Fluorescence studies on the interaction of furocoumarins with DNA in the dark

The dark interaction of furocoumarins with DNA has been studied by a fluorescence quenching technique. The intrinsic fluorescence of psoralen, 4,5', 8-trimethylpsoralen (TMP) and 8-methoxypsoralen (8-MOP) was quenched to an appreciable extent upon their noncovalent binding to DNA molecule. The analysis of the binding data revealed that TMP binds to DNA with higher efficiency than 8-MOP and psoralen, their apparent Scatchard binding constants being 13.2 X 10(5) M-1, 7.1 X 10(5) M-1, and 12.2 X 10(5) M-1, respectively. The interaction of furocoumarins with DNA was strongly dependent on the conformational stability of DNA in the particular interaction media. The perturbation of DNA structure by changing the ionic environment decreased its interaction with furocoumarins. However, the interaction was facilitated by the presence of an electron-donating moiety in the parent compound, psoralen.