SKIN‐PHOTOSENSITIZING FUROCOUMARINS: PHOTOCHEMICAL INTERACTION BETWEEN DNA AND —O14CH3 BERGAPTEN (5‐METHOXY‐PSORALEN)

Abstract— For some years the mechanism of the photosensitizing effects displayed by some furocoumarins on various biological substrates (human and guinea‐pig skin, bacteria cultures, mammalian cells adapted to in vitro growth, viruses) have been studied. Recently it has been pointed out that a photoreaction occurs between the photosensitizing furocoumarins and DNA after irradiation at 3655 Å. By use of a labeled furocoumarin, i.e.—O14CH3 bergapten or 5‐methoxy‐psoralen, this has been confirmed and more extensively studied. During the irradiation a stable combination of the furocoumarin with native DNA takes place with a quantum yield of 5·2 × 10‐3. It is probable that the reactive sites of DNA are the pyrimidine bases. Yeast‐RNA and the same DNA after heat‐denaturation or in the presence of high NaCl concentration photoreact at a much reduced rate. This photoreaction may explain some various biological photosensitizing effects produced by furocoumarins.