EPR study of dipeptides containing tyrosine

Previous work on tyrosyl radicals in polycrystalline tyrosine-containing peptides by means of FT-IR (Fourier-transform infrared) spectroscopy provided evidence for a migration of unpaired spin density from the phenoxyl ring to the terminal amino group suggesting an interaction between the π system of the tyrosyl radical and the amino group. The aim of presented study was to test that such an interaction occurs and whether it has an effect on the character of forming radicals. Polycrystalline dipeptides Tyr-Gly, Tyr-Leu, Tyr-Met were irradiated in a 60Co-γ source. Radicals were identified by the electron paramagnetic resonance. The main EPR signals observed at low temperature 77-180 K in dipeptides containing N-terminal tyrosine were anisotropic singlets with very similar gav=2.0045 assigned to the tyrosyl radicals. The presence of N-centered radicals suggests that spin delocalization from the phenoxyl radical to the amino group might occur by a through-space mechanism