Preparation of a novel psoralen containing deoxyadenosine building block for the facile solid phase synthesis of psoralen-modified oligonucleotides for a sequence specific crosslink to a given target sequence.

4,5',8-Trimethylpsoralen was attached to the C8-position of deoxyadenosine via a sulfur atom and a five carbon atom linker. The modified deoxyadenosine was then converted to a protected phosphoramidite and used as unusual as a building block for solid phase oligodeoxyribonucleotide synthesis. The efficiency of the photoreaction of a psoralen-modified oligonucleotide to a complementary matrix strand reached more than 90% within a 1 hour irradiation time at a wavelength of 345 nm.