On the basis of the structure of DNA-psoralen bis adducts (formed by psoralen with two thymines on opposite strands), a psoralen-oligonucleotide conjugate was designed to photoinduce a cross-link between the two DNA strands at a specific sequence. Psoralen was attached via its C-5 position to a 5'-thiophosphate group of an 11-mer homopyrimidine oligonucleotide. The 11-mer binds to an 11-base-pair homopurinehomopyrimidine sequence of a DNA fragment, where it forms a triple helix. Upon near-UV-irradiation, the two strands of DNA are crosslinked at the TpA step present at the triplex-duplex junction. The reaction is specific for the homopurine'homopyrimidine DNA sequence and requires both oligonucleotide recognition of the DNA major groove and intercalation of psoralen at the triplex-duplex junction. The yield of the photoinduced cross-linking reaction is quite high (>80%). Such psoralen-oligonucleotide conjugates are probes of sequencespecific triple-helix formation and could be used to selectively control gene expression or to induce site-directed mutations.

To study the photoreactions between thymidine and psoralen and to devise a synthetic route to prepare thymidine-5-methoxypsoralen adducts, we have synthesized a compound in which thymidine is linked to a 5-methoxypsoralen derivative by a short flexible chain located to allow only the formation of syn intramolecular [2+2] cycloadducts. Irradiation at 365 nm carried out at usual concentrations (10-2-10-3 M ) resulted in a regioselective dimerization involving the 3,4 double bonds. Extreme dilution (c I 5 x M ) was necessary to observe the intramolecular photoaddition, which produces exclusively the 3,4-trans-syn adduct. These results are compared to those obtained previously with similar heterodimers that lead exclusively to the 3,4-cis-anti thymine or thymidine-psoralen adduct on exposure to near UV light.