The chemistry of flavins and flavoproteins III. The reaction of dihydrolipoic acid with flavins

1. The reaction between dihydrolipoic acid and a number of flavin derivatives has been found to be of first order in both reactants.

2. The rates are strongly pH dependent and are of first order in OH- concentration as well as being generally base catalysed.

3. The polarographic half-wave potentials of flavin derivatives and their reactivities towards NADH correlate reasonably well with their reactivities towards dihydrolipoic acid.

4. On the basis of these observations and on some electron spin resonance evidence a two-step mechanism is suggested for the reaction, the first being a fast dissociation of one of the -SH groups of dihydrolipoic acid into its anion followed by a second-order reaction between this anion and the flavin, probably by a two-electron transfer.

5. The pK_a of the -SH-groups of the acid has been measured spectrophotometrically, and the value derived (10.7) is in good agreement with the value arrived at from the kinetic studies.