Mechanistic examination of C α –C β tyrosyl bond cleavage: Spectroscopic investigation of the generation of α‐glycyl radical cations from tyrosyl (glycyl/alanyl)tryptophan

In this study, dissociative one-electron transfer dissociation of [CuII (dien)Y(G/A)W]•2+ [dien = diethylenetriamine; Y(G/A)W = tyrosyl (glycyl/alanyl)tryptophan] was used to generate the tripeptide radical cations [Y(G/A)W]•+ ; subsequent loss of the Tyr side chain formed [Gα• (G/A)W]+ . The π-centered species [YGWπ• ]+ generated the α-centered species [Gα• GW]+ through Cα -Cβ bond cleavage, as revealed using infrared multiple photon dissociation (IRMPD) measurements and density functional theory (DFT) calculations. Comparisons of experimental and theoretical IR spectra confirmed that both the charge and spin densities of [Y(G/A)Wπ• ]+ were delocalized initially at the tryptophan indolyl ring; subsequent formation of the final [Gα• (G/A)W]+ structure gave the highest spin density at the α-carbon atom of the N-terminal glycine residue, with a proton solvated by the first amide oxygen atom. The IRMPD mass spectra and action spectra of the [Gα• (G/A)W]+ species were all distinctly different from those of their isomeric [G(G/A)Wπ• ]+ species. The mechanism of formation of the captodative [Gα• (G/A)W]+ species-with the charge site separated from the radical site-from [Y(G/A)Wπ• ]+ has been elucidated. DFT calculations suggested that the Cα -Cβ bond cleavage of the tyrosine residue in the radical cationic [Y(G/A)Wπ• ]+ precursor involves (a) through-space electron transfer between the indolyl and phenolic groups; (b) formation of proton-bound dimers through Cα -Cβ cleavage of the tyrosine residue; and (c) a concerted proton rearrangement from the phenolic OH group to the carboxyl group and formation of the α-carbon-centered product [Gα• (G/A)W]+ through hydrogen bond cleavage. The barriers for the electron transfer (a), the Cα -Cβ cleavage (b), and the protonation rearrangement (c) were 12.8, 26.5, and 10.3 kcal mol-1 , respectively.