A FLASH-PHOTOLYSIS INVESTIGATION OF FLAVIN PHOTOSENSITIZATION OF PURINE NUCLEOTIDES

Evidence for the existence of a reactive triplet excited state of lumiflavin has been obtained by the flash-photolysis technique. The triplet state is formed in high yield on the irradiation of flavin solutions in water or chloroform by visible light, and it has been demonstrated that it can transfer its energy to a second molecular species. The flavin-sensitised oxidation of two purine nucleotides, adenylic and guanylic acids, has been studied by flash-photolysis and by long-term irradiation, and the results suggest a triplet-triplet mechanism for the transfer of energy from the excited flavin to the nucleotide. Approximate absorption spectra of the triplet state and of a semiquinone of the flavin have been calculated from the complex transient absorption curves observed on flashing the flavin solution. The triplet decays by a first-order process where k₁= 1·1 × 10^-3. The chemiluminescence spectrum of skatole is identical with the fluorescence spectrum of o-formamidoacetophenone in the same environment Similar results for 2,3-dimethylindole lead to the identification of the acylamide anion as the emitter in indole chemiluminescence. A peroxide ring cleavage excitation mechanism is proposed. 10⁴ sec^-1 and the semiquinone by a second-order process where k₂= 0·75 × 10⁹ 1.m^-1 sec^-1. The rate constants and extinction coefficients obtained enable decay curves to be calculated which fit satisfactorily those measured with the kinetic-flash apparatus.