7227that free-radical transfer and trapping reactions can be treated with the same model when nuclear relaxation processes are included.$ Radical transfer has also been discussed in ref 4 and 8.We consider the formation of a geminate radical pair (RP) by a sudden reaction of the precursor molecule "M with electron spin multiplicity m. Because of singlet-triplet mixing via the isotropic hyperfine coupling, the time evolution of the electron spin wave function depends on the nuclear spin states of RP.*t415 Consequently, the probability of cage collapse to combination or disproportionation products is a function of the nuclear spin states. Concurrent with radical pair collapse, diffusion-controlled separation into free radicals occurs with probability wd. This competition results in enrichment of certain nuclear spin states in the cage products and depletion of the same states in the free radicals.lo The degree of sorting can be calculated from the steady-state concentration of R P which for the nuclear spin state i is given by where k is the rate constant for the formation of RP, wt is the nuclear spin state dependent probability of cage collapse to be calculated from eq 9, ref 5 or eq 1, ref 6 for m = 1 or 3, respectively, and we, covers all cage product formation resulting from nuclear spin independent singlet-triplet mixing in RP. The enhancement factor, of an nmr transition between states i a n d j in the cage product is then given by (l), where (ZtJo is the nuclear spin expectation value at t hermoequili brium.[RP1i = ["MIk/(wi f wer + wd) acid oxidase proceeds by a two-step mechanism and the CY hydrogen is lost as a proton; (2) Porter and Bright" observed that Vma, for L-amino acid oxidase showed both a solvent deuterium isotope effect (expected if protonation of N-1 of flavin occurs in the rate-determining step) and an isotope effect in the cleavage of the CY carbon-hydrogen bond; (3) for the reactions catalyzed by succinate dehydrogenase and acyl-CoA dehydrogenases, a kinetic deuterium isotope effect of differing magnitudes has been found for the removal of each of the hydrogens from the substrate, and one hydrogen exchanges more rapidly than the others with the medium ; l8 (4) there is no evidence for free-radical intermediates in the dehydrogenation step of any flavoenzyme-catalyzed reaction or in any model flavin reaction; 3-10 (5) hydride transfer from many typical substrates of flavoenzymes is without chemical analogy.The distinctive features of the general mechanism proposed here for flavoenzyme-catalyzed dehydrogenations are: (1) both hydrogens are transferred as protons and (2) the substrate forms a covalent compound with the flavin ring system; the formation and breakdown of this intermediate provide a mechanism for electron transfer. Thus, the mechanism is closely related to that of most nonredox enzymic reactions. As in such cases, suitably placed acid and base groups on the enzyme surface would be expected to catalyze the flavoenzyme dehydrogenations, and may be largely responsible for the rapid...

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