| The pigment melanin, found in the body's skin, hair, and eyes, protects us from photosensitized chemical reactions by quickly converting light energy into heat---so quickly, it is generally believed, as to prevent intersystem crossing into excited triplet states. Melanin does, however, contribute to photochemical reactions, including the generation of semiquinone-like radicals and, in the presence of oxygen, the production of cytotoxic superoxide anion and hydrogen peroxide. We present X-band time-resolved electron paramagnetic resonance (TREPR) data of eumelanins and pheomelanins from various sources on the timescale of tens of nanoseconds to hundreds of microseconds. In eumelanins we find a largely symmetric low-field absorption, high-field emission (A/E) signal which rises and falls in the first microsecond and has a radical pair dipolar splitting indicative of a spin-correlated radical pair at distances within the window of 17.5 and 30 angstroms. Thereafter emerges a second, mostly emissive, signal. The early signal is presumed to come from an excited singlet precursor while the latter seems to come from an excited triplet state. This later signal changes in time in a way that indicates that the distance between the radical pairs increases in a manner which can loosely be termed diffusive, an idea supported by the presence F-pair polarization at late time. TREPR spectra of pheomelanins, in the presence of zinc ions, give a wholly emissive signal suggestive of non-interacting radicals from a triplet precursor, though that signal becomes A/E upon zinc removal with EDTA. Possible mechanisms for this phenomenon are discussed, as are alternative explanations for emissive polarization in both types of melanins which would not require excited triplets. |
