title: In vitro characterization of the reaction of four psoralen derivatives with DNA.
abstract: Four psoralen derivatives were radiolabeled and used for in vitro DNA binding studies. The derivatives were compared for their dark-binding ability to DNA, photoreactivity, and for unwinding angles. The dark-binding dissociation constants we determined were 1.4 X 10(-3) M for 8-methoxypsoralen (8-MOP), 3.5 X 10(-4) M for 5-methoxypsoralen (5-MOP), and 5.5 X 10(-4) M for 5-methylisopsoralen (5-MIP). We did not detect any dark binding to DNA for 3-carbethoxypsoralen (3-CP). Photoaddition experiments indicated that the relative rates of photoaddition by psoralen to DNA (measured as psoralens bound per base pair per second) are 4.4 X 10(-5) for 5-MIP, 9.2 X 10(-6) for 5-MOP, 7.8 X 10(-6) for 8-MOP, and 4.6 X 10(-6) for 3-CP. We found the peak level of binding (for an initial base pair-to-psoralen ratio of 22) to be 27, 32.2, 31.2, and 1,538 base pairs per psoralen bound for 5-MOP, 8-MOP, 5-MIP, and 3-CP, respectively. In addition, 3-CP adducts could be photoreversed by prolonged irradiation at 360 nm. After 10 hours of irradiation, the amounts of 3-CP bound to DNA had fallen to less than 50% of the peak amount bound. In the same time, the amount of 8-MOP and 5-MOP bound had fallen to 95% of their peak values, and 5-MIP had fallen to 85% of its peak value. We also performed unwinding angle experiments to determine the amount of unwinding of the DNA helix induced per photobound derivative molecule; the unwinding angles +/- 3 degrees were 25 for 5-MOP, 28 for 8-MOP, 26 for 3-CP, and 18 for 5-MIP.
mesh_terms: D004247:DNA; D011564:Furocoumarins; D007700:Kinetics; D010777:Photochemistry; D013057:Spectrum Analysis
chemical_list: D011564:Furocoumarins; D004247:DNA