Department of Microbiology and Immunology, Division of Infectious Diseases, Albert Einstein College of Medicine, Bronx, New York, United States of America.
PLoS One. 2012;7(2):e30299. doi: 10.1371/journal.pone.0030299. Epub 2012 Feb 16.
Melanins are notoriously difficult to study because they are amorphous, insoluble and often associated with other biological materials. Consequently, there is a dearth of structural techniques to study this enigmatic pigment. Current models of melanin structure envision the stacking of planar structures. X ray diffraction has historically been used to deduce stacking parameters. In this study we used X ray diffraction to analyze melanins derived from Cryptococcus neoformans, Aspergillus niger, Wangiella dermatitides and Coprinus comatus. Analysis of melanin in melanized C. neoformans encapsulated cells was precluded by the fortuitous finding that the capsular polysaccharide had a diffraction spectrum that was similar to that of isolated melanin. The capsular polysaccharide spectrum was dominated by a broad non-Bragg feature consistent with origin from a repeating structural motif that may arise from inter-molecular interactions and/or possibly gel organization. Hence, we isolated melanin from each fungal species and compared diffraction parameters. The results show that the inferred stacking distances of fungal melanins differ from that reported for synthetic melanin and neuromelanin, occupying intermediate position between these other melanins. These results suggest that all melanins have a fundamental diffracting unit composed of planar graphitic assemblies that can differ in stacking distance. The stacking peak appears to be a distinguishing universal feature of melanins that may be of use in characterizing these enigmatic pigments.
黑色素因其无定形、不溶和常与其他生物材料相关联而难以研究。因此，用于研究这种神秘色素的结构技术非常匮乏。目前的黑色素结构模型设想了平面结构的堆叠。X 射线衍射在历史上被用于推断堆叠参数。在这项研究中，我们使用 X 射线衍射分析了来自新生隐球菌、黑曲霉、皮炎外瓶霉和毛头鬼伞的黑色素。对黑色素化新生隐球菌包裹细胞中的黑色素进行分析时，偶然发现囊多糖的衍射光谱与分离的黑色素相似，这一发现排除了这种可能性。囊多糖光谱主要由一个宽的非布拉格特征主导，这与来自重复结构基序的起源一致，这种基序可能来自分子间相互作用和/或凝胶组织。因此，我们从每种真菌物种中分离黑色素并比较衍射参数。结果表明，真菌黑色素的推断堆叠距离与合成黑色素和神经黑色素的报道不同，在这些其他黑色素之间占据中间位置。这些结果表明，所有黑色素都具有由平面石墨组装组成的基本衍射单元，其堆叠距离可以不同。堆叠峰似乎是黑色素的一个独特的普遍特征，可用于表征这些神秘的色素。
