Unimolecular chemistry of metal ion-coordinated α-dipeptide radicals

The Li+ complexes of the isomeric α-dipeptide radicals H2NCHC(O)NHCH2COOH (GlyGly) and H2NCH2C(O)NHCHCOOH (GlyGly) are formed in the gas phase from the isomeric complexes [PheGly + Li]+ and [GlyPhe + Li]+, respectively, via homolytic cleavage of the corresponding benzyl side chains. The isomers undergo distinctively different reactions upon collisionally activated dissociation (CAD) and, hence, represent unique, non-interconverting species. The investigation of deuterated isotopomers and of dipeptide radicals with Ala residues permits complete elucidation of the dissociation pathways of the radical complexes. The majority of reactions observed are promoted by the radical site, with the location of the unpaired electron playing an important role in the types of reactions taking place. Analogous differences are found for dilithiated complexes of GlyGly and GlyGly, in which the COOH termini are derivatized to COO−Li+ salt bridges. Density functional theory calculations confirm that the lithiated and dilithiated α-dipeptide radicals have distonic character; the radical is largely localized on the N- or C-terminal α-C atom and the charge is largely localized on the metal ions. In the most stable conformers, the Li+ ion(s) are bound between the amide carbonyl and C-terminal carbonyl (or carboxylate) groups. Theory predicts a higher thermodynamic stability for the complexes of the N-terminal radical GlyGly, as reflected by the significantly higher yield, with which these complexes are formed (from their PheGly precursors), compared to the GlyGly complexes.