Pigments indole

Biochemistry and pharmacology of melanine pigments, indole derivatives 99MI14. 

Halogenated pyrroles can serve as the aryl halide in Stille couplings with organotin reagents. Scott has used this idea to prepare a series of 3-vinylpyrroles, which are important building blocks for the synthesis of vinyl-porphyrins, bile pigments, and indoles [77]. Although 3-chloro-and 3-bromopyrroles fail completely or fared poorly in this chemistry, 3-iodopyrroles 101 work extremely well to yield 3-vinylpyrroles 102. 

MEKC has been applied for the study of the effect of Monascus pigments on the decomposition of the mutagenic 3-hydroxyamino-l-methyl-5H-pyrido[4,3-b]indole (Trp-P-2(NHOH)). The chemical structures of yellow and red pigments are shown in Fig. 2.165. 

T. Watanabe, T.K. Mazumder, A. Yamamoto, S. Nagai, S.Arimoto-Kobayashi, H. Hayatsu and S. Terabe, A simple and rapid method for analyzing the Monascus pigment-mediated degradation of mutagenic 3-hydroxyamino-l-methyl-5H-pyrido[4,3-b]indole by in-capillary micellar electrokinetic chromatography. Mutat. Res. 444 (1999) 75-83. 

An additional impetus for pyrrole and indole research derives from the recognition of the physiological significance of these ring systems. In the case of pyrroles the early work centered around hemin from hemoglobin and the bile pigments. The structure of chlorophyll was also shown to be pyrrole-derived. The biosynthetic connection between simple pyrroles and the macrocyclic hemin and chlorin have remained of interest up to the present (75ACR201). 

The phenol-oxidizing enzyme tyrosinase has two types of activity (/) phenol o-hydroxylase (cresolase) activity, whereby a monophenol is converted into an o-diphenol via the incorporation of oxygen, and (2) cathecholase activity, whereby the diphenol is oxidized. The two reactions are illustrated in Figure 2-6, in the conversion of tyrosine (2.40) to L-DOPA (3,4-dihydroxyphenylalanine (2.41), dopaquinone (2.42), and indole-5,6-quinone carboxylate (2.43), which is further converted to the brown pigment... 

Catechol melanin, a black pigment of plants, is a polymeric product formed by the oxidative polymerization of catechol. The formation route of catechol melanin (Eq. 5) is described as follows [33-37] At first, 3-(3, 4 -dihydroxyphe-nyl)-L-alanine (DOPA) is derived from tyrosine. It is oxidized to dopaquinone and forms dopachrome. 5,6-Dihydroxyindole is formed, accompanied by the elimination of C02. The oxidative coupling polymerization produces a melanin polymer whose primary structure contains 4,7-conjugated indole units, which exist as a three-dimensional irregular polymer similar to lignin. Multistep oxidation reactions and coupling reactions in the formation of catechol melanin are catalyzed by a copper enzyme such as tyrosinase. Tyrosinase is an oxidase con-... 

Asterriquinone, the pigment isolated earlier from Aspergillus terreus, proves to be one of eleven closely related pigments, based on a bisindolyl-p-benzoquinone skeleton substituted at positions 1, 2 or 7 of the indole rings by isoprene or... 

A conducting, polymeric film of poly(indole-5-carboxylic acid) has been employed for covalent immobilization of tyrosinase, which retains catalytic activity and catalyzes oxidation of catechol to the quinone <2006MI41>. Poly(l-vinylpyrrole), polyfl-vinylindole), and some methyl-substituted compounds of poly(l-vinylindole) are of potential interest as photorefractive materials with a relatively low glass-transition temperature and requiring a lower quantity of plasticizer in the final photorefractive blend <2001MI253>. Polymers of 5,6-dihydroxyindoles fall within the peculiar class of pigments known as eumelanins and their chemistry has been reviewed <2005AHC(89)1>. 

The position of the sugar residue in the dihydroindole portion can be determined by methylation of the pigment with diazomethane and subsequent alkaline degradation of the resulting neobetanin derivative. In the case of 5-0-glycosides, 5-hydroxy-6-methoxyindole-2-carboxylic acid is obtained, whereas the 6-O-glyco-sides yield 6-hydroxy-5-methoxyindole-2-carboxylic acid (47,48). The indole carboxylic acids are easily identified by TIjC. 

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