FIGURE 7-18 Chitin. (a) A short segment of chitin, a homopolymer of N-acetyl-d-glucosamine units in (01 n4) linkage. (b) A spotted June beetle (Pellidnota punetatia), showing its surface armor (exoskeleton) of chitin.

ple, free rotation about both C—O bonds linking the residues (Fig. 7-16a), but as in polypeptides (see Figs 4-2, 4-9), rotation about each bond is limited by steric hindrance by substituents. The three-dimensional structures of these molecules can be described in terms of the dihedral angles, $ and $, made with the glycosidic bond (Fig. 7-19), analogous to angles $ and $ made by the peptide bond (see Fig. 4-2). Because of the bulki-ness of the pyranose ring and its substituents, their size and shape place constraints on the angles $ and $; certain conformations are much more stable than others, as can be shown on a map of energy as a function of $ and $ (Fig. 7-20).

The most stable three-dimensional structure for starch and glycogen is a tightly coiled helix (Fig. 7-21), stabilized by interchain hydrogen bonds. In amylose (with no branches) this structure is regular enough to allow crystallization and thus determination of the structure by x-ray diffraction. Each residue along the amylose chain forms a 60° angle with the preceding residue, so the helical structure has six residues per turn. For amylose, the core of the helix is of precisely the right dimensions to accommodate iodine in the form I3- or I5-(iodide ions), and this interaction with iodine is a common qualitative test for amylose.

For cellulose, the most stable conformation is that in which each chair is turned 180° relative to its neighbors, yielding a straight, extended chain. All —OH groups are available for hydrogen bonding with neighboring chains. With several chains lying side by side, a stabilizing network of interchain and intrachain hydrogen bonds produces straight, stable supramolecular

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