Gallas J M, Littrell K C, Seifert S, Zajac G W, Thiyagarajan P
Division of Earth and Physical Sciences, University of Texas at San Antonio, San Antonio, Texas 78249, USA.
Biophys J. 1999 Aug;77(2):1135-42. doi: 10.1016/S0006-3495(99)76964-X.
Melanin, the ubiquitous biological pigment, provides photoprotection by efficient filtration of light and also by its antioxidant behavior. In solutions of synthetic melanin, both optical and antioxidant behavior are affected by the aggregation states of melanin. We have utilized small-angle x-ray and neutron scattering to determine the molecular dimensions of synthetic tyrosine melanin in its unaggregated state in D(2)O and H(2)O to study the structure of melanin aggregates formed in the presence of copper ions at various copper-to-melanin molar ratios. In the absence of copper ions, or at low copper ion concentrations, tyrosine melanin is present in solution as a sheet-like particle with a mean thickness of 12.5 A and a lateral extent of approximately 54 A. At a copper-to-melanin molar ratio of 0.6, melanin aggregates to form long, rod-like structures with a radius of 32 A. At a higher copper ion concentration, with a copper-to-melanin ratio of 1.0, these rod-like structures further aggregate, forming sheet-like structures with a mean thickness of 51 A. A change in the charge of the ionizable groups induced by the addition of copper ions is proposed to account for part of the aggregation. The data also support a model for the copper-induced aggregation of melanin driven by pi stacking assisted by peripheral Cu(2+) complexation. The relationship between our results and a previous hypothesis for reduced cellular damage from bound-to-melanin redox metal ions is also discussed.
黑色素，这种普遍存在的生物色素，通过有效过滤光线及其抗氧化行为提供光保护作用。在合成黑色素溶液中，光学和抗氧化行为均受黑色素聚集状态的影响。我们利用小角X射线和中子散射来确定合成酪氨酸黑色素在D(2)O和H(2)O中未聚集状态下的分子尺寸，以研究在不同铜与黑色素摩尔比下铜离子存在时形成的黑色素聚集体的结构。在没有铜离子或铜离子浓度较低时，酪氨酸黑色素以片状颗粒形式存在于溶液中，平均厚度为12.5 Å，横向尺寸约为54 Å。当铜与黑色素的摩尔比为0.6时，黑色素聚集形成半径为32 Å的长棒状结构。在较高铜离子浓度下，铜与黑色素的比例为1.0时，这些棒状结构进一步聚集，形成平均厚度为51 Å的片状结构。有人提出，添加铜离子引起的可电离基团电荷变化是聚集的部分原因。数据还支持一个由π堆积辅助外周Cu(2+)络合驱动的铜诱导黑色素聚集模型。我们还讨论了我们的结果与先前关于与黑色素结合的氧化还原金属离子减少细胞损伤的假设之间的关系。
