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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

208 Biopolymers

homopolymers, or from several kinds of monomers, giv- B-B-B-. Examples of all these kinds of biopolymer ar-
ing copolymers. For many biopolymers the repeat units of rangements are known.
the macromolecule are not identical in structure to the A polymer sample may consist of a collection of iden-
monomers, because a small molecule such as water is tical molecules (i.e., with the same sequence of monomer
eliminated from the monomers during incorporation into residues, the same position and length of branches [where
the polymer. This leaves monomer residues or repeat units present] and the same degree of polymerization). On the
smaller than the original monomers (e.g., consider three other hand, polymers are known where the molecules
hypothetical monomers becoming linked at the start of are heterogeneous with respect to number and sequence
synthesis of a polymer, as in I). of monomer residues and distribution of branches. Pro-
teins and nucleic acids, for example, fall in the first cate-
HO ROH HO R OH HO R OH gory, while polysaccharides and lignin come in the second
category.
One of the characteristics of polymers, apart from
molecular size, which distinguishes them from substances
HO R O R O R OH 2H 2 O made of small molecules is the importance of nonco-
valent bonding between two parts of the same polymer
monomer residue
or molecule or between two separate molecules. This non-
repeat unit covalent bonding can be of various kinds. Hydrogen bond-
ing arises when hydrogen atoms are shared between two
I atoms such as oxygen or nitrogen which carry partial neg-
ative charges. Hydrogen bonds are particularly common
The monomer is HOROH, while the repeat unit is in biopolymers, but they can be disrupted easily by heat or
R O . Molecules consisting of a small number (<20) changes in acidity (or pH) of the environment of the poly-
of repeat units are called oligomers, but there is no gener- mer. Many groups attached to polymer main chains can
ally accepted value for the number of repeat units which ionize, and electrostatic attractions, or ionic bonding, can
distinguishes a large oligomer from a small polymer. In occur between positively and negatively charged group-
this article the molecules of the polymers discussed gener- ings. Conversely, groups carrying like charges repel each
ally have a degree of polymerization (the number of repeat other. Ionic bonding is important in many proteins and in-
units in the polymer molecule) of more than 30. organic polymers but again can be lessened by changes in
Where polymers are synthesized from monomers con- pH of the environment. In proteins, it is often found that
taining two chemically reactive, or functional, groups, the hydrocarbon groups associate with each other. This hy-
resultant polymer molecules are linear. If monomers have drophobic interaction excludes water from the vicinity of
more than two functional groups, however, branched or the hydrocarbon groupings. In fact, it is this exclusion of
even network polymers may be formed (Fig. 1). water which provides the driving force for the association.
Carbon atoms are an important constituent of a biopoly- In addition to these kinds of bonding, all atoms in close
mer chain. Such atoms are usually bonded to four atoms contact attract each other weakly. Such forces are called
or chemical groupings arranged tetrahedrally in space. van der Waals forces and can be important in the interiors
When the four groups attached to one carbon atom are of highly folded and compact polymer molecules.
different, two possible arrangements in space, or config-
urations, are possible (Fig. 2). Biosynthesis of polymers
is under strict stereochemical control, however, and usu- II. BIOPOLYMERS
ally only one possible configuration is incorporated into
a biopolymer. Thus, proteins, which are polymers of α- A large number of polymers, differing widely in structure
amino acids, are synthesized from L-α-amino acids only, and function, are synthesized in living organisms. It is
and not from D-α-amino acids (see Fig. 2). convenient, therefore, to discuss biopolymers in groups,
In copolymers, where more than one monomer residue rather than as a whole. The polymers described below have
(say A and B) is present, these residues can be arranged been assigned to groups on the basis of structure, but at
along the polymer chain in different ways, giving for ex- best this division is approximate.
ample an apparently random sequence A–B-A-A-B-A- Although individual polymers are important, in living
B-B-B-A-A-, or an alternating sequence A-B-A-B-A-B- organisms it is often the interactions between biopolymers
A-B-, or a sequence consisting of a block of one residue which confer on a tissue its form and function. Much work
followed by a block of the other A–A-A-A-A-A-A-B-B- is now being carried out to elucidate these interactions.