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332 Carraher’s Polymer Chemistry

aldehydes that react with either a lysine residue or with one another through an aldol condensation
and dehydration resulting in a cross-link. This process continues throughout our life, resulting in
our bones and tendons becoming less elastic and more brittle. Again, a little cross-linking is essen-
tial, but more cross-linking leads to increased fracture and brittleness.
Collagen is a major ingredient in some “gelation” materials. Here, collagen forms a triple helix
for some of its structure while other parts are more randomly flowing single-collagen chain seg-

ments. The bundled triple-helical structure acts as the rigid part of the polymer, while the less
ordered amorphous chains act as a soft part of the chain. The triple helix also acts as a noncova-
lently bonded cross-link.

10.2.5 ELASTIN

Collagen is found where strength is needed, but some tissues, such as arterial blood vessels and
ligaments need materials that are elastic. Elastin is the protein of choice for such applications.
Elastin is rich in glycine, alanine, and valine and it is easily extended and fl exible. Its confor-
mation approaches that of a random coil so that secondary forces are relatively weak, allowing
elastin to be readily extended as tension is applied. The structure also contains some lysine side
chains that are involved in cross-linking. The cross-linking is accomplished when four lysine side
chains are combined to form a desmosine cross-link. This cross-link prevents the elastin chains

from being fully extended and causes the extended fiber to return to its original dimensions
when tension is removed. One of the areas of current research is the synthesis of polymers with
desired properties based on natural analogues. Thus, elastin-like materials have been synthesized
using glycine, alanine, and valine and some cross-linking. These materials approach elastin in
its elasticity.
We are beginning to understand better how we can utilize the secondary structure of poly-
mers as tools of synthesis. One area where this is being applied is “folded oligomers.” Here, the
secondary structure of the oligomer can be controlled through its primary structure and the use
of solvents. Once the preferred structure is achieved, the oligomers are incorporated into larger
chains eventually forming synthetic polymers with several precise structures “embedded” within

them. The secondary structure of the larger polymers can also be influenced by the nature of the
solvent allowing further structural variety. Further, other species, such as metal ions, can be added
to assist in locking in certain desired structures and they can also be used to drive further structure
modifi cations.

10.2.6 TERTIARY STRUCTURE

The term tertiary structure is used to describe the shaping or folding of macromolecules. These
larger structures generally contain elements of the secondary structures. Often, hydrogen bond-

ing, salt bridges, posttranslational modifications, and disulfide cross-linking lock in such structures

(Figure 10.6). As noted above, proteins can be divided into three broad groups—fibrous or fi brillar
proteins, membrane proteins, and globular proteins.
10.2.7 GLOBULAR PROTEINS

There is a wide variety of so-called globular proteins. Almost all globular proteins are soluble in
acidic, basic, or neutral aqueous solutions. Many globular proteins are enzymes. Many of these have
varieties of alpha- and beta-structures imbedded within the overall globular structure. Beta sheets
are often twisted or wrapped into a “barrel-like” structure. They contain portions that are beta-sheet
structures and portions that are in an alpha-conformation. Further, some portions of the globular
protein may not be conveniently classified as either an alpha or beta structure.

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