Fluorescent and non fluorescent C4-cycloadducts in the photoreaction at 365 nm between psoralen-3H and DNA

In the C4-photocycloaddition at 365 nm between 3H-psoralen and native DNA, the furocoumarin reacts with the pyrimidine bases of the macromolecule mostly in the 4′-5′-positions (fluorescent photocompounds). The non-fluorescent cycloadditions (addition in the 3—4 position) form in a ratio of 1: 3 compared with the preceding ones.