Color structure-based

Four anthocyanin species exist in equilibrium under acidic conditions at 25°C/ according to the scheme in Figure 4.3.3. The equilibrium constant values determine the major species and therefore the color of the solution. If the deprotonation equilibrium constant, K, is higher than the hydration constant, Kj, the equilibrium is displaced toward the colored quinonoidal base (A), and if Kj, > the equilibrium shifts toward the hemiacetalic or pseudobase form (B) that is in equilibrium with the chalcone species (C), both colorless." - Therefore, the structure of an anthocyanin is strongly dependent on the solution pH, and as a consequence so is its color stability, which is highly related to the deprotonation and hydration equilibrium reaction constant values (K and Kj,). 

Some simple azo compounds, because of restricted rotation about the ( N=N ) double bond, are capable of exhibiting geometrical isomerism. The geometrical isomerism of azobenzene, the simplest aromatic azo compound which may be considered as the parent system on which the structures of most azo colorants are based, is illustrated in Figure 3.1. The compound is only weakly coloured because it absorbs mainly in the UV region giving a 2m.lx value of 320 nm in solution in ethanol, a feature which may be attributed to the absence of auxochromes (see Chapter 2). 

Heterocycles structurally based on the phenalene ring system frequently possess distinctive colors. With nitrogen as the central atom we have the unstable 9a-azaphenalene (20), (76JCS(Pl)34i). The cyclazine nomenclature is commonly applied to this and related compounds thus, (20) is (3.3.3)cyclazine. Further aza substitution is possible, e.g. as in the heptaazaphenalene (21). Another well-known and important representative of this family is yellow colored perimidine. 

Fig. 8.9 Composite protein structure-based pharmacophore used for virtual screening. Donor features and their corresponding interaction partners in the protein are colored blue, acceptor features and their correspond ing interaction partners are colored red and the acceptor/donor feature and the corre-...

Colorant formation may, therefore, be regarded as a polymerization process, which, as it proceeds, results in a shift of the absorption to the long wave lengths (red) and in a reduction in the intensity of the absorption. Such shifts are satisfactorily explained by structures, based on the conjugated a-dicarbonyl groups, in which the general conjugation is in-... 

Figure 12 Comparison of experimentally determined RDCs and RDCs back-calculated from the crystal structure (A), an NOE-derived structural model (B) and the (NOE+RDC)-refined structure (C) of cyclosporin A. (D) Evaluation of the resulting models for the average structures show a flat geometry in the crystal, an already bent backbone for the structure based only on NOE data, and a further twist in the RDC-refined backbone, which does not violate any NOE data. (Reproduced from ref. 230 with permission from Wiley-VCH Verlag GmbH Co. KGaA (Copyright).) (See Plate 2 in Color Plate Section)...

One of the oldest known methods of producing wash-fast colors involves the use of metallic hydroxides, which form a link, or mordant (L. mordere, to bite), between the fabric and the dye. Other substances, such as tannic acid, also function as mordants. The color of the final product depends on both the dye used and the mordant. For instance, the dye Turkey Red (alizarin) is red with an aluminum mordant, violet with an iron mordant, and brownish-red with a chromium mordant. Some important mordant dyes possess a structure based on triphenylmethane, as do Crystal Violet and Malachite Green. 

Additional information may come from a variety of sources. Certainly a proposed structure based on plausible chemistry and the total molecular mass is helpful. However, one must be careful to keep in mind that proposed structures are based on preliminary data only and thus may not be consistent with subsequently collected data. If LC-NMR is to be performed, then it is essential to obtain details of the chromatographic method to be used. Other information may include color to suggest conjugation, IR absorption to detect carbonyl stretches, and relative chromatographic retention time to evaluate polarity compared to the parent and other known compounds. 

Figure 8.2 Three-dimensional NIR chemical imaging of a corn kernel, (a) False-color image based on the structures identified in the NIR imaging data set. Reprinted with permission from Ref [9] 2007, John Wiley Sons,...

Cursory examination of the NMR sample, when dissolved in DMSO, allowed the deduction that an 11-cryptolepinyl moiety was contained in the structure based on the intense purple color, which is characteristic of the extended conjugation of cryptolepine. Because of the relatively small size of the sample, 100 pg, all of the NMR spectral data were recorded using a sealed sample of the degradant isolate in 150 pi of d -DMSO in a sealed 3 mm NMR tube using a 5 mm 500 MHz gradient inverse-detection triple resonance cryogenic NMR probe. 

Table 2 Set Membership Established for Atoms in Both Query and Target Structures Based on the Selected Properties (Node Color, Degree, Edge Color, and Order, Except Connectivity), with Reference to Fig. 2.

---