Photoreaction between 4,7-dimethylallopsoralen and DNA: isolation of a fluorescent cycloadduct between the furocoumarin and thymine.

A new fluorescent adduct has been isolated from the products of hydrolysis of DNA irradiated in the presence of 4,7-dimethylallopsoralen. The close similarity of the U.V. absorption spectrum of this compound to that of a synthetic 4',5'-dihydroallopsoralen shows that the 4',5'- double bond initially present in the 4,7-dimethylallopsoralen was saturated in the photoreaction with DNA, and this is consistent with a C4 cycloaddition at this level. The irradiation of the photoadduct at 254 nm provoked its photodissociation into the parent compounds, that is, thymine and 4,7-dimethylallopsoralen. These data are consistent with a cycloadduct between the 4',5'- double bond of 4,7-dimethnylallopsoralen and the 5,6- double bond of thymine.