Exploratory photochemistry of 5-azido-8-alkoxy-substituted psoralens free and bound to DNA.

5-Azido-8-alkoxy psoralens were synthesized. Laser flash photolysis (LFP: XeF, 351 nm, 55 mJ, 17 ns) of the azides in acetonitrile or benzene solution produces the triplet nitrene and a small amount of ketenimine. Laser flash photolysis of the azides in methanol or aqueous solution cleanly produces the triplet nitrene. In aqueous solution containing highly polymerized calf thymus DNA, LFP produces a mixture of triplet nitrene and ketenimine corresponding to photolysis of free and bound psoralen, respectively. The two transients decay slowly but at different rates. Assignment of the transient spectra were secured by matrix EPR and UV-visible spectroscopy. The triplet nitrene lifetime is the same in buffer and in the presence and absence of calf thymus DNA. The results explain why psoralen azides are unable to efficiently nick plasmid DNA pBR322 upon UV activation.