title: Three different tyrosyl radicals identified in L-tyrosine HCl crystals upon gamma-irradiation: magnetic characterization and temporal evolution.
abstract: High-frequency electron paramagnetic resonance (EPR) and X-band electron-nuclear double resonance (ENDOR) spectroscopies were used to investigate the effect of gamma-irradiation on single crystals of L-tyrosine hydrochloride at room temperature. The oxidation product is the tyrosyl radical formed by hydrogen abstraction from the phenolic group; interestingly, on freshly irradiated crystals, two tyrosyl radicals were identified, characterized by slightly different magnetic parameters. In particular, one of the two radicals, with a gxx value of 2.00621, has its phenoxyl oxygen strongly hydrogen-bonded to one or more donors; to our knowledge, this is the lower gxx value reported for tyrosyl radicals. These two oxidation radicals are found to evolve very slowly to a third, single more stable radical conformation. To interpret the experimental data, a possible molecular scenario is presented, where the process of radical formation can be seen as a hydrogen atom transfer or a proton-coupled electron transfer. These processes seem to be controlled by the specific network of hydrogen-bond interactions present in the crystal. The results are discussed in relation to their relevance for the interpretation of EPR spectra of tyrosyl radicals in biological systems.
authors.affiliation: Università degli Studi di Padova, Dipartimento di Scienze Chimiche, Via Marzolo 1, I-35131 Padova, Italy. annalisa.maniero@unipd.it
authors.forename: Anna Lisa
authors.initials: AL
authors.lastname: Maniero
authors.forename: Vasile
authors.initials: V
authors.lastname: Chis
authors.forename: Alfonso
authors.initials: A
authors.lastname: Zoleo
authors.forename: Marina
authors.initials: M
authors.lastname: Brustolon
authors.forename: Alberto
authors.initials: A
authors.lastname: Mezzetti
mesh_terms: D003460:Crystallization; D005609:Free Radicals / Q000737:chemistry; D005720:Gamma Rays; D006851:Hydrochloric Acid / Q000737:chemistry; D008280:Magnetics; D008958:Models, Molecular; D008968:Molecular Conformation; D011522:Protons; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D005609:Free Radicals; D011522:Protons; D014443:Tyrosine; D006851:Hydrochloric Acid