FLAVIN-PHOTOREDUCTION BY CYANIDE: NUCLEOPHILIC ADDITION IN THE EXCITED TRIPLET STATE

— Flavin photochemistry as well as biochemistry consists of competitive 1e− - and 2e− -reduction pathways, depending on the nature of the substrate. We show that cyanide ion is a photosubstrate which suppresses 1e−-oxidoreduction and leads to exclusive formation of 6- and 9-cyano-1,5-dihydroflavin. The photoreduction mechanism is thus revealed as a nucleophilic addition of cyanide ion at the excited flavin triplet. Preparative photochemistry and isolation and characterization of cyanoflavins have been done, as well as thorough mechanistic studies by conventional flash photolysis. In contrast, nitrite ion is shown to be a normal photosubstrate for flavins leading to exclusive 1e− -transfer followed by back donation.