The dependence of the photochemically induced reduction of lumiflavin on pH, light intensity, and concentration of sulphydryl compounds

The spectral changes which occur on photolysis of flavin solutions containing β-mercaptoethanol (RSH) and lipoamide (L(SH)2) are in accord with the formation of dihydroflavin, except at pH values below 3 for RSH. Here an oxygen-stable product is formed, which has λmax 360 nm and is probably a C-4a substituted dihydroflavin. The fraction of flavin present when FlH2 reaches a photostationary statef(FlH2)ps is dependent on pH and on the absorbed light intensity Ia. At certain pH values the actual photostationary concentration of FlH2 is linear in the concentration of RSH, but for the lipoamide system shows a more complex dependence on concentration. The initial quantum yields are independent of absorbed light intensity in the pH range 7 to 10 for RSH, and increase strongly with pH in the region 7 to 9 for both sulphydryl compounds. In the pH range 5–6 they are linear in sulphydryl concentration. For RSH the results can be explained by a mechanism in which flavin triplet abstracts a hydrogen atom from RSH to ...