Organic Chemistry

980 CHAPTER 23 Amino Acids, Peptides, and Proteins

Theoretically, one should be able to make as long a peptide as desired with this tech-

nique. Reactions do not produce 100% yields, however, and the yields are further de-

creased during the purification process. After each step of the synthesis, the peptide

must be purified to prevent subsequent unwanted reactions with leftover reagents. As-

suming that each amino acid can be added to the growing end of the peptide chain in an

80% yield (a relatively high yield, as you can probably appreciate from your own expe-

rience in the laboratory), the overall yield of a nonapeptide such as bradykinin would be

only 17%. It is clear that large polypeptides could never be synthesized in this way.

Number of amino acids 23456789

Overall yield 80% 64% 51% 41% 33% 26% 21% 17%

PROBLEM 23

What dipeptides would be formed by heating a mixture of valine and N-protected leucine?

PROBLEM 24

Suppose you are trying to synthesize the dipeptide Val-Ser. Compare the product that would

be obtained if the carboxyl group of N-protected valine were activated with thionyl chloride

with the product that would be obtained if the carboxyl group were activated with DCC.

PROBLEM 25

Show the steps in the synthesis of the tetrapeptide Leu-Phe-Lys-Val.

PROBLEM 26

a. Calculate the overall yield of bradykinin if the yield for the addition of each amino acid

to the chain is 70%.

b. What would be the overall yield of a peptide containing 15 amino acid residues if the

yield for the incorporation of each is 80%?

23.10 Automated Peptide Synthesis

In addition to producing low overall yields, the method of peptide synthesis described

in Section 23.9 is extremely time-consuming because the product must be purified at

each step of the synthesis. In 1969, Bruce Merrifield described a method that revolu-

tionized the synthesis of peptides because it provided a much faster way to produce

peptides in much higher yields. Furthermore, because it is automated, the synthesis re-

quires fewer hours of direct attention. Using this technique, bradykinin was synthesized

with an 85% yield in 27 hours. Subsequent refinements in the technique now allow a

reasonable yield of a peptide containing 100 amino acids to be synthesized in four days.

In the Merrifield method, the C-terminal amino acid is covalently attached to a

solid support contained in a column. Each N-terminal blocked amino acid is added one

at a time, along with other needed reagents, so the protein is synthesized from the

C-terminal end to the N-terminal end. Notice that this is opposite to the way proteins

are synthesized in nature (from the N-terminal end to the C-terminal end;

Section 27.13). Because it uses a solid support and is automated, Merrifield’s method

of protein synthesis is called automated solid-phase peptide synthesis.

N-protected amino acid

Merrifield automated solid-phase synthesis of a tripeptide

CH 3

CH 3

CH 3 CClCH 2

O

O C

R

NHCHCO−

O

+

resin

R. Bruce Merrifieldwas born in

1921 and received a B.S. and a Ph.D.

from the University of California, Los

Angeles. He is a professor of chem-

istry at Rockefeller University. Merri-

field received the 1984 Nobel Prize in

chemistry for developing automated

solid-phase peptide synthesis.

BRUI23-959-998r2 29-03-2003 1:36 PM Page 980