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Naturally Occurring Polymers—Animals 329

Our fingernails are also composed of alpha-keratin, but keratin with a greater amount of sul-

fur cross-links giving a more rigid material. In general, for both synthetic and natural polymers,
increased cross-linking leads to increased rigidity.
The other major structural feature is pleated sheets. Two kinds of pleated sheets are found. When
the chains have their N→C directions running parallel they are called parallel beta sheets. The
N→C directions can run opposite to one another giving what is called an antiparallel beta sheet.
The beta-keratin (Figure 10.5) that occurs in silk produced by insects and spiders is of the antipar-
allel variety. While alpha-keratin is especially rich in glycine and leucine, beta-keratine is mostly
composed of glycine and alanine with smaller amounts of other amino acids, including serine and
tyrosine. Size-wise, leucine offers a much larger grouping attached to the alpha carbon than does
alanine. The larger size of the leucine causes the alpha-keratine to form a helical structure to mini-
mize steric factors. By comparison, the smaller size of the alanine allows the beta-keratine to form
sheets. This sheet structure is partially responsible for the “softness” felt when we touch silk. While
silk is not easily elongated because the protein chains are almost fully extended, beta keratin is

flexible because of the low secondary bonding between sheets, allowing the sheets to flow past one
another.
10.2.3.1 Silk
Silk is a protein fiber that is woven into fiber from which textiles are made, including clothing and

high-end rugs. It is obtained from the cocoon of silkworm larvae. While most silk is harvested
from commercially grown silkworms, some is still obtained from less well-established sources.

Silk was first developed as early as 6,000 BC. The Chinese Empress Xi Ling-Shi developed the

process of retrieving the silk filaments by floating the cocoons on warm water. This process and the

silkworm itself were monopolized by China until about 550 AD when two missionaries smuggled
silkworm eggs and mulberry seeds from China to Constantinople (Istanbul). First reserved to use by
the Emperors of China, its use eventually spread to the Middle East, Europe, and North America,
but now its use is worldwide. The history of silk and the silk trade is interesting and can be obtained
at http://en.wikipedia.org/wiki/Silk.
The early work focused on a particular silkworm, Bombyx mori, which lives on mulberry bushes.
There are other silkworms each with its on special properties, but in general, most silk is still
derived from the original strain of silkworm. Crystalline silk fiber is about four times stronger than

steel on a weight basis.
In the silk fibroin structure, almost every other residue is glycine with either alanine or serine

between them, allowing the sheets to fit closely together (Table 10.1). While most of the fi broin exists
as beta sheets, regions that contain more bulky amino acid residues interrupt the ordered beta struc-
ture. Such disordered regions allow some elongation of the silk. Thus, in the crystalline segments of
silk fibroin, there exists directional segregation using three types of bonding: covalent bonding in the

first dimension, hydrogen bonding in the second dimension, and hydrophobic bonding in the third

dimension. The polypeptide chains are virtually fully extended; there is a little puckering to allow for
optimum hydrogen bonding. Thus, the structure is not extensible in the direction of the polypeptide
chains. By comparison, the less specific hydrophobic (dispersive) forces between the sheets produce

considerable flexibility. The crystalline regions in the polymers are interspersed with amorphous
regions in which glycine and alanine are replaced by amino acids with bulkier pendent groups that
prevent the ordered arrangements to occur. Furthermore, different silk worm species spin silks with
differing amino acid compositions and thus, with differing degrees of crystallinity. The correlation
between the extent of crystallinity and the extension at the break point is given in Table 10.2.
Another natural key that illustrates the tight relationship between structure and property is found

in spider webs. The composition within a spider web is not all the same. We can look briefly at two of
the general types of threads. One is known as the network or frame threads, also called the dragline
fabric. It is generally stiff and strong. The second variety is the catching or the capture threads that

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