Reactivity of Peroxy Radicals Coordinated to Vitamin B12 Studied by Electron Spin Resonance Spectroscopy

Abstract

One-electron transfer from chelated Co(II)(3d7) of vitamin B12 (cyanocobalamine) to tert-butylhydroperoxide forms at ambient temperature in non-polar solvents peroxy radicals stabilized by π-coordination to Co(III)(3d6) of the oxidized vitamin. In the absence of oxygen, the peroxy radicals manifest themselves as singlet ESR lines at g = 2.0174. After addition of sterically hindered phenols, apart from the decreased original singlet line, a new signal of stable phenoxy radicals, formed by H-abstraction from the phenols, is observed. Phenoxy radicals generated from unhindered phenols remain stabilized by σ-coordination to the cobalt(III) centre, giving rise to an ESR signal split into eight lines due to interaction of the unpaired electron with the magnetic moment of the 59Co nucleus (I = 7/2) of vitamin B12. Large molecules with unhindered OH groups, such as testosterone or cholesterol cannot be coordinated after the primary radical attack and the generated radicals disappear by recombination. By this technique, the sensitivity of biological targets to primary peroxy radical attack as well as the efficiency of different antioxidants (vitamin E, Stobadine) can be tested.