| Melanin is a pigment that is ubiquitous in the biological world. In addition to its obvious function of adornment, melanin is believed to serve as a metal reservoir and sunscreen. This work examined the metal binding behavior, the chemical structures and the interaction with light of natural melanins isolated from several different sources.;Sepia eumelanin was used as a model to study the metal binding behavior of eumelanins. Infrared spectroscopic analysis suggests that Mg(II), Ca(II) and Zn(II) are mainly bound to carboxyl groups and Cu(II) binds to hydroxyl groups at pH ∼4. The effect of metal content on the aerobic reactivity was examined by analyzing the ability of melanin to nick DNA. Of the metals studied, Cu(II)- and Fe(III)-loaded melanin showed the most damaging effects to DNA.;The photoreactivity of melanin is most important in two parts of the body: the skin and the eye. We first studied melanosomes from hair, which correlate well with the pigment in the skin. Chemical analysis showed that eumelanin was the major pigment in the melanosomes in black hair, while the melanosomes from red hair contained both pheomelamn and eumelanin. Photoelectron emission microscopic (PEEM) analyses on the two melanosomes revealed that the photoionization threshold of eumelanin was 4.4 eV and that pheomelanin was 3.8 eV, respectively. The low threshold of pheomelanin may contribute to the high incidence of skin cancer in redheads.;We then examined the physical and chemical properties of bovine ocular melanosomes. The collective data showed that iris melanosomes had a higher degree of crosslinking and conjugation than choroid and RPE melanosomes of the same age group. The content of carboxyl groups and the degree of conjugation in melanosomes were smaller in older samples. These tissue and age-related differences are likely to result in variation of efficiency in melanin functions such as metal binding and light absorption.;Finally, we examined the photoionization properties of human RPE melanosomes and lipofuscin, another important pigment in the RPE. RPE melanosomes became more pro-oxidant with aging and lipofuscins were more pro-oxidant than melanosomes. |
