Use of magnetic field effects to study coenzyme B12-dependent reactions

Abstract

This chapter summarizes the theory of magnetic field effects (MFEs) on chemical and enzymatic reactions and outlines the experimental methods necessary to use this powerful technique to study a variety of enzymatic and chemical reactions with radical pair (RP) intermediates. There are at least three mechanisms by which a magnetic field can change the rate of intersystem crossing in radical pair intermediates—the hyperfine interaction mechanism, the Δg mechanism, and the level crossing mechanism. The use of magnetic field effects to probe enzymatic reactions is usually limited to static (DC) magnetic fields that do not vary with time. Magnetic field effects provide a new method to study biological reactions with radical pair intermediates. Coenzyme B 12 -dependent enzymes with radical pair intermediates are well suited for study by this technique, as the RP is reversibly formed and the hyperfine interactions from the paramagnetic Co(II) provide a mechanism by which magnetic field-dependent intersystem crossing can occur. Not all enzymes with radical intermediates exhibit magnetic field-dependent chemistry—only the subset of enzymes with a pair of radicals.