Physical and photochemical properties of ocular melanin

Recent advances in the current state of knowledge about the properties of physiological and synthetic melanin are reviewed in the context of that pigment's optical properties, the physical structure that confers some of its unusual properties, its supportive role in the visual process (by absorbing excess light and reducing intraocular light scatter), its role in some (but not all) laser interactions with ocular tissue, its protective properties (by absorbing potentially phototoxic short-wavelength visible light), and its photoinducible free radical properties. The ability of melanin to form a long-lived radical during visible light irradiation may serve as a protective mechanism against light damage by transforming optical energy into chemical potential energy that can be dissipated in a chain of coupled redox reactions. If a cellular event such as antioxidant depletion occurs that disrupts this chain, however, the melanin radical may promote photo-oxidative damage in ocular tissue. Thus melanin may play two opposing roles in the eye: one protective and the other potentially damaging.