Photobiological activity of sulphur and selenium analogues of psoralen

Eight psoralen analogues, in which sulphur or selenium replaces one or both intracyclic oxygen atoms, were synthesized. Photoreaction with M13mp19 RF DNA in the presence and absence of oxygen (wavelength, greater than 320 nm) was studied. The damaged viral DNA was transfected into Escherichia coli and scored for infectivity towards Ca-treated wild-type E. coli. This allowed a comparative evaluation to be made of the heteropsoralens in terms of the photoreaction with DNA and the photodynamic effect. Most of the seleno- and thio-psoralens show very high photoactivity towards DNA compared with psoralen and 8-methoxypsoralen (8-MOP). Their photoreactivity is due mainly to a [2 + 2] photoreaction, since only a minor influence of molecular oxygen could be detected. Some of the studied seleno- and thio-psoralens are very efficient DNA photoinactivating agents and show great promise in photochemotherapy.