It is believed that while eumelanin plays photoprotective and antioxidant role in pigmented tissues, pheomelanin being more photoreactive could behave as a phototoxic agent. Although the metal ion-sequestering ability of melanin might be protective, transition metal ions present in natural melanins could affect their physicochemical properties. The aim of this research was to study iron binding by pheomelanin and analyze how such a binding affects selected properties of the melanin. Synthetic pheomelanin (CDM), prepared by enzymatic oxidation of DOPA in the presence of cysteine was analyzed by electron paramagnetic resonance (EPR) spectroscopy, spectrophotometry, chemical analysis, and time-resolved measurements of singlet oxygen phosphorescence. Iron broadened EPR signal of melanin and increased its optical absorption. Iron bound to melanin exhibited EPR signal at g = 4.3, typical for high-spin iron (III). Iron bound to melanin significantly altered the kinetics of melanin photodegradation, which in turn modified the accessibility and stability of the melanin–iron complexes as indicated by the release of iron from melanin induced by diethylenetriaminepentaacetic acid and KCN. Although bound to melanin iron little affects initial stages of photodegradation of CDM, the effect of iron becomes more pronounced at later stages of melanin photolysis.

中文翻译：

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铁对合成褐黑素选择特性的影响。

据信，虽然真黑色素在色素组织中发挥光保护和抗氧化作用，但更具光反应性的褐黑素可能表现为光毒剂。尽管黑色素的金属离子螯合能力可能具有保护作用，但天然黑色素中存在的过渡金属离子会影响其理化性质。本研究的目的是研究褐黑素对铁的结合，并分析这种结合如何影响黑色素的选定特性。通过电子顺磁共振 (EPR) 光谱、分光光度法、化学分析和单线态氧磷光的时间分辨测量分析了在半胱氨酸存在下通过酶促氧化多巴制备的合成褐黑素 (CDM)。铁拓宽了黑色素的 EPR 信号并增加了其光吸收。g  = 4.3，高自旋铁 (III) 的典型值。与黑色素结合的铁显着改变了黑色素光降解的动力学，这反过来又改变了黑色素-铁复合物的可及性和稳定性，如二亚乙基三胺五乙酸和 KCN 诱导黑色素释放铁所表明的。虽然与黑色素铁结合对 CDM 光降解的初始阶段影响很小，但在黑色素光解的后期阶段，铁的作用变得更加明显。