The rates of reaction of 5-ethyl-3-methyllumiflavinium perchlorate and.5-ethyl-3-methyllumiflavinyl radical with a NADH analogue, N-benzyl-1,4-dihydronicotinamide, were measured anaerobically in tert-butanol and .5% acetonitrile/ 95% tert-butanol-solutions at 30C. The biphasic kinetics observed for the reaction of flavin radical with dihydronicotinamide were interpreted in terms.of both a leC and a 2e-mechanism; the former was found to be inadequate based on experimental requirements of the mechanism. The dihydronicotinamide reacts preferentially with oxidized flavin rather than flavin radicals even when the concentration of oxidized flavin. is at a concentration 5 orders of magnitude less than that of radical. These studies show that the "hydride" reduction of oxidized flavin by BNAH is more facile than is the le-reduction of flavin radical by BNAH.The oxidation of reduced pyridine nucleotides and their analogues is far from completely understood, especially when a flavin moiety is used as the oxidant as it is in the flavin coenzymes ofthe respiratory chain (1). Suelter and Metzler (2) studied the nonenzymatic reduction of several flavin analogues by 1-propyl-1,4-dihydronicotinamide and concluded that hydride transfer was occurring. Their deduction was based on the observations that: (i) electron-withdrawing.substituents on the. nicotinamide decreased the oxidation rate; (ii) the 4,4-dideutero-1,4-dihydronicotinamide reacted slower than its protio analogue by a factor of 0.31; and (iii) the oxidation rate increased with increasing solvent polarity. Radda and Calvin (3) pointed out that radical detection by ESR is not a useful. observation because the comproportionation reaction between oxidized flavinmononucleotide and reduced flavinmononucleotide (FMN and FMNH2, respectively) provides the flavin radical, FMNHF-.[1]Thus, the presence ofa flavin radical does not provide evidence for a le-transfer mechanism. In further studies (4-6), these results were confirmed and the N5 of the flavin. moiety was proposed to be the position accepting the hydride ion. The reaction mechanism for net hydride transfer. from dihydronicotinamides to flavin was later formulated to involve preequilibrium complex formation along the reaction path. This was based on the lack of correlation of log(kr te) for the redox reaction with the E112 potentials for a series of flavins and the good correlation between log(krale) with log(Ke) for complex formation ofthe flavins with tryptophan (7,8 (19)(20)(21)(22)(23)(24). A multistep rather than a single-step Htransfer has been suggested in order. to account for. the observations that product isotope effects have been found to be significantly larger than the kinetic ones. This observation-can be explained if an intermediate is formed along the reaction path for "hydride. transfer. " In most cases a pyridinyl radical was invoked. The direct determination of isotope ratios of products is inherently more difficult in the dihydronicotinamide reduction of flavins because of the exchangable nat...

098ChemInform Abstract The equilibrium constants for mono-and bis-ligation of imidazole with the title porphyrin complex are determined. Kinetic data for the reaction of p-cyano-N,N-dimethylaniline N-oxide with the Mn(III) complex in the presence and absence of imidazole in dry CH2Cl2 at 25 rc C under anaerobic conditions are given. Imidazole ligation increases the rate constant for oxygen atom transfer from the N-oxide to the porphyrin complex. Bis-imidazole ligated species are blocked to reaction with N-oxides. The behavior of the various ligated higher valent manganese-oxo porphyrins as oxidants is examined. They are shown to be the principal epoxidizing agent in reaction with cis-cyclooctene. Epoxidation reactions are not rate controlling, and epoxide is formed in competitive processes that involve thereaction of higher valent manganese-oxo porphyrin species with p-cyano-N,N-dimethylaniline (and its oxidation products) and alkene.

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The equilibrium constants for mono‐ and bis‐ligation of imidazole with the title porphyrin complex are determined.