Psoralen synthesis. Improvements in furano ring formation. Application to the synthesis of 4,5',8-trimethylpsoralen

Coumarins from Opopanax chironium. New dihydrofuranocoumarins and differential induction of apoptosis by imperatorin and heraclenin

Opopanax chironium is a rich source of furano- and dihydrofuranocoumarins, whose accumulation in all plant parts and especially the roots is presumably responsible for the poisonous properties of the species. The presence of two distinct chemotypes was evidenced, with the one from Sicily affording the new dihydrofuranocoumarins 5d and 5e, while extracts from the Sardinian chemotype showed powerful apoptotic activity, which was traced to the prenylated furanocoumarins heraclenin (2a) and imperatorin (2b). Despite a close structural similarity, compounds 2a and 2b induced apoptosis in Jurkat leukemia cells in mechanistically different ways.

Synthesis of psoralens and analogues

This review presents briefly the various classical syntheses of psoralens and angelicins which generally start from preformed suitably substituted coumarin or benzofuran derivatives. However, the discovery of the photocyclo-C-4 addition of psoralens to pyrimidine bases prompted new developments in this area. In particular, improved synthesis of psoralens and substituted angelicins, and synthesis of new analogues such as furochromenes, pyridopsoralens, pyrano benzopyrandiones, dioxinocoumarins and aza-psoralens were described recently. This review focuses on the chemical syntheses of these various compounds.

Site specificity of psoralen-DNA interstrand cross-linking determined by nuclease Bal31 digestion

A novel method for determination of psoralen photo-cross-linking sites in double-stranded DNA is described, which is based on a pronounced inhibition of Bal31 exonuclease activity by psoralen-DNA interstrand cross-links. The results using a 51 base pair fragment of plasmid pUC19 and a 346 base pair fragment of pBR322 show that 5'-TA sequences are preferred cross-linking sites compared to 3'-TA sequences. They also indicate that sequences flanking the 5'-TA site influence the cross-linking efficiency at the site. The DNA photo-cross-linking by 4,5',8-trimethylpsoralen and 8-methoxypsoralen was analyzed, and these two psoralens showed identical site specificity. The 5'-TA preference is rationalized on the basis of the local DNA structure in terms of the pi-pi electronic interaction between the thymines and the intercalated psoralens, as well as on the base tilt angles of the DNA.

The reaction of the psoralens with deoxyribonucleic acid

Psoralen photochemistry is specific for nucleic acids and is better understood at the molecular level than are all other methods of chemical modification of nucleic acids. These compounds are used both for in vivo structure analysis and for photochemotherapy since they easily penetrate both cells and virus particles. Apparently, natural selection has selected for membrane and virus penetrability during the evolution of these natural products. Most cells are unaffected by relatively high concentrations of psoralens in the absence of ultraviolet light, and the metabolites of the psoralens have thus far not created a problem. Finally, psoralens form both monoadduct and cross-links in nucleic acid helices, the yield of each being easily controlled by the conditions used during the photochemistry.

Psoralen photochemistry and nucleic acid structure

Many new psoralen derivatives have been synthesized in an effort to enhance their water solubility and their binding to nucleic acids. Availability of the very soluble strongly binding compounds has improved our abilities to follow the optical changes associated with the photochemistry of psoralens with DNA. Changes in both absorbance and fluorescence are presented in this review. A kinetic model for the photochemistry concludes that the detailed kinetics is dominated by the equilibrium constant for intercalation of the psoralen in the DNA, the quantum yield for photoaddition to DNA once intercalated and the quantum yield for photodestruction of the drug in water. With these 3 parameters the kinetics of photochemistry is predictable. The values of these parameters for numerous derivatives of 8-methoxypsoralen and 4,5',8 trimethylpsoralen are presented. Application of this photochemistry to a study of nucleic acid secondary structure in chromatin, fd bacteriophage, and in ribosomes is reviewed.