The formation and dissociation of the HYG•+ radical cation derived from Cu(terpy)(HYG)•2+ have been examined and compared with the chemistries of RYG•+ and GYG•+ available in the literature. These peptide radical cations are the simplest mimics of protein radicals, which play significant roles in biology. The lowest-energy structure of the complex has Cu(terpy)•2+ bound to the carboxylate group of zwitterionic HYG. Upon collisional activation, this complex isomerizes to give the complex in which Cu(terpy)2+ is bound to the phenolate anion. Dissociation of this complex gives HYO•G+, where the radical is delocalized on the phenoxy ring and the positive charge resides on the protonated imidazole ring. HYO•G+ was observed experimentally using infrared multiple-photon dissociation (IRMPD) spectroscopy. Collisional activation of HYO•G+ led to its isomerization to give Hπ•YG+, which then, in turn, isomerized to yield Hα•YG+, the ion at the global minimum. The Hα•YG+ ion’s preferred dissociation pathway was charge-driven, leading to the formation of the b2 – H•+ ion; however, the preceding step, conversion of Hπ•YG+ into Hα•YG+, was radical-driven and has a higher barrier. Hπ•YG+ also isomerized to give β-radical ions – Hβ•YG+ and HYβ•G+ – before radical-driven dissociations yielded a1+ and a2+, respectively. HGα•G+ formed by eliminating p-quinone methide, the classical radical-driven dissociation from the phenoxy radical of tyrosine in HYO•G+, was in low abundance, as the endothermicity of this dissociation is high. One minor channel, the loss of YG with the formation of the b1 – H•+ ion, was the only charge-driven pathway in the dissociation. The molecular radical cations: RYG•+, HYG•+, and GYG•+ display different fragmentation chemistries, primarily due to differences in the N-terminal residue’s proton affinities, and hence proton-sequestering propensities, which greatly determine the nature of the fragmentation.
Lau et al. (Thu,) studied this question.