982 CHAPTER 23 Amino Acids, Peptides, and ProteinsThe solid support to which the C-terminal amino acid is attached is a polystyrene
resin similar to the one used in ion-exchange chromatography (Section 23.5), except
that the benzene rings have chloromethyl substituents instead of sulfonic acid sub-
stituents. Before the C-terminal amino acid is attached to the resin, its amino group
is protected with t-BOC to prevent the amino group from reacting with the resin.
The C-terminal amino acid is attached to the resin by means of an reaction—its
carboxyl group attacks a benzyl carbon of the resin, displacing a chloride ion
(Section 10.4).
After the C-terminal amino acid is attached to the resin, the t-BOC protecting group
is removed (Section 23.9). The next amino acid, with its amino group protected with
t-BOC and its carboxyl group activated with DCC, is added to the column.
A huge advantage of the Merrifield method of peptide synthesis is that the
growing peptide can be purified by washing the column with an appropriate solvent
after each step of the procedure. The impurities are washed out of the column be-
cause they are not attached to the solid support. Since the peptide is covalently
attached to the resin, none of it is lost in the purification step, leading to high yields
of purified product.
After the required amino acids have been added one by one, the peptide can be re-
moved from the resin by treatment with HF under mild conditions that do not break
the peptide bonds.
Merrifield’s technique is constantly being improved so that peptides can be
made more rapidly and more efficiently. However, it still cannot begin to compare
with nature: A bacterial cell is able to synthesize a protein thousands of amino
acids long in seconds and can simultaneously synthesize thousands of different
proteins with no mistakes.
Since the early 1980s, it has been possible to synthesize proteins by genetic engi-
neering techniques. Strands of DNA can be introduced into bacterial cells, causing the
cells to produce large amounts of a desired protein (Section 27.13). For example, mass
quantities of human insulin are produced from genetically modified E. coli. Genetic
engineering techniques also have been useful in synthesizing proteins that differ in one
or a few amino acids from the natural protein. Such synthetic proteins have been used,
for example, to learn how a change in a single amino acid affects the properties of a
protein (Section 24.9).PROBLEM 27Show the steps in the synthesis of the peptide in Problem 25, using Merrifield’s method.23.11 Protein Structure
Protein molecules are described by several levels of structure. The primary struc-
tureof a protein is the sequence of amino acids in the chain and the location of all
the disulfide bridges. The secondary structuredescribes the regular conformation
assumed by segments of the protein’s backbone. In other words, the secondary
structure describes how local regions of the backbone fold. The tertiary structure
describes the three-dimensional structure of the entire polypeptide. If a protein has
more than one polypeptide chain, it has quaternary structure. The quaternary
structureof a protein is the way the individual protein chains are arranged with re-
spect to each other.
Proteins can be divided roughly into two classes. Fibrous proteinscontain
long chains of polypeptides that occur in bundles. These proteins are insoluble in
water. All the structural proteins described at the beginning of this chapter, such as
keratin and collagen, are fibrous proteins. Globular proteinsare soluble in water
and tend to have roughly spherical shapes. Essentially all enzymes are globular
proteins.SN 2BRUI23-959-998r2 29-03-2003 1:36 PM Page 982