Complex carbohydrates. See

Gums are complex carbohydrates exuded from plants, or produced by the decomposition of vegetable matter, that have been used since remote times as adhesives, sizes, and binders (see Table 73). Most gums are tasteless and odorless solids that either dissolve or swell in water to form adhesive, viscous mucilages. When the water evaporates from a mucilage, the... 

Heteropolysaccharides are complex carbohydrates composed of more than one kind of simple sugar. See Table 22.4 for the structure of the acyl functional group. 

Tentative localization of acetyl groups at the 0-4 atoms of sialic acids in native complex carbohydrates is possible with the aid of siali-dases, which are almost, or completely, inactive with these compounds or, after isolation of the sialic acids, by acylneuraminate pyruvate-lyase, which also shows little activity with these substrates (see Section VI). 

The substantial progress made in synthesis of the complex carbohydrates occurring in medicinally important molecules68-72 is largely due to the discovery of new oxidative procedures that permit ready preparation of aldosuloses. Branched-chain sugars were obtained by nucleophilic additions to various lcetopentoses and ketohexoses subsequent condensation with purines and pyrimidines then afforded the desired natural, or synthetic, antibiotics (see, for example, Refs. 19 and 73). 

In alcoholic solution, carbohydrates possibly complex with undissociated molecules (ion pairs) of salt, as well as with free cations. Data from optical rotation experiments1 suggest that, even in aqueous solution, undissociated molecules of salt may be associated with the carbohydrate (see Section II, 7, p. 230). On the other hand,/ree anions do not appear to complex with carbohydrates in solution (see Section II, 8, p. 234). 

Figure 15.3. Overlaid HSQC spectra of biopolymers on IHSS peat. (A) Biopolymers lignin (gray), amylopectin (red), albumin (blue) and cuticle (green) overlaid on each other. (B) All biopolymers are illustrated in black. (C) IHSS humic acid extract from peat. (D) Biopolymers (black) overlaid on IHSS peat (green). The highlighted areas in 2D are those not well represented by biopolymers in the HA, namely complex carbohydrates and p- ydroxybcnzoatcs from lignin [see Kelleher and Simpson (2006) for more details]. See color insert. Reprinted from Kelleher, B. R, and Simpson, A. J. (2006). Humic substances in soils Are they really chemically distinct Environ. Sci. Technol. 40,4605-4611, with the permission of the American Chemical Society.

Amino acids link together linearly to form proteins, nucleotides link linearly to form RNA and DNA, and sugars link in a more complicated way to form complex carbohydrates. The specific sequence in which these units link together to form the final polymeric macromolecule is called its primary structure. In a way that is still very ill-understood, the primary structure ultimately controls the macromolecule s three-dimensional structure and thereby largely determines its function. There is therefore great interest in analyzing primary structural information in order to detect similarities and relationships between macromolecules. However, as we shall see later, although similar primary structures imply similar three-dimensional structures, it is possible for three-dimensional structures to resemble each other without any sequence similarity. 

Macrolide antibiotics, in some instances, may be considered as hydroxy acid derivatives. In addition, many of them have carbohydrates attached, often with unique structural features, as illustrated by erythromycin (20).53 Although some of these antibiotics are semi-synthetic, all are derived by a fermentation process where the antibiotic is formed as a secondary metabolite. This approach, of course, alleviates the need to perform complex carbohydrate chemistry (see Chapter 19). 

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