The photoreaction of furocoumarins with cell substrates of high molecular weight are limited on nucleic acids. Both molecules are jointed together via a cyclobutane ring system. The 5.6 double bonds of thydine or uracil and the 3.4 and/or 4',5' positions of furocoumarins act as binding sites. Proteins, polysaccharides, and lipides behave quite inert. 2. The photoreactions of cancerogenic hydrocarbons with nucleic acids are similar to those of furocoumarins. 3. Photochemically induced cross-links of DNA strands by furocoumarins are to a high degree dependent on the secondary structure of nucleic acids. Double stranded DNA is much more favoured to bind furocoumarins than single stranded DNA or RNA. 4. Lethal and mutagenic effects in cells caused by photoreactions between furocoumarins and DNA are well explainable on molecular level. It consists a satisfactory correlation between skin sensitization and binding tendency of furocoumarins to DNA. Possibilities of repair of furocoumarine induced DNA damages are discussed.
