Photosensitisers from plants

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Abstract
The absorption of solar radiation by certain phytochemicals leads to excitation to the singlet state. If unquenched, intersystem crossing occurs, and a longer‐lived triplet state arises. Triplet states may directly induce damage to biological molecules by type I reactions, or by interaction with triplet or ground state oxygen (3O2) generate singlet oxygen (1O2) in a type II reaction.1O2reacts with lipids, proteins and nucleic acids, and thus induces considerable cellular damage if unquenched. Chlorophyll is the most widespread plant photosensitiser, and damage to chloroplasts and plant cells will occur if, for example, photosynthesis is inhibited and excited chlorophyll unquenched. The endogenous formation of chlorophyll precursors from δ‐aminolaevulinic acid fed to darkened leaves has been shown to have a phytotoxic effect in subsequently illuminated plants. In animals a number of breakdown products of chlorophyll, such as phylloerythrin, promote secondary sensitisation, and a condition in sheep known as geeldikkop is induced if these compounds are not metabolised further in the liver. Hypericism is a well‐known case of primary photosensitisation in which the ingested molecule acts as the photosensitiser. The active compound hypericin is produced in the glands of variousHypericumspecies and is offensive to grazing animals and insect predators. Damage is promoted by the photosensitised generation of1O2. A related compound, fagopyrin from buckwheat, is similarly toxic, as also are a number of fungal quinones such as cercosporin. This compound is produced byCercosporaspecies and promotes a leaf‐spot disease in certain crop plants. A large number of furanocoumarins such as psoralen and xanthotoxin are particularly offensive to phytophagous insects. Phototoxicity may involve ultraviolet sensitised binding to DNA as well as type II reactions. Numerous other polyacetylenes and thiophenes are also activated by ultraviolet radiation, and have a role as plant protectants.