Formation of inter-strand cross-linkings in the photoreactions between furocoumarins and DNA.

A furocoumarin intercalated between two base pairs of native DNA can photoreact with two pyrimidine bases engaging both its 4',5'- and its 3,4-double bond. This fact was evidenced studying the behaviour of the fluorescence acquired by DNA during irradiation at 365 nm. When this double reaction occurs, a cross-linking is formed between the two strands of DNA, as denaturation and renaturation experiments have demonstrated. The various furocoumarin derivatives have a very diverse ability to form cross-linkages, which is independent of their photobinding capacity. This different ability is due mainly to the structure of furocoumarins and to the steric relationships that the intercalated furocoumarin molecules have with the stacked pyrimidine bases.