Photosensitizers plants

Poulton, G. A. and Ashwood-Smith, M. J. 1983. Photosensitizing plant products. In Stitch, H. (Ed.), Carcinogens and Mutagens in the Environment, Vol. II. Chemical Rubber Publishing Co., Cleveland, OH... 

Light-absorbing chemicals in lichens or oak moss (from a lichen) are said to cause photoallergic dermatitis [137, 138], although this is not experimentally reproducible, at least not with atranorin and lichen mixtures [139]. Table 5 lists a number of putative causes of phytophotodermatitis, but omits those caused by ingestion and those reported but doubted from available evidence by Mitchell and others. The subject has been reviewed by Pathak [125] and others. There are also photosensitizing plants that do not contain psoralens, e.g., St. John s wort (Hypericum perforatum). 

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

Hypericin from St. John s wort is another phenolic compound with multiple rings and multiple double bonds. This compound readily absorbs UV light and is a primary photosensitizing agent that will result in severe sunburn in species that either ingest the plant or come in contact with plant dust or leaf extracts. 

Downum, K. R., Villegas, S., Rodriguez, E., and Keil, D. J. (1989). Plant photosensitizers a survey of their occurrence in arid and semiarid plants from North America.. Journal of Chemical Ecology 15,345-355. 

Severe phototoxicity has been reported in cattle and sheep grazing on the plant (the veterinary term is hy-pericism) but not in humans taking therapeutic doses. However, photosensitivity does appear to be a problem... 

The Role of Natural Photosensitizers in Plant Resistance to Insects... 

Despite the hypothesis that the evolutionary significance of phototoxic secondary substances may be linked to their ability to discourage Insect herbivores, most research has been directed toward their effects on human skin and range animals (42). In an attempt to extend our knowledge of insect photosensitizers we have screened a number of plant secondary substances (TABLE TI) for their photosensitizing activity to 4th instar mosquito larvae Aedes atropalpus under solar simulating lamps. 

In addition to organic dyes, natural dyes extracted from plants can be used as photosensitizers [36,140,141]. A nanocrystalline 2 solar cell using a san-talin dye extracted from red sandalwood can produce 1.8% efficiency under 80 mW/cm2 irradiation [141]. Cherepy et al. reported that a nanocrystalline 2 solar cell using flavonoid anthocyanin dyes extracted from blackberries could convert sunlight to electrical power with an efficiency of 0.6% (Jsc = 1.5-2.2 mA/cm2 and Voc = 0.4-0.5 V) under AM 1.5 [36]. 

Only one class of benzofuran derivative, natural or synthetic, shows this property—the furocoumarins-, these are important for plant biosynthesis, since they take part in the elaboration of chlorophyll.308 13 Among linear furocoumarins, psoralene (27a) is particularly active. Angelicin (28), another fundamental furocoumarin heterocycle with an angular structure, has only about 12% of the photosensitizing activity of 27a.74-75 On the other hand, it has antibiotic activity. Furochromones have no photosensitizing properties. 

Sakamoto, A., Takeba, G., Shibata, D. Tanaka, K. (1990). Phytochrome-mediated activation of the gene for cytosolic glutamine synthetase (GSi) during inhibition of photosensitive lettuce seeds. Plant Molecular Biology 15, 317-23. 

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