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Naturally Occurring Polymers—Animals 325

Almost all of the sulfur needed for healthy bodies is found in amino acids as cysteine and methi-
onine. Sulfur serves several important roles, including as a cross-linking agent similar to that served
by sulfur in the cross-linking, vulcanization, of rubber. This cross-linking allows the various chains,
which are connected by these cross-links, to “remember” where they are relative to one another. This
cross-linking allows natural macromolecules to retain critical shapes to perform necessary roles.
The most widely used technique for producing polypeptides with specific sequences is the solid

phase technique developed by Nobel Prize winner Bruce Merrifield in which all reactions take
place on the surface of cross-linked polystyrene beads. The process begins with the attachment
of the C-terminal amino acid to the chloromethylated polymer. Nucleophilic substitution by the
carboxylate anion of an N-blocked protected C-terminal amino acid displaces chloride from the
chloromethyl group forming an ester, protecting the C site while attaching it to the solid support.
The blocking group is removed by addition of acid, and the polymer containing the nonprotected
N-terminus is washed to remove unwanted byproducts. A peptide bond is formed by condensation
to an N-blocked, protected amino acid. Again, the solid phase system is washed removing byprod-
ucts. The blocking group is removed and the site is ready for attachment by another amino acid.
This cycle is repeated eventually producing the desired polypeptide without isolation of intermedi-
ate products. These steps are outlined below:
R O R
−HCl H +
PS−Ph−CH 2 −Cl + HOOC−CH−NH−BG -------- PS−Ph−CH 2 −O−C−CH− NH−BG --------
(10.7)
O R
PS−Ph−CH 2 −O−C−CH−NH  -----  

where PS = polystyrene bead, Ph = phenyl group, and BG = blocking group.

10.2 LEVELS OF PROTEIN STRUCTURE

The shapes of macromolecules, both synthetic and natural, can be described in terms of primary,
secondary, tertiary, and quaternary structure (Figure 10.1). Protein structure will be used to illus-
trate these structures.

FIGURE 10.1 Four levels of structure elucidation. From left to right: primary, secondary, tertiary, and
quaternary structures illustrate using a globular protein segment.

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