Quinones pigments

Although they are pigments, quinones make only small contributions to the colors of tissues and organisms that produce them. Quinones play an important role in the coloration of some fungi, lichens, insects (Coccidae), and echinoderms, but they rarely contribute to the external colors of higher plants. 

Enzymatic browning reactions, which have been known for almost 100 years, are complex enzyme catalysed reactions involving the oxidation of phenohc compounds by some oxidoreductases in the presence of air oxygen. Oxidation products are quinones that are transformed by subsequent enzymatic and spontaneous (non-enzymatic) reactions into various coloured pigments. Quinones and other reaction products in plants have microbistatic effects and are able to prevent the spread of microbial and viral infections. Polymeric insoluble reaction products are an effective mechanical barrier against further erosion at the injury site of a plant. 

Most vat dyes are based on the quinone stmcture and are solubilized by reduction with alkaline reducing agents such as sodium dithionite. Conversion back to the insoluble pigment is achieved by oxidation. The dyes are appHed by either exhaust or continuous dyeing techniques. In both cases the process is comprised of five stages preparation of the dispersion, reduction, dye exhaustion, oxidation, and soaping. 

In order that the rate of cure of phenolic moulding compositions is sufficiently rapid to be economically attractive, curing is carried out at a temperature which leads to the formation of quinone methides and their derivatives which impart a dark colour to the resin. Thus the range of pigments available is limited to blacks, browns and relatively dark blues, greens, reds and oranges. 

Quinones are colored p-benzoquinone, for exanple, is yellow. Many occur naturally and have been used as dyes. Alizarin is a red pigment extracted from the roots of the madder plant. Its preparation from anthracene, a coal tar derivative, in 1868 was a significant step in the development of the synthetic dyestuff industry. 

Light absorption of quinones depends on their skeletons and is also strongly influenced by the presence of various substituents. The absorption spectra of 1.4-benzoquinone presents an intense absorption band (band I) at 240 nm, a medium-intensity absorption band (band II) around 285 nm, and a weak absorption band (band III) at 434 nm. Due to their weak absorption levels in the visible region, unsubstituted benzoquinones are not important as pigments. 

Chapter 13 is devoted to the PLC of namral pigments, which encompass fla-vonoids, anthocyanins, carotenoids, chlorophylls and chlorophyll derivatives, porphyrins, quinones, and betalains. Chromatography of pigments is especially difficult because many are photo- and air-sensitive and can degrade rapidly unless precautions are taken. 

Despite the importance of the oxidative polymerization of 5,6-dihydroxyin-dole, in the biosynthesis of pigments, little experimental data are known on the oxidation chemistry of the oligomers of 1. For such reasons, three major dimers of 1, such as 2-4 (Scheme 2.9), have been computationally investigated at PBEO/ 6-31+G(d,p) level of theory both in gas and in aqueous solution (by PCM solvation model) to clarify the quinone methide/o-quinone tautomeric distribution. 

The chemistry of quinone dyes has been discussed in a series of books entitled The Chemistry of Synthetic Dyes, edited by Venkataraman.1 The general chemistry of quinoid compounds has been discussed by Patai.2 There have been many books that cover quinoid compounds as dyes and pigments but very few discuss the chemistry of the corresponding leuco dyes. Traditional vat dyes are applied to cellulosic fiber in the leuco form. The chemistry of the leuco form of vat dyes is rather simple. Some leuco quinones are quite stable in the solid state and can be stored for a year. Other leuco dyes are unstable in solution and gradually undergo aerial oxidation. 

Drosophila Ddc is expressed primarily in the CNS and the hypoderm, the epithelial layer of the fly that secretes the cuticle. In the CNS, Ddc is expressed in a small subset of neurons that produce either dopamine or serotonin (Budnik and White, 1988 Valles and White, 1988). In the hypoderm, Ddc expression leads to synthesis of dopamine, which is further metabolized into quinones that have a vital function in the cross-linking, hardening, and pigmentation of the fly cuticle (Wright, 1987). The developmental profile of DDC activity in these two tissues is quite different (Hirsh, 1986). DDC is first detected during late embryo-... 

The flow of electrons occurs in a similar manner from the excited pigment to cytochromes, quinones, pheophytins, ferridoxins, etc. The ATP synthase in the mitochondria of a bacterial system resembles that of the chloroplast—chloroplast proton translocating ATP synthase [37]. 

Although treated as separate classes in the Colour Index, these structural types are closely related and the few diphenylmethane dyes such as auramine (1.28 Cl Basic Yellow 2) are now of little practical interest. Commercial usage of the triarylmethane dyes and pigments has also declined considerably in favour of the major chemical classes. They were formerly noteworthy contributors to the acid, basic, mordant and solvent ranges, primarily in the violet, blue and green sectors. Numerous structural examples are recorded in the Colour Index. The terminal groupings can be amine/quinonimine, as in auramine and crystal violet (1.29 Cl Basic Violet 3), hydroxy/quinone, or both. The aryl nuclei are not always benzenoid (section 6.5). 

The BASF route started from hydroquinone, which was converted to 2,5-dihydroterephthalic acid by a Kolbe-Schmitt reaction. One mole of this acid was treated with two moles of an arylamine, both components being in the form of a suspension in aqueous methanol. This was added to a small amount of a solution of vanadium(III) chloride and sodium chlorate. Gentle heating gave a 95% yield of 2,5-bis(arylamino)benzo-l,4-quinone-3,6-dicarboxylic acid. Ring closure to the trans-quinacridonequinone took place in the presence of concentrated sulphuric acid at 60-80 °C. This was then reduced to the required crude pigment by zinc or aluminium powder in caustic soda under pressure,in an aluminium chloride/urea melt or by the use of a sulphuric acid/polyphosphoric acid mixture. 

Both molecules favor the o-quinone-hydrazone form over the o-hydroxyazo form. Corresponding studies on a selection of monoazo pigments and several (i-naphthol and Naphthol AS pigments are described in the respective chapters. 

P.R.206 is a mixed crystal type and consists of unsubstituted quinacridone and quinacridone quinone. The ratio between the two components as well as the crystal modification is not yet known. P.R.206 affords a very dull, yellowish shade of red, referred to as maroon. The pigment is considerably weaker than perylene pigments. All commercially available types of P.R.206 are more or less transparent and are used mostly in metallic finishes for automobiles, to which they lend reddish shades of copper. The pigment is often found to be difficult to disperse. The finishes frequently exhibit rheological problems, especially at high pigment concentration. 

The exact physical properties of P.O.49, i.e., mainly the crystal modification of this quinacridone/quinacridone quinone pigment remain to be published. P.O.49, like P.R.209, is a specialty product for metallic shades. It is used to produce shades of gold in finishes, which are considerably more yellowish than those of P.O.48. 

This class includes polycarbocyclic compounds which are at least formally derived from the anthraquinone structure. The products are considered members of the higher condensed carbocyclic quinone series, which even in the absence of additional substituents provide yellow to red shades. Halogenation is frequently found to afford cleaner shades and improved fastness properties. Heading the list of such derivatives are pyranthrone, anthanthrone, and isoviolanthrone pigments. 

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