A laser flash photolysis study of the photoreduction of the lumiflavin triplet state

The efficiency of several compounds as electron donors for the photochemical reduction of lumiflavin has been studied with regard to their use in the photoproduction of hydrogen peroxide. Laser flash photolysis was employed in order to study the rate constants and nature of the primary reactions between the flavin triplet state and each donor in the pH range 3-14. A substantial contribution of two-electron transfer from donor to flavin triplet state was ruled out; only one electron equivalent (either hydrogen atom or electron depending on the substrate) transfer was significant. The nature of the secondary reactions of the flavin semiquinone radicals was also investigated. Several different decay modes were distinguished depending on the donor, including a back reaction to regenerate the starting materials, transfer of a second electron from donor to flavin and disproportionation of the flavose- miquinone radical. The results are discussed in terms of the relative efficiency of each donor in generating H 2 O 2 upon illumination in the presence of flavins.