Department of Chemical Sciences, University of Naples Federico II, Napoli (NA), Italy.
Biomacromolecules. 2012 Aug 13;13(8):2379-90. doi: 10.1021/bm3006159. Epub 2012 Jul 17.
Establishing structure-property relationships in the black insoluble eumelanins, the key determinants of human pigmentation and skin photoprotective system, is a considerable conceptual and experimental challenge in the current drive for elucidation of the biological roles of these biopolymers and their application as advanced materials for organoelectronics. Herein, we report a new breakthrough toward this goal by the first detailed investigation on the nanoscale level of the oxidative polymerization of 5,6-dihydroxyindole (DHI), a model process of eumelanin synthesis. On the basis of a combined use of spectrophotometry, dynamic light scattering (DLS), and small-angle neutron scattering (SANS) investigations, it was possible to unveil the dynamics of the aggregation process before precipitation, the key relationships with visible light absorption and the shape of fundamental aggregates. The results indicated a polymerization mechanism of the type: Polymer(n) + DHI(x) = Polymer(n+x), where DHI(x) indicates monomer, dimer, or low oligomers (x ≤ 5). During polymerization, visible absorption increases rapidly, reaching a plateau. Particle growth proceeds slowly, with formation of 2-D structures ~55 nm thick, until precipitation occurs, that is, when large aggregates with a maximum hydrodynamic radius (R(h)) of ~1200 nm are formed. Notably, markedly smaller R(h) values, up to ~110 nm, were determined in the presence of poly(vinyl alcohol) (PVA) that was shown to be an efficient aggregation-preventing agent for polymerizing DHI ensuring water solubilization. Finally, it is shown that DHI monomer can be efficiently and partially irreversibly depleted from aqueous solutions by the addition of eumelanin suspensions. This behavior is suggested to reflect oxidant-independent competing pathways of polymer synthesis and buildup via monomer conversion on the active aggregate surface contributing to particle growth. Besides filling crucial gaps in DHI polymerization, these results support the attractive hypothesis that eumelanins may behave as a peculiar example of living biopolymers. The potential of PVA as a powerful tool for solution chemistry-based investigations of eumelanin supramolecular organization and for technological manipulation purposes is underscored.
建立黑色不溶性真黑色素的结构-性质关系，这些真黑色素是人类色素沉着和皮肤光保护系统的关键决定因素，这是当前阐明这些生物聚合物的生物学作用及其作为有机电子学先进材料应用的一个相当大的概念和实验挑战。在此，我们通过对真黑色素合成模型过程 5,6-二羟基吲哚（DHI）的氧化聚合的首次详细的纳米级研究取得了这一目标的新突破。基于分光光度法、动态光散射（DLS）和小角中子散射（SANS）研究的结合，有可能揭示沉淀前的聚合过程动力学、与可见光吸收和基本聚集体形状的关键关系。结果表明聚合机制为：Polymer(n) + DHI(x) = Polymer(n+x)，其中 DHI(x) 表示单体、二聚体或低聚物（x ≤ 5）。聚合过程中，可见光吸收迅速增加，达到一个平台。颗粒生长缓慢，形成约 55nm 厚的 2-D 结构，直到沉淀发生，即形成最大水动力半径（R(h)）约为 1200nm 的大聚集体。值得注意的是，在存在聚乙烯醇（PVA）的情况下，确定了明显较小的 R(h) 值，低至约 110nm，表明 PVA 是聚合 DHI 的有效聚集体防止剂，确保聚合物在水中的溶解。最后，证明通过添加真黑色素悬浮液可以有效地将 DHI 单体从水溶液中部分不可逆地耗尽。这种行为被认为反映了氧化还原剂独立的聚合物合成和通过单体在活性聚集体表面的转化来构建的竞争途径，有助于颗粒生长。这些结果除了填补 DHI 聚合中的关键空白外，还支持了一个有吸引力的假设，即真黑色素可能表现为一种特殊的活生物聚合物的例子。强调了 PVA 作为基于溶液化学的真黑色素超分子组织研究和技术处理目的的有力工具的潜力。
