Three different tyrosyl radicals identified in L-tyrosine HCl crystals upon gamma-irradiation: magnetic characterization and temporal evolution.
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.
Maniero
Anna Lisa
AL
Università degli Studi di Padova, Dipartimento di Scienze Chimiche, Via Marzolo 1, I-35131 Padova, Italy. annalisa.maniero@unipd.it
Chis
Vasile
V
Zoleo
Alfonso
A
Brustolon
Marina
M
Mezzetti
Alberto
A
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*
D005609:Free Radicals; D011522:Protons; D014443:Tyrosine; D006851:Hydrochloric Acid