Reduction of substituted flavins by .CO2- and cyclic disulphide anions.

5-Deazariboflavin was observed to undergo one electron-equivalent reduction on reaction with .CO2-, DS2-. or a mixture of .CO2- and eaq-. This is attributed to formation of .FlH, followed by dimerization: 2 5-DRFlH.----(5-DRFlH)2 a process not previously confirmed. In contrast, 3-deazariboflavin, 1-deazariboflavin 7-chlororiboflavin, 2-thioriboflavin, lumiflavin, flavin adenine dinucleotide and riboflavin all underwent two electron-equivalent reductions to FlH2 or FlH-species via the disproportionation: 2.FlH----FlH2+Fl Spectra of the reduced species are reported for 3-deazariboflavin, 1-deazarboflavin and 2-thioriboflavin. The anion radicals of dithiothreitol (DS2-.) and lipoamide (LS2-.) did not effect reduction beyond the two electron equivalence point. However, .CO2- was seen to effect further reactions with the dihydroforms of the higher redox potential flavins 7-chlororiboflavin and 2-thioriboflavin at pH 3 to 10, but for the other flavins only at or below pH 2. Except for the sulphur-containing 2-thioriboflavin, the protonated DS2.H and LS2.H radicals were unable to reduce any of the flavins at pH less than or equal to 3. This cause a 'falloff' in the yield of the reduced flavin which is in accord with the reaction rate constants and radical pK values. Despite their differences in absorbance, the reactions of DS2.H and LS2.H towards flavins were similar in all aspects examined. Several new rate constants are reported.