phase basicities.

The mobility of the radical center in three isomeric triglycine radical cations[G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) has been investigated theoretically via density functional theory (DFT) and experimentally via tandem mass spectrometry. These radical cations were generated by collision-induced dissociations (CIDs) of Cu(II)-containing ternary complexes that contain the tripeptides YGG, GYG, and GGY, respectively (G and Y are the glycine and tyrosine residues, respectively). Dissociative electron transfer within the complexes led to observation of [Y(*)GG](+), [GY(*)G](+), and [GGY(*)](+); CID resulted in cleavage of the tyrosine side chain as p-quinomethide, yielding [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+), respectively. Interconversions between these isomeric triglycine radical cations have relatively high barriers (> or = 44.7 kcal/mol), in support of the thesis that isomerically pure [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) can be experimentally produced. This is to be contrasted with barriers < 17 kcal/mol that were encountered in the tautomerism of protonated triglycine [Rodriquez C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The CID spectra of [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) were substantially different, providing experimental proof that initially these ions have distinct structures. DFT calculations showed that direct dissociations are competitive with interconversions followed by dissociation.

The fragmentation mechanisms of protonated triglycine and its first-generation dissociation products have been investigated using a combination of density functional theory calculations and threshold collision-induced dissociation experiments. The activation barrier measured for the fragmentation of protonated triglycine to the b(2) ion and glycine is in good agreement with a calculated barrier at the B3LYP/6-31++G(d,p) level of theory reported earlier [Rodriquez, C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The b(2) ion fragments to the a(2) ion via a transition state structure that is best described as acylium-like. Contrary to what is commonly assumed, the lowest energy structure of the a(2) ion is not an iminium ion, but a cyclic, protonated 4-imidazolidone. Furthermore, fragmentation of the b(2) to the a(1) ion proceeds not via a mechanism that results in HNCO and H(2)C=C=O as byproducts, as have been postulated, but via a transition state that contains an incipient a(1) ion and an incipient carbene. The fragmentation of a(2) to a(1) proceeds via a transition state structure that contains the a(1) ion, CO and an imine as incipient components.