Sharp Kink of DNA at Psoralen-cross-link Site Deduced from Crystal Structure of Psoralen-Thymine Monoadduct

Psoralens are a class of tricyclic aromatic heterocycles that have been used as chemical probes to study DNA and RNA structures (for review, see Hearst 1981). On exposure to UV light, psoralens form chemical cross-links to the DNA bases in three steps (Isaacs et al. 1977; Johnston et al. 1977). The first step is intercalation of psoralen between two adjacent base pairs. The second step is formation of a monoadduct, i.e., one psoralen photoreacts to one strand of DNA. The third step is the cross-linking of the same psoralen to the other strand of DNA.