Polymeric pigment formation

DHA can be reduced to RAA by chemical agents, such as hydrogen sulfide or enzymatically, by dehydroascorbic acid reductase. The conversion of DHA to diketogulonic acid (DKG) is irreversible and occurs both aerobically and anaerobically, particularly during heating. This reaction results in loss of biological activity. The total oxidation of RAA may result in the formation of furfural by decarboxylation and dehydration. With subsequent polymerization, the formation of dark-colored pigments results. These compounds affect the color and flavor of certain foods, such as citrus juices, and decrease nutritive value. 

Auto-oxidation of catecholamines also leads to formation of the polymeric pigment neuromelanin that increases with age and is responsible for the dark coloration of DA-pro-ducing cells in the substantia nigra and the norepinephrine neurons of the locus coeruleus (42). A physiological role for this polymer is not established, and there is no evidence to support its involvement either in normal loss of DA neurons with aging or in the pathophysiology of PD. 

Pigment formation can be used to visually detect mutants constitutive for enzymes of the purine pathway (Dorfman, 1969). The technique uses an auxotroph blocked early in the pathway which forms red colonies because of the polymerization into a red pigment of accumulated S-amino-4-imidazole ribonucleotide. Normally, high levels of adenine in the medium inhibit pigment formation because AMP represses the pathway. After mutation of the auxotroph, the population is plated on agar containing adenine derepressed mutant colonies are red. 

Although not generally described as such, certain reactions of oxidation hair dyes are examples of in situ polymerizations in hair. These consist of the oxidation of electron-rich aromatic amine and phenol monomers that condense with each other and perhaps even attach to amino acid residues of hair. The net result, at least with products containing p-phenylenediamine (PPD) and resorcinol, is the formation of polyindophenol-type polymeric pigments [81-83] that render color to the hair. (See the discussion on oxidation hair dyes in Chapter 6 and its references for additional details.)... 

Biosynthetic studies provided evidence that pigment formation involves the oxidative polymerization of cysteinyldopas of type 55CD (5-S-cysteinyldopa) via... 

FIGURE 35.14 Syringic acid is released from the flavyUum structure of malvidin-3-glucoside in the polymerized pigment by alkaline fusion throngh the formation of a hydrated chemical form in which the pyran (C ring) is broken in two steps ([124] figure reproduced with kind permission). 

Important reactions leadingto the formation ofbrown polymeric pigments are reactions of quinones with thiol and amino groups... 

Melanomas are among the deadliest forms of cancer as they have a high recurrence but as yet no effective chemotherapy.(7, 2) The drug resistance of melanoma has been attributed to the presence of melanin, a redox-active polymeric pigment formed from the oxidation of tyrosine within cells.(5) The formation of melanin itself depends on fine control of oxidative chemistry a peroxide-dependent enzyme, tyrosinase, catalyzes two successive reactions, the hydroxylation of tyrosine and die oxidation of the product L-dopa, Scheme 1.(4) The product of dopa oxidation cyclizes to a 5,6-dihydroxyindole (DHI) intermediate, which is highly reactive and gives rise to black eumelanin polymers by a pathway dependent on further oxidation by oxygen.(5)... 

Lead ll) oxide, PbO, exists in two forms as orange-red litharge and yellow massicot. Made by oxidation of Pb followed by rapid cooling (to avoid formation of Pb304). Used in accumulators and also in ceramics, pigments and insecticides. A normal hydroxide is not known but hydrolysis of lead(II) oxyacid salts gives polymeric cationic species, e.g. [Pb OfOH) ] and plumbates are formed with excess base. 

Acetal formation, microwaves in, 76 557 Acetalization, of PVA, 25 602-603 Acetal polymerization, 74 271 Acetal resins, 70 183-185 Acetal resins, formaldehyde in, 72 122 Acetals, 2 64 70 529 aroma chemicals, 3 253 inorganic pigment applications, 7 372t organic pigment applications, 7 368t typical soluble dye applications, 7 376t Acetaminophen, 4 701. See also AT-Acetyl-p-aminophenol (acetaminophen)... 

The chemistry is both wide ranging and interesting. It involves carbohydrate chemistry, the chemistry of inorganic pigments, organic resins —both natural and synthetic—and many other organic and polymeric additives. The sheet formation process also involves a considerable amount of colloid and surface chemistry. Polymer chemistry and environmental and analytical chemistry also play an important part. 

PPO catalyses the dependent oxidation of phenolics to quinones. The secondary reactions of quinones lead to the formation of polymeric brown or black pigments, which are responsible for significant post-harvest losses of fruits and vegetables [72]. Finally, induced PPO activity consists of both systemic and localized components. Systemic induction of PPO in tissues in response to all types of injuries may represent a broad, defensive role for PPO in protection of juvenile tissues from subsequent attack by a broad spectrum of pathogens and pests [71]. 

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-... 

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