Institut de Chimie Radicalaire, Spectrométries Appliquées à la Chimie Structurale, CNRS, UMR 7273, Aix-Marseille University, 13397 Marseille, France.
Chemistry. 2012 Aug 20;18(34):10689-700. doi: 10.1002/chem.201200277. Epub 2012 Jul 12.
Melanin is the most widespread pigment in the animal kingdom. Despite its importance, its detailed structure and overall molecular architecture remain elusive. Both eumelanin (black) and pheomelanin (red) occur in the human body. These two melanin compounds show very different responses to UV-radiation exposure, which could relate to their microscopic features. Herein, the structural properties and motional behavior of natural eu- and pheomelanin extracted from black and red human hair are investigated by means of solid-state NMR spectroscopy. Several 1D and 2D NMR spectroscopic techniques were combined to highlight the differences between the two forms of the pigment. The quantitative analysis of the (1) H NMR wide-line spectra extracted from 2D (1) H-(13) C LG-WISE experiments revealed the presence of two dynamically distinguishable components in both forms. Remarkably, the more mobile fraction of the pigment showed a higher mobility with respect to the proteinaceous components that coexist in the melanosome, which is particularly evident for the red pigment. An explanation of the observed effects takes into account the different architecture of the proteinaceous matrix that constitutes the physical substrate onto which melanin polymerizes within the eu- and pheomelanosomes. Further insight into the molecular structure of the more mobile fraction of pheomelanin was also obtained by means of the analysis of 2D (1) H-(13) C INEPT experiments. Our view is that not only structural features inherent in the pure pigment, but also the role of the matrix structure in defining the overall melanin supramolecular arrangement and the resulting dynamic behavior of the two melanin compounds should be taken into account to explain their functions. The reported results could pave a new way toward the explanation of the molecular origin of the differences in the photoprotection activity displayed by black and red melanin pigments.
黑色素是动物界分布最广泛的色素。尽管黑色素非常重要，但其详细结构和整体分子结构仍然难以捉摸。真黑色素（黑色）和褐黑色素（红色）都存在于人体中。这两种黑色素化合物对紫外线辐射的反应非常不同，这可能与其微观特征有关。在此，通过固态 NMR 光谱研究了从黑、红发中提取的天然真黑色素和褐黑色素的结构特性和动态行为。结合使用了几种 1D 和 2D NMR 光谱技术，以突出两种色素之间的差异。从 2D (1) H-(13) C LG-WISE 实验中提取的 (1) H 宽线谱的定量分析表明，这两种形式的色素都存在两种可动态区分的成分。值得注意的是，色素中更具流动性的部分相对于共存于黑素体中的蛋白质成分具有更高的流动性，对于红色色素尤其如此。对观察到的效应的解释考虑了构成真黑素体和褐黑素体中黑色素聚合的物理基质的蛋白质基质的不同结构。通过分析 2D (1) H-(13) C INEPT 实验，还进一步了解了褐黑色素更具流动性部分的分子结构。我们的观点是，不仅要考虑到纯色素固有的结构特征，还要考虑到基质结构在定义黑色素超分子排列的整体结构和两种黑色素化合物的最终动态行为方面的作用，以解释它们的功能。所报道的结果可能为解释黑、红两种黑色素在光保护活性方面的差异的分子起源开辟了一条新的途径。
