Hydroxyl Radical Reactions with Phenol as a Model for Generation of Biologically Reactive Tyrosyl Radicals

Reactions of the hydroxyl radical with phenol, as model for the generation of biologically harmful tyrosyl radicals, have been investigated using high level quantum chemical techniques. Hybrid Hartree−Fock density functional theory (HF-DFT) calculations, using the B3LYP functional, and standard ab initio HF and second-order Moller−Plesset perturbation theory (MP2) calculations are employed to locate addition complexes, transition state structures, and products for OH addition to the various positions in the aromatic ring, as well as for direct abstraction of the phenolic hydrogen and for the observed water elimination from 1,2-dihydroxy-cyclohexadienyl. Although the different methods yield some variation in terms of barrier heights and addition complex geometries, a consistent picture is provided by the DFT and spin projected MP2 data. In summary the present calculations support previous experimental observations reporting essentially diffusion-controlled mechanisms for ring OH addition and direct hydrogen abstraction, but a significantly slower rate for water elimination of 1,2-dihydroxy-cyclohexadienyl. The computed energy surfaces are analyzed in relation to explicit product mixtures and reaction rates previously reported experimentally for OH radical additions to tyrosine and to phenol.