Free radicals from eumelanins: quantum yields and wavelength dependence.

Formation of light-induced free radicals from natural eumelanin (from bovine eyes) and synthetic melanin (from oxidation of 3,4-dihydroxyphenylalanine) has been studied by electron spin resonance spectroscopy. Action spectra measured for natural melanins are very similar to that found for synthetic melanin, and are unaffected by the removal of associated protein. A comparison of action spectra with optical absorbance spectra shows that the former has a more marked wavelength dependence, suggesting that the chromophore that is most active in free-radical production is not the major melanin chromophore that absorbs visible light. Measurements of quantum yields for free-radical production have been made over a wavelength range from 600 to 230 nm. The efficiency of radical production from natural eumelanin is about three times greater than from the synthetic material. Although production of the melanin radicals detected is independent of oxygen, some correlation with oxygen consumption is evident; quantum yields for radical production are approximately three times those for oxygen consumption obtained under similar conditions. Possible reasons for this are discussed.