Reactivity of free and coordinated radicals in biology and chemical carcinogenity. I. Coordinated phenoxy radicals generated by hydrogen transfer from hydroxy derivatives of 3,4-benzopyrene.

The tautomeric keto form of 6-, 7- and 8-hydroxy-3,4-benzopyrene (BP) prevails in nonpolar solvents at laboratory temperature, in contrast with the enol form of 9-HO-BP and 3-HO-BP, as it was shown according to the ESR study of H-transfer reactions initiated by tert. butyl peroxy radicals coordinated upon the hydroxy derivative of cobalt(III)-acetylacetonate [HO-Co(acac)2]. A further hydroxylation of the keto form of 6-HO-BP, mainly in the position 3, in the thermal interval 40-60 degrees C and the presence of oxygen was observed. Because of lack of steric hindrance in the neighborhood of the position 3 or 9 the primarily formed phenoxy radical after H-abstraction remains stabilized as sigma-coordinated radical on CoIII. Such a radical complex can be destroyed after addition of polar solvents (e.g. methanol). During autooxidation of BP in aerated solutions, similarly as during enzymatic oxidation, a relatively great concentration of high-stable radicals accumulates in nonpolar solvents and in the absence of peroxides. The paramagnetic species is interpreted as a radical pair of two nondissociated semiquinones. In discussion of the carcinogenic activity of oxidative products of BP, not only the enzymatically formed, but also the randomly formed radical intermediates in the first steps of autooxidation must be taken into consideration, which reactivity to biological targets is mediated with the polarity of the biological medium.