The effects of different DNA sequences on the photoreaction of various 
furocoumarin derivatives was investigated from a quantitative point of view 
using a number of self-complementary oligonucleotides. These contained 5'-TA and 
5'-AT residues, having various flanking sequences. The furocoumarins included 
classical bifunctional derivatives, such as 8-methoxy- and 5-methoxypsoralen, as 
well as monofunctional compounds, such as angelicin and benzopsoralen. Taking 
into an account the thermodynamic constant for noncovalent binding of each 
psoralen to each DNA sequence, the rate constants for the photobinding process 
to each fragment were evaluated. The extent of photoreaction is greatly affected 
by the DNA sequence examined. While sequences of the type 5'-(GTAC)n are quite 
reactive towards all furocoumarins, 5'-TATA exhibited a reduced rate of 
photobinding using monofunctional psoralens. In addition terminal 5'-TA groups 
were the least reactive with 5- and 8-methoxypsoralen, but not with angelicin or 
benzopsoralen. Also 5'-AT-containing fragments exhibited remarkably variable 
responses toward monofunctional or bifunctional psoralen derivatives. As a 
general trend the photoreactivity rate of the former is less sequence-sensitive, 
the ratio between maximum and minimum being less than 2 for the examined 
fragments. The same ratio is about 3.4 for 8-methoxypsoralen and 6.2 for 
5-methoxypsoralen. This approach, in combination with footprinting studies, 
appears to be quite useful for a quantitative investigation of the process of 
covalent binding of psoralens to specific sites in DNA.