MAGNETIC-FIELD EFFECTS IN ADENOSYLCOB(III)ALAMIN PHOTOLYSIS - RELEVANCE TO B(12) ENZYMES

The magnetic field dependence of geminate radical pair recombination following the photolysis of adenosylcob(III)alamin (AdoCbl(III)) has been studied by continuous-wave photolysis and picosecond laser flash photolysis techniques. The net quantum yield (phi) for continuous-wave 514-nm photolysis of AdoCbl(III) is decreased 2-fold at 80 mT in aqueous 75% glycerol and 1.8-fold at 50 mT in aqueous 20% Ficoll-400. No magnetic field dependence is observed in H2O. Glycerol is a microviscosigen that enhances geminate radical pair recombination by decreasing diffusion, whereas Ficoll-400 is a macroviscosigen that enhances geminate radical pair recombination by forming ''cage'' structures in solution. The rate constant for AdoCbl(III) geminate radical pair recombination following photolysis at 532 nm is increased from 1 x 10(9) s-1 to 4 x 10(9) s-1 at 50 mT in 75% glycerol and from 1 x 10(9) s-1 to x 10(9) s-1 at 80 mT in H2O, indicating that the true rate for geminate radical pair recombination is being observed and that the rate for geminate radical pair recombination is magnetic field sensitive. The magnetic field-induced increase in the recombination rate constant and the corresponding decrease in net phi is consistent with the geminate radical pair being born in the singlet spin state. The picosecond quantum yield, phi(ps), is 0.7 +/- 0.1. Within 2 ns after the photolyzing pulse, 75% of the geminate radical pair population recombines. From nanosecond transient photolysis studies (Chen, E.; Chance, M. R. J. Biol. Chem. 1990, 265, 12987-12994), a large fraction of the remaining 25% of the radical pair population remains in the solvent cage and recombines on a slower time scale to give an overall fraction of recombination greater than 0.9. The extreme lability of the C-Co bond and the high rate of geminate radical pair recombination makes AdoCbl(III) well designed to be a transient radical source for enzymatic catalysis. An inherently high rate of geminate radical pair recombination also suggests that the rate of reaction of AdoCbl(III)-dependent enzymes may be affected by magnetic fields near 50-80 mT at subsaturating substrate concentrations.