Essential structural features

Polyene and polymethine dyes are two structurally related groups of dyes which contain as their essential structural feature one or more methine (-CH=) groups. Polyene dyes contain a series of conjugated double bonds, usually terminating in aliphatic or alicyclic groups. They owe their colour therefore simply to the presence of the conjugated system. In polymethine dyes, electron-donor and electron-acceptor groups terminate either end of the polymethine chain, so that they may be considered as typical donor-acceptor dyes. 

In Chapters 3-6, the commercially important chemical classes of dyes and pigments are discussed in terms of their essential structural features and the principles of their synthesis. The reader will encounter further examples of these individual chemical classes of colorants throughout Chapters 7 10 which, as a complement to the content of the earlier chapters, deal with the chemistry of their application. Chapters 7, 8 and 10 are concerned essentially with the application of dyes, whereas Chapter 9 is devoted to pigments. The distinction between these two types of colorants has been made previously in Chapter 2. Dyes are used in the coloration of a wide range of substrates, including paper, leather and plastics, but by far their most important outlet is on textiles. Textile materials are used in a wide variety of products, including clothing of all types, curtains, upholstery and carpets. This chapter deals with the chemical principles of the main application classes of dyes that may be applied to textile fibres, except for reactive dyes, which are dealt with exclusively in Chapter 8. 

The most commonly employed routes for the preparation of the / -sulfatoethylsulfone group, which is the essential structural feature of vinylsulfone reactive dyes, are illustrated in Scheme 8.5. One method of synthesis involves, initially, the reduction of an aromatic sulfonyl chloride, for example with sodium sulfite, to the corresponding sulfinic acid. Subsequent condensation with either 2-chloroethanol or ethylene oxide gives the / -hydroxyethylsulfone, which is converted into its sulfate ester by treatment with concentrated sulfuric acid at 20 30 °C. An alternative route involves treatment of an aromatic thiol with 2-chloroethanol or ethylene oxide to give the /Miydroxyethylsulfonyl compound which may then be converted by oxidation into the /Miydroxyethylsulfone. 

The porous membranes consist of a porous metal or ceramic support with porous top layers which can have different morphologies and microstructures. Their essential structural features are presented in Figures 2.1 and 2.2 and are discussed later (Section 2.2). 

We have encountered examples of simple lipid bilayers earlier. These bilayers are composed largely of amphipathic molecules. In water, they have their hydrophobic parts occupying the center of the bilayer and their hydrophilic parts occupying the bilayer surface. Such bilayers form a continuous and essential structural feature of virtually all biological membranes. We need to distinguish between that layer which faces out from the cell and is in contact with the external environment, the exoplasmic leaflet, and that which faces in and is in contact with the cellular contents, the cytoplasmic leaflet. As we shall see, these two aspects of the lipid bilayer are quite distinct. 

The reactivity of the 2,5-anhydrides of aldoses is determined by two essential structural features that do not exist in the sugars, namely, the presence of an oxolane ring and of a carbonyl group (most frequently, free) a to the ring-oxygen atom. These two characteristics make the 2,5-anhydroaldoses closer to tetrahydro-2-furaldehyde than to the aldoses, where only in exceptional cases is the carbonyl group not masked by the formation of an intramolecular, five- or six-membered, hemiacetal ring. 

This description of the behavior of polyethylenimines demonstrates that it is possible to construct synthetic polymers with traits analogous to those of enzymes. The essential structural features in the effective polymers are high local density of functional groups and hydrophobic apolar domains of submicroscopic size, all embedded in a gossamerlike mac-romolecular framework readily permeated by the aqueous solvent. Such structures can be catalytically effective under ambient conditions, that is, at room temperature and pressure in aqueous environments near physiological pH 7. 

Although there are thousands of different protein functions and millions of protein sequences, the number of basic 3D structural folds is believed not to exceed about 30 (Brandon, 1999). Nature uses the same fold over and over again about 10% of all enzymes have the a//3-barrel as an essential structural feature (Figure 11.2a). a-helices on the outside and [3-sheets on the inside form an n-fold (usually 8-fold) symmetry with a hole in the middle that extends deep into the center of the enzyme structure. 

At present, little can be said concerning the structure of villalstonine, except that it appears to be the first dimeric base in the Alstonia series. Its constituent chromophores are probably z-unsubstituted indole and dihydroindole nuclei these may be contained in tetra- and hexahydro-/3-carboline ring systems, but not even this can yet be regarded as established. The speculation that villalstonine may be regarded as composed of C19 and C21 units, the latter bearing the essential structural features of coronaridine (35), seems somewhat hasty on the basis of the published data. (See note added in proof, p. 201.)... 

The X-ray structures of the bci complexes from bovine heart, chicken heart, and the yeast Saccharomyces cerevisiae have been determined at 3.0 - 2.3 A resolution [3-6] therefore, it is possible to explore the interaction between the complex and quinones and inhibitors at molecular level. First, we will give an overview of essential structural features of the bci complex. As the non-catalytic subunits are not involved in the coordination of the redox centers or the formation of quinone binding sites, only the structure of the three eatalytic subunits will be discussed in the following section (Figure 3) for a more detailed description see [19]. In order to avoid confusion, the residue numbering of the yeast bci complex will be used even if other organisms have been studied. 

Many years ago Hopper and McKenzie (1974) noted structural similarities between equine and echidna lysozymes. They also obtained some evidence, albeit controversial, of a weak ability of echidna lysozyme to act as a modifier in the lactose synthase system. More recently, McKenzie and White (1987) noted very weak lytic activity in a variety of a-lactalbumin preparations. Also, Teahan et al. (1986, 1990) confirmed certain essential structural features for Ca(II) binding in echidna lysozymes I and II and noted the potential binding of Ca(II) by equine and pigeon lysozymes. D. C. Shaw and R. Tellam (quoted by Godovac-Zimmermann et al., 1987) made preliminary fluorometric observations that indicated binding of Ca(II) by echidna and equine lysozymes. 

Three models which have appeared recently are discussed in this section (Fig. 1). They are representative of a variety of models which have been proposed [15,16,121,124,283] and are intended to provide an illustration of current thinking on 02-evolution mechanisms. Only the essential structural features of these models are summarized in Fig. T, the details of the structural transitions involved in the S-state changes are discussed in the original publications but have been omitted for clarity. 

The initial chemistry carried out around the endothelin and sarafotoxin structures, by modification of the amino acid sequences, generated a range of peptide agonists. These compounds, with a wide range of relative potencies, have been used to probe the essential structural features of the natural pharmacologically active agents. 

Figure 13.6. (a) Linear dissolution kinetics observed for the dissolution of 6-AI2O3, representative of processes whose rates are controlled by a surface reaction and not by a transport step. (Data from Furrer and Stumm, (1986).) (b) Linear dissolution kinetics of frame silicates. Minerals used were pyroxenes and olivines their essential structural feature is the linkage of Si04 tetrahedra, laterally linked by bivalent cat-Fe -, Ca ). Plotted ate... 

A recent development in glycogen chemistry is the recognition that multiple branching is an essential structural feature. Enzymic experiments provide the only means of differentiating between singly- and multiply-branched structures, and methods for the qualitative and quantitative assessment of A. B. (degree of multiple branching) have been devised. 

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