Potentially useful antimicrobial and antiviral phototoxins from plants

The wide range of naturally occurring compounds from microorganisms and plants which are phototoxic in UV-A light (320-400 nm) includes cinnamyl esters, coumarins and furanocoumarins, furanochromones, benzofurans, alkaloids, based on tryptophan or phenylalanine, extended naptho-and anthraquinones, polyacetylenes and their thiophene derivatives. The cellular targets are cell membranes, e.g., acetylenes, or the nucleus, e.g., furanocoumarins. Compounds reacting with cell membranes in light either generate singlet oxygen, e.g., alpha-terthienyl, or react by a free radical mechanism, e.g., phenylheptatriyne, or do both. Those that react with nucleic acids intercalate with these macromolecules forming photoadducts, e.g., furanocoumarins, furanochromones and furanoquinolines. With others, such as the beta-carbolines, the explanation for their photogenotoxicity is unknown. A number of these natural photosensitizers have been examined with bacteria, yeasts, and viruses and the mechanisms of phototoxicity have been elucidated. Some of the sulfur-containing acetylenes may be useful in photochemotherapy.