Quinoid pigments

As frequently mentioned in the literature, anthocyanins co-exist in equilibrium in four different forms. The pH conditions shift this equilibrium toward a variety of structural forms, with the direct consequences of color changes of these pigments. As a rule, at pH above 4, yellow compounds (chalcone form), blue compounds (quinoid base), or colorless compounds (methanol form) are produced. Anthocyanins have the highest stabilities at a pH between 1 and 2 since the flavylium cation is the most stable predominant form. 

Saito, K. and Kawasaki, H. Comparative studies on the distribution of quinoidal chalcone pigments in extracts from insect wastes and intact tissues of dyer s saffron florets, Zeits. Lebens, Unter. Forsch., 194, 131, 1992. 

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. 

A simple, fast and specific color test for urea nitrate was reported recently by Almog et al. It is based on the reaction between urea nitrate and ethanolic solution ofp-dimethylaminocinnamaldehyde (p-DMAC) (9) under neutral conditions [91]. A red pigment is formed within 1 min from contact. Its structure has also been determined by the same group, by X-ray crystallography [92]. It appears to be a resonance hybrid between a protonated Schiffbase (10) and a quinoid system (10a) (Eq. (14)). The limit of detection on filter paper is 0.1 mg/cm. Urea itself, which is the starting material for urea nitrate, does not react with p-DMAC under the same conditions. Other potential sources of false-positive response such as common fertilizers, medications containing the urea moiety and various amines, do not produce the red pigment with p-DMAC. 

Absorption spectra have also been used in the reexamination of pH-dependent color and structural transformations in aqueous solutions of some nonacylated anthocyanins and synthetic flavylium salts." ° In a recent study, the UV-Vis spectra of flower extracts of Hibiscus rosasinensis have been measured between 240 and 748 nm at pH values ranging from 1.1 to 13.0." Deconvolution of these spectra using the parallel factor analysis (PARAFAC) model permitted the study of anthocyanin systems without isolation and purification of the individual species (Figure 2.21). The model allowed identification of seven anthocyanin equilibrium forms, namely the flavylium cation, carbinol, quinoidal base, and E- and Z-chalcone and their ionized forms, as well as their relative concentrations as a function of pH. The spectral profiles recovered were in agreement with previous models of equilibrium forms reported in literature, based on studies of pure pigments. 

Commercial anthraquinoid vat dyes are dye preparations that consist of a vat-table colored pigment and a dispersing agent. Such vattablepigments are polycyclic quinoid compounds that contain two or more carbonyl groups in a closed system of conjugated double bonds (Section 3.4). 

Fig. 25. Some representative quinoid pigments from crinoids.

Thiophene, the foundation of this book, had a tricky birth. It masqueraded as benzene from 1879 to 1882, when Maeyer [1] uncovered the subterfuge. It turned out that coal tar-derived benzene, when treated with isatin and sulfuric acid, produced a beautifully deep-blue precipitate, named indophenine. This pigment was claimed by Baeyer in 1879 [2] to be a qualitative test for benzene and was the product of the indophenine reaction . The pigment s structure was eventually shown to consist of a quinoid form of bithiopene, shown below. 

Fedoreev SA, Krivoshchekova OE, Denisenko VA, Gorovoi PG et al. 1979 Quinoid pigments of far eastern representatives of the family Boraginaceae. Khim Prir Soedin 15 625-630... 

A polymorph of the quinoid red crystal form of fluorescein was one of the first examples of a complex molecule whose structure was determined by a real space approach based on the Monte-Carlo method. The same method has more recently been used to solve the structure of the (3-form of the latent pigment boc-DPP (Figure 8-6). The kinetics of the thermal fragmentation to DPP differs for both forms. The more reactive a-form crystallizes (less ordered) with three conformation-ally different half-molecules in the asymmetric unit. This structure was initially solved from single crystal data. However, it could be improved substantially by Rietveld refinement, thus demonstrating the potential of this technique . ... 

In processed fruit and vegetables, the situation is more complex. Anthocyanins in plants (pH of from 2.5 to 7.5) occur as a purplish-red neutral quinoid base, but in food products they may be in media of different pH. However, they are mostly stabilised by inter co-pigmentation (interactions with other flavonoids) or intra copigmentation (acylated forms), or by interactions with other food components. Many products therefore retain their original colour... 

Quinones represent a group of about 200 yellow, red, brown and almost black pigments with variable structure. They include simple quinones, dimers, trimers and condensation products that mutually differ in the number of hydroxyl groups and other substituents. The naturally occurring quinoid pigments are mostly derived from ... 

In the past, some quinoid pigments were used as textile and leather dyes (that were later replaced by synthetic dyes) and many plants were specially grown for this function. Some quinoid pigments are now used for cosmetic and pharmaceutical purposes and also as food additives. Compared with other pigments, natural quinoid pigments are of less importance. 

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