The Interaction of Light with Biological Molecules

Because the absorption of light energy enables a molecule to undergo photochemistry, there is the chance that molecules will participate in undesirable reactions that may cause biological damage. There are many molecules which absorb light and cause biological damage by acting as photosensitizers (also called photodynamic agents). Examples of photosensitizers are cerosporin, a molecule produced by some phytopathogenic fungi which cause membrane damage to plant cells in the light, and fagopyrins from buckwheat leaves that cause photosensitization of grazing animals. Most photosensitizers form high yields of the triplet state and then use one of two mechanisms to produce biological damage. The first mechanism is for the excited photosensitizer to interact directly with another molecule to liberate an electron or hydrogen atom, forming a free radical. The organic free radicals can react within the aqueous cell environment to form hydrogen peroxide and superoxide. The second mechanism for biological damage can occur when energy transfers from the triplet photosensitizer to molecular oxygen (O₂), also in the triplet state, forming singlet oxygen. The superoxide, peroxide, singlet oxygen and free radical products of photosensitization reactions are able to damage many important cellular components, especially membrane lipids.