The different levels of structure
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص667-668
2025-12-17
52
The different levels of structure
The primary structureof a macromolecule is the sequence of small molecular residues making up the polymer. The residues may form either a chain, as in polyethylene, or a more complex network in which cross-links connect different chains, as in cross linked polyacrylamide. In a synthetic polymer, virtually all the residues are identical and it is sufficient to name the monomer used in the synthesis. Thus, the repeating unit of polyethylene is -CH2CH2-, and the primary structure of the chain is specified by denoting it as -(CH2CH2)n-. The concept of primary structure ceases to be trivial in the case of synthetic copolymers and biological macromolecules, for in general these substances are chains formed from different molecules. For example, proteins are polypeptides formed from different amino acids (about twenty occur naturally) strung together by the peptide link,-CONH-. The determination of the primary structure is then a highly complex problem of chemical analysis called sequencing. The degradation of a polymer is a disruption of its primary structure, when the chain breaks into shorter components. The secondary structure of a macromolecule is the (often local) spatial arrange ment of a chain. The secondary structure of an isolated molecule of polyethylene is a random coil, whereas that of a protein is a highly organized arrangement determined largely by hydrogen bonds, and taking the form of random coils, helices (Fig. 19.13a), or sheets in various segments of the molecule. The loss of secondary structure is called denaturation. When the hydrogen bonds in a protein are destroyed (for instance, by heating, as when cooking an egg) the structure denatures into a random coil. The tertiary structure is the overall three-dimensional structure of a macro mole cule. For instance, the hypothetical protein shown in Fig. 19.13b has helical regions connected by short random-coil sections. The helices interact to form a compact tertiary structure. The quaternary structure of a macromolecule is the manner in which large molecules are formed by the aggregation of others. Figure 19.14 shows how four molecular subunits, each with a specific tertiary structure, aggregate together. Quaternary structure can be very important in biology. For example, the oxygen-transport protein haemoglobin consists of four subunits that work together to take up and release O2.
Fig. 19.13 (a) A polymer adopts a highly organized helical conformation, an example of a secondary structure. The helix is represented as a cylinder. (b) Several helical segments connected by short random coils pack together, providing an example of tertiary structure.
Fig. 19.14 Several subunits with specific tertiary structures pack together, providing an example of quaternary structure.
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