The gold and cream colors of cultured Akoya pearls, as well as natural yellow nacre of pearl oyster shells, are thought to arise from intrinsic yellow pigments. While the isolation of the yellow pigments has been attempted using a large amount of gold pearls, the substance concerned is still unknown. We report here on the purification and characterization of yellow pigments from the nacre of Akoya pearl oyster shells. Two yellow components, YC1 and YC2, were isolated from the HCl-methanol (HCl-MeOH) extract from nacreous organic matrices obtained by decalcification of the shells with ethylenediaminetetraacetic acid (EDTA). Energy-dispersive X-ray and infrared spectroscopy analyses suggested that YC1 and YC2 precipitated under basic conditions are composed of Fe-containing inorganic and polyamide-containing organic compounds, respectively. YC1 solubilized under acidic conditions exhibited positive reactions to KSCN and K₄[Fe(CN)₆] reagents, showing the same ultraviolet-visible absorption spectrum as those of Fe(III)-containing compounds. In addition, X-ray absorption fine structure analysis supported the compound in the form of Fe(III). The total amount of Fe was approximately 2.6 times higher in the yellow than white nacre, and most Fe was fractionated into the EDTA-decalcifying and HCl-MeOH extracts. These results suggest that Fe(III) coordinated to EDTA-soluble and insoluble matrix compounds are mainly associated with yellow color development not only in the Akoya pearl oyster shells but also in the cultured Akoya pearls.

中文翻译：

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三价铁负责Akoya珍珠牡蛎壳珍珠层中的黄色发展。

养殖的Akoya珍珠的金色和奶油色以及珍珠贝的天然黄色珍珠层被认为是由固有的黄色颜料产生的。尽管尝试使用大量金珍珠分离黄色颜料，但有关物质仍未知。我们在这里报告了从Akoya珍珠牡蛎壳的珍珠质中黄色颜料的纯化和表征。从通过用乙二胺四乙酸（EDTA）脱壳获得的珍珠质有机基质的HCl-甲醇（HCl-MeOH）萃取物中分离出两种黄色成分YC1和YC2。能量色散X射线和红外光谱分析表明，在碱性条件下沉淀的YC1和YC2分别由含铁的无机化合物和含聚酰胺的有机化合物组成。₄ [Fe（CN）₆ ]试剂，其紫外可见吸收光谱与含Fe（III）的化合物相同。另外，X射线吸收精细结构分析支持Fe（III）形式的化合物。黄色中的Fe总量比白色珍珠母高约2.6倍，并且大多数Fe被分馏到EDTA脱钙和HCl-MeOH提取物中。这些结果表明，与EDTA可溶和不可溶的基质化合物配位的Fe（III）不仅在Akoya珍珠贝中而且在培养的Akoya珍珠中均与黄色发展有关。