EXPLORATORY PHOTOCHEMISTRY OF 5-AZIDO-8-ALKOXY-SUBSTITUTED PSORALENS FREE AND BOUND TO DNA

5-Azido-8-alkoxy psoralens were synthesized. Laser flash photolysis (L FP: XeF, 351 nm, 55 mJ, 17 ns) of the azides in acetonitrile or benzen e solution produces the triplet nitrene and a small amount of ketenimi ne. Laser flash photolysis of the azides in methanol or aqueous soluti on cleanly produces the tripler nitrene. In aqueous solution containin g highly polymerized calf thymus DNA, LFP produces a mixture of triple t nitrene and ketenimine corresponding to photolysis of free and bound psoralen, respectively. The two transients decay slowly but at differ ent rates. Assignment of the transient spectra were secured by matrix EPR and UV-visible spectroscopy. The tripler nitrene lifetime is the s ame in buffer and in the presence and absence of calf thymus DNA. The results explain why psoralen azides are unable to efficiently nick pla smid DNA pBR322 upon UV activation.