Photoinduced Bond Cleavage Reactions in Biologically Active B12 Coenzymes and Model Complexes. An FT-EPR Study

D.M MARTINO; C. KIARIE; H. VAN WILLIGEN

Bariloche

Congreso; 88° Reunión Anual de la Asociación Física Argentina; 2003

AFA

Free alkyl radicals generated by the photo-induced homolytic cleavage of the Co–C bond in methyl (Me) and 5’-adenosyl (Ad) cobalamines, and methyl cobaloximes were studied using Fourier Transform Electron Paramagnetic Resonance (FT-EPR). Cobaloximes consist of a cobalt atom coordinated to a four nitrogen in the equatorial plane and to an alkyl group (R= methyl) and a base (B = pyridine or H2O) in the axial positions.           Spectra of the Me- and Ad- radicals generated by photo excitation of cobalamines in aqueous solutions show Chemically Induced Dynamic Electron Polarization (CIDEP) produced in precursor radical pairs. This effect can be accounted for in terms of bond cleavage via the cobalamine singlet excited state. The spectrum of CH3• produced by photo-excitation of methyl-cobalamine is wavelength dependent. This confirms earlier observations that bond cleavage can occur via two reaction channels involving distinct transient intermediates.           CIDEP patterns in the FT-EPR spectra of CH3• generated by the photo-excitation of cobaloximes show a striking dependence on the identity of the axial ligand, as well as on the nature of the solvent (viscosity, polarity and coordination ability). The effect is attributed tentatively to shifts in energy levels of the cobalt complexes induced by changes in axial coordination. Consequently, the spin polarization pattern is attributed to the relative contribution of two reaction channels. A dominant contribution from a singlet excited state reaction channel giving rise to a Radical Pair mechanism (RPM CIDEP) in some solvents, or a triplet excited state channel producing a Triplet Mechanism (TM CIDEP) in others.