OXIDATIVE-DEGRADATION OF MELANINS TO PYRROLE ACIDS - A MODEL STUDY
OXIDATIVE-DEGRADATION OF MELANINS TO PYRROLE ACIDS - A MODEL STUDY
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DOI：
10.1016/0040-4020(95)00259-b
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发表时间：
1995-05-15
期刊：
影响因子：
2.1
通讯作者：
PROTA, G
中科院分区：
文献类型：
--
作者：
NAPOLITANO, A;PEZZELLA, A;PROTA, G
利用以5，6-二羟基吲哚(DHI)和5，6-二羟基吲哚-2-羧酸(DHICA)为主要生物合成前体制备的合成色素，研究了黑色素最具特征的降解产物--吡咯-2，3，5-三羧酸(PTCA)的来源。在报道的条件下，即酸性高锰酸盐，氧化降解DHI-和DHICA-黑色素分别以0.15%和3.0%的w/w产率获得PTCA。以碱性过氧化氢为氧化剂，PTCA产率可显著提高7%。在此条件下，还得到了吡咯-2，3-二元酸(PDCA)。对一些模型吲哚低聚物(1-4)的氧化降解研究提供了明确的证据，表明PTCA可能来自颜料聚合物中2-连接的DHI-单元以及DHICA衍生单元，而PDCA则来自2-位未被取代的DHI-单元。二聚体1氧化为PTCA的监测表明，DHICA是中间体。因此，提出了黑色素聚合物中2-取代DHI-单元降解为PTCA的机理路线，这也解释了从模型低聚物中观察到的吡咯酸生成的产率。
The origin of pyrrole-2,3,5-tricarboxylic acid (PTCA), the most characteristic degradation product of melanins, was investigated by use of synthetic pigments prepared from the major biosynthetic precursors, 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Under the reported conditions, i.e. acidic permanganate, oxidative degradation of DHI- and DHICA-melanins afforded PTCA in 0.15 and 3.0% w/w yield, respectively. A significant improvement of PTCA yields up to 7% was obtained using alkaline hydrogen peroxide as the oxidising agent. Under these conditions pyrrole-2,3-dicarboxylic acid (PDCA) was also obtained in significant yields. Investigation of the oxidative degradation of some model indole oligomers (1-4) provided unambiguous evidence that PTCA may originate from 2-linked DHI-units in the pigment polymer as well as from DHICA-derived units, whereas PDCA arises from DHI-units not substituted at 2-position. Monitoring of the oxidation of dimer 1 to PTCA showed the intermediate formation of DHICA. Accordingly, a mechanistic route is proposed for the degradation of 2-substituted DHI-units in melanin polymer to PTCA which also accounts for the observed yields of formation of the pyrrole acid from the model oligomers.