Photoaddition of 4,6-dimethyltetrahydrobenzoangelicin to thymine in DNA: X-ray studies and experiments with model oliganucleotides

The crystal structures of 4,6-dimethyltetrahydrobenzoangelicen (THBA), a fu rocoumarin analog, and of its furan-side cis-syn cycloadduct with thymine f ormed in the photoreaction with DNA, have been determined. The crystal stru cture of the latter compound contained only one enantiomeric form correspon ding to the addition to a 5'-XpT site. Contrary to most psoralen derivative s studied, THBA showed higher photoreactivity toward synthetic oligonucleot ides containing that sequence than toward those with the 5'-TpX sequence.