The Flavin‐sensitized Photooxidation of Nucleotides—ii. The Participation of the Flavin Triplet State

Abstract— –A study has been made of the effects of a series of nucleotides upon the electronic excited states of lumiflavin in order to determine the mechanism of their flavin‐sensitized oxidation. A hydrogen‐abstraction mechanism is ruled out, because if the nucleotide acts as a reducing agent for the excited dye molecules, it should increase the rate of reduction of the dye when the irradiation is carried out in the absence of oxygen. However, each of the nucleotides studied was found to reduce the rate of anaerobic photoreduction. While oxidation by an intermediate species such as the dye ‘moloxide’ or singlet oxygen is not entirely ruled out, our evidence suggests that the initial reaction is between the nucleotide and the flavin triplet. This results in a loss of the triplet excitation energy and is a very efficient reaction, guanosine monophosphate shewing 36 per cent of the triplet quenching efficiency of potassium iodide. The relative rates of reaction of the nucleotides with the flavin triplet exactly parallels their quantum yields of sensitized photo‐oxidation. The formation of ground‐state complexes between flavin and nucleotide and the participation of the singlet excited state of the flavin are not considered to be important.