macromolecule

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macromolecule,

term that may refer either to a crystalcrystal,
a solid body bounded by natural plane faces that are the external expression of a regular internal arrangement of constituent atoms, molecules, or ions. The formation of a crystal by a substance passing from a gas or liquid to a solid state, or going out of solution (by
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 such as a diamond, in which the atoms are identical and held by covalent bonds (see chemical bondchemical bond,
mechanism whereby atoms combine to form molecules. There is a chemical bond between two atoms or groups of atoms when the forces acting between them are strong enough to lead to the formation of an aggregate with sufficient stability to be regarded as an
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) of equal strength, or to one of the units that compose a polymerpolymer
, chemical compound with high molecular weight consisting of a number of structural units linked together by covalent bonds (see chemical bond). The simple molecules that may become structural units are themselves called monomers; two monomers combine to form a dimer,
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. Macromolecules such as proteins and nucleic acids are vital to the functions of living cells.

Macromolecule

 

literally, a large molecule; the molecule of a polymer. The structure of macromolecules may be described as a repetition of identical (in a homopolymer) or different (in a copolymer) structural units called monomeric (repeating) units.

In a linear macromolecule the monomeric units are joined covalently into a chain whose length is characterized by the degree of polymerization (the number of repeating units) or by the molecular weight. The aggregate of the macromolecules of a given polymer, as distinct from the molecules of a substance of low molecular weight, consists of a set of chains that (in homopolymers, for example) have the same chemical structure but different lengths. For homopolymers this set is described quantitatively by the distribution function according to the degree of polymerization (or the molecular-weight distribution). In a homologous series of copolymers of the same average composition, the macromolecules also exhibit compositional heterogeneity (heterogeneity of the composition proper) and configu-rational heterogeneity (differences in the alternation of different types of units). Constructed of hundreds to millions of elementary units, each macromolecule is a miniature statistical ensemble that is subject to the laws of thermodynamics of small systems and manifests such properties of macroscopic physical bodies as variability in size (geometric) and shape that is not due to chemical conversions.

This variability is associated with one of the most important properties of macromolecules—their flexibility, that is, the capacity of polymeric chains to change their conformation as a result of the intramolecular micro-Brownian thermal motion of the repeating units (in the case of thermodynamic flexibility) or under the action of external mechanical (in particular, hydrodynamic) factors (kinetic flexibility). Flexibility is a function of the possibility of rotation of the atoms of the chain and of the units as a whole around simple (single) bonds. The flexibility of macromolecules is distinguished from mobility, which is limited by external factors, such as interaction with a solvent or with neighboring macromolecular chains. A direct measure of flexibility is the magnitude of the inhibition potential of the internal rotation of atoms and repeating units, which depends on the structure of the units and is explained by quantum mechanics.

The thermodynamic flexibility of macromolecules is determined by their geometrical dimensions and stereochemical characteristics. The principal stereochemical characteristic of the macromolecule is its configuration (the entire spatial distribution of the atoms that form the macromolecule), which is determined by the lengths of the bonds and by the magnitudes of the valent angles and which cannot be changed without breaking chemical bonds. It is known that macromolecules of the same total configuration may assume several conformations. Thus, conformation is a variable statistical quantity that characterizes the spatial arrangement of the atoms and atomic groups when the valent angles are constant but the bond orientations variable.

A change in orientation occurs as a result of relative rotations of the atoms and groups under the effect of the thermal motion of the repeating units. In the absence of interaction with other macromolecules (for example, in dilute solutions), a hypothetical polymeric chain that is at first extended acquires the conformation of a statistical coil, as a result of a series of elementary rotations. The dimensions of such a coil are expressed, for example, by the root-mean-square of the distance between its ends. Comparison of these dimensions with those that the macromolecule would have acquired in the absence of any inhibition of internal rotation (calculated theoretically) makes it possible to evaluate thermodynamic flexibility. The dimensions of the macromolecule required for flexibility calculations may be found by light scattering or by hydrodynamic methods, and certain configurational characteristics may be found by dynamo-optical or electro-optical methods (birefringence in flux, the Kerr effect).

Unlike thermodynamic (equilibrium) flexibility, kinetic flexibility is not a constant characteristic of macromolecules but depends rather on the speed of external deformational actions. The influence of the speed of action on the macromolecule’s kinetic flexibility can be calculated if the macromolecule’s relaxation spectrum is known. There is a definite connection between equilibrium and kinetic flexibility, for, in the final analysis, both are determined by the inhibition potential.

From the standpoint of statistical physics, the capacity of a macromolecule for deformation may be characterized by the conformational set (also called the statistical weight, or conformational entropy). With a decrease in the degree of polymerization, there is also a decrease in the number of possible conformations. The relatively short macromolecules of oligomers, or multimers, are never deformable, but only because they consist of a small number of units. Their inhibition potential— the ultimate measure of their flexibility—is the same as that of longer chains.

Configuration can also be described in terms of statistical weight, as is apparent in the case of copolymers. The number of ways in which the different units may be distributed along a chain determines the configurational entropy of a macromolecule, and the negative value of this magnitude is a measure of the information that the macromolecule can contain. The significance of the macromolecule’s capacity to store information—one of its most important characteristics—became clear only after the discovery of the genetic code.

The unique mechanical properties of polymers (in particular, their high elasticity) are associated with the equilibrium and kinetic flexibility of macromolecules. The conformational entropy of polyelectrolytes and copolymers makes possible the transformation of chemical energy into mechanical energy. Associated with configurational entropy is the capacity of macromolecules to form stable secondary molecular structures that attain a high degree of perfection and possess specific properties in the macromolecules of the most important biopolymers (proteins and nucleic acids). Instead of the term “configurational entropy,” the term “configurational information” may be used when referring to biopolymers. Configurational information determines the uniqueness (the nonstatisticity, in contrast to synthetic macromolecules) of the conformations of protein macromolecules and predetermines their capacity to act as enzymes and oxygen carriers, for example. In synthetic copolymers, secondary molecular structures arise as a result of the selective interaction of the various types of monomeric units distributed in a particular way along the chain. These structures are only moderately specific but can serve as the simplest models of memory on the macromolecular level.

REFERENCES

Vol’kenshtein, M. V. Konfiguratsionnaia statistika polimernykh tsepei. Moscow-Leningrad, 1959.
Vol’kenshtein, M. V. Molekuly i zhizn’ Moscow, 1965.
Tsvetkov, V. N., V. E. Eskin, and S. Ia. Frenkel’. Struktura makromolekul v rastvorakh. Moscow, 1964.
Morawetz, H. Makromolekuly v rastvore. Moscow, 1967. (Translated from English.)
Birshtein, T. M., and O. B. Ptitsyn. Konformatsii makrontolekul Moscow, 1964.
Flory, P. Statisticheskaia mekhanika tsepnykh molekul Moscow, 1971. (Translated from English.)
Frenkel’, S. Ia. “Gibkost’ makromolekul.” In Entsiklopediia polimerov, vol. 1. Moscow, 1972.
“Makromolekula.” In Entsiklopediia polimerov, vol. 2. Moscow (in press).
S. IA. FRENKEL’

macromolecule

[¦mak·rō′mäl·ə‚kyül]
(organic chemistry)
A large molecule in which there is a large number of one or several relatively simple structural units, each consisting of several atoms bonded together.

macromolecule

any very large molecule, such as a protein or synthetic polymer
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