The interaction of two water-soluble furocoumarins, 8-(omega-diethyl 
aminopropyloxy)psoralen hydrochloride (I) and its 5-isomer (II), with DNA has 
been investigated by spectroscopic, equilibrium dialysis, hydrodynamic and 
chiroptical techniques. Both compounds intercalate into the polynucleotide 
double helix. From the dependence of the binding on ionic strength, ion release 
and binding free energy corrected for counterion release have been 
quantitatively estimated. It is shown that the differences in DNA-affinity 
observed for compounds I and II arise primarily from non electrostatic 
contributions. The binding process is exothermic, with slightly different van't 
Hoff enthalpies for the examined furocoumarins. Helix lengthening and dichroic 
effects indicate different intercalation geometries for the isomeric compounds. 
These studies allow a possible explanation for the finding that isomer I 
exhibits largely better DNA-photobinding properties, while isomer II is by far 
more effective as an antiviral agent.