Isolation and characterization of the photoadducts of 5,7-dimethoxycoumarin and adenosine

Photoreaction of 5-methoxypsoralen with thymidine. Isolation and characterization of a pyrone-side monoadduct involving the pyrimidine methyl group

The UVA-mediated photoreaction of 5-methoxypsoralen (5-MOP) with thymidine has been investigated in the dry state. Under these conditions, the main products are 5-MOP pyrone-side monoadducts to thymidine. We report the isolation and characterization of an unusual 5-MOP-thymidine photoproduct. The assignment of the photoadduct was achieved on the basis of extensive spectroscopic measurements (UV, mass spectrometry (MS), 1H and 13C nuclear magnetic resonance (NMR), nuclear Overhauser effect (NOE) experiments). The formation of the photoadduct, which is rationalized in terms of a radical mechanism, appears to involve, in a covalent bond, the C-4 pyrone moiety of 5-MOP and the methyl group of thymidine.

Adenosine-mediated photoreaction of 4,5',8-trimethylpsoralen with alcohols

Irradiation of thin films consisting of 4,5',8-trimethylpsoralen (TMP), adenosine and small amounts of alcohols led to TMP-alcohol photoadducts in addition to TMP-adenosine photoadducts. Four TMP-ethanol and two TMP-methanol adducts have been separated and characterized. Covalent bonds were formed between the 4-carbon of TMP and the alpha-carbon to the hydroxy group in the alcohols. The TMP-alcohol photoadducts were formed only in the TMP film containing small amounts of alcohol and adenosine. Furthermore, no photoadduct of TMP and ribose was detected upon photolysis of a TMP-ribose film, suggesting that the adenine moiety plays a specific role in the reaction. The interaction of adenosine with psoralens in a dry film may be related to the DNA sequence selectivity observed for the photoreaction of psoralens with DNA.

Light-activated plant defence

Sunlight provides the energy required for all biochemical, physiological and developmental processes necessary for plant growth, reproduction and survival. The role of light in photosynthesis and photomorphogenesis has been appreciated for some time; however, the role of light in plant defence is a comparatively recent realization. Between 75-100 photosensitizers or phototoxins, molecules that become toxic in the presence of light, have been extracted from higher plant tissues. These biologically-active compounds have diverse biosynthetic origins and belong to at least 15 different phytochemical classes (i.e. acetophenones, acetylenes, benzophenanthrenes, β-carbolines, coumarins, extended quinones, furanochromones, furanocoumarins, furanoquinolines, isoquinolines, lignans, pterocarpans, quinolines, sesquiterpenes and thiophenes). Of more than 100 angiosperm families assayed, phototoxins and/or phototoxic activity have been reported in c. 40 families representing 32 orders and 8 subclasses of the Magnoliophyta. Most of these allelochemicals are acute toxins with little organism-specificity. As such, they are effective biocides capable of killing a wide-range of potentially harmful organisms including: viruses, pathogenic bacteria and fungi, nematodes and herbivorous insects, as well as competing plant species. This review focuses on the phytochemistry, taxonomic occurrence and toxicological consequences of phototoxic metabolites in flowering plants. The available information suggests that phototoxic phytochemicals: (i) are broad-spectrum allelochemicals capable of protecting plants against a variety of detrimental organisms in nature, (ii) represent a successful defensive strategy in both primitive and advanced plant taxa, and (iii) may be an effective defence under a variety of photoenvironmental conditions. CONTENTS Summary 401 I. Introduction 401 II. Chemistry/biochemistry of plant photosensitizers 402 III. Botanical distribution of photosensitizers 403 IV. Toxicology 405 V. Biological activity/defensive role of photosensitizers 408 VI. Concluding remarks 413 References 415.

Transient absorption spectra and quenching of coumarin excited states by nucleic acid bases

Triplet-triplet absorption spectra of coumarin show different profiles and maxima in ethanol from those in water, which are very similar to those reported in benzene. Long-lived transient species other than triplet states were generated as initial photoproducts between coumarins and nucleic acid bases. The excited singlet and triplet states of coumarins were quenched by nucleic acid bases. Adenine base quenched the excited singlet state of coumarins more efficiently than thymine base. However, photocycloadducts of furocoumarins are formed predominantly with thymine, and not with adenine. Moreover, it is reported that the poly[dA-dT].poly[dA-dT] sequence region is the most favourable site for the photocycloaddition reaction of furocoumarins. The results imply that adenine contributes to singlet-state photocycloaddition reaction of furocoumarins with thymine, probably through an adenine-furocoumarin-thymine termolecular interaction.

A novel photoadduct of 4,5',8-trimethylpsoralen and adenosine

A novel photoadduct of 4,5',8-trimethylpsoralen (TMP) and adenosine was isolated and purified by reverse-phase liquid chromatography. The structure of the photoproduct was determined by various spectral methods and found to be a TMP-adenosine 1:1 adduct resulting from the covalent bond formation between the carbon C(4) of TMP and ribose 4'-carbon of adenosine.

Photoreaction of 4,5',8-trimethylpsoralen with adenosine

The near-UV induced photoreaction of 4,5',8-trimethylpsoralen (TMP) with adenosine was investigated in a dry film state. Four major photoadducts were isolated and purified by reverse-phase liquid chromatography. The structures of the photoproducts were elucidated on the basis of spectroscopic methods, including UV, FT-IR, mass spectrometry (FAB and EI methods) and 1H-NMR analysis. These photoproducts were characterized to be TMP-adenosine 1:1 adducts, which resulted from the covalent bond formation between the carbon C(4) of TMP and ribose 1' or 5' carbon of adenosine. Of the photoadducts, one photoadduct (V) was the major product, reflecting some selectivity in the photoreaction of TMP with adenosine in the solid state.

Photoreactions of furocoumarins with biomolecules

Recent aspects of the photoreactions of linear and angular furocoumarins with DNA and related compounds, including [2 + 2] cycloaddition to pyrimidine bases, covalent attachment to the osidic moiety of adenine nucleosides and photodynamic effects, are surveyed. Reactions of photoexcited furocoumarins with proteins and unsaturated lipids and the possible biological roles of the resulting adducts are also presented and discussed.

A new class of psoralen photoadducts to DNA components: isolation and characterization of 8-MOP adducts to the osidic moiety of 2'-deoxyadenosine

The near-UV-induced photoreaction of the bifunctional 8-methoxypsoralen (8-MOP) with 2'-deoxyadenosine (dAdo) was investigated in the dry state. Four main monoadducts of 8-MOP to 2'-deoxyadenosine were separated by high performance liquid chromatography and subsequently characterized by soft ionization mass spectrometry (fast atom bombardment and plasma desorption mass spectrometries) and extensive 1H NMR analysis including nuclear Overhauser effect (NOE) measurements. These new types of furocoumarin-nucleic acid component which appear to be specific to 2'-deoxyadenosine were shown to result from recombination of the 3,4-dihydropyron-4-yl radical of 8-MOP with 2'-deoxyadenosyl radical either at the 1' or the 5' position.