Laser flash photolysis studies of electron transfer between semiquinone and fully reduced free flavins and horse heart cytochrome c.

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Abstract
Laser flash photolysis was used to determine the second-order rate constants for the reduction of horse heart cytochrome c by the semiquinone and fully reduced forms of various flavin analogs. Substitution in the dimethylbenzene ring of the flavin causes appreciable changes in the rate constants, whereas substitutions at the N-10 position do not. Placing a charged phosphate group in the N-10 ribityl side chain leads to only small ionic strength effects on the rate constants, whereas a charged group attached to the dimethylbenzene ring produces a large ionic strength effect. These results can be accounted for by assuming that a productive collision between flavin and cytochrome involves an orientation that positions the aromatic ring.sbd.N-5 region of the flavin toward the heme crevice and the N-10-pyrimidine ring region away from it. These observations have implications for mechanistic understanding of biological electron transfer reactions and are discussed in this context.

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