The Foundations of Chemistry

Prefixes di-, tri-, and so on are used to indicate the number of amino acid monomers that

are joined to form the peptide. The compound indicated in the preceding formula is a

dipeptide.

Twenty different 
-amino acids (see Section 27-13) are found widely in nature. Each

amino acid is commonly given a three-letter abbreviation. These acids differ in the R

groups attached to the 
-carbon (the carbon adjacent to the XCOOH group). Differ-

ences in R groups lead to differences in the properties of the 
-amino acids. The amino

acids are usually classified by their R groups, or side chains, according to two criteria. The

first classification depends on whether the R groups are polar or nonpolar, the second

describes whether the R group contains an acidic or basic group. The 20 common amino

acids are shown, according to these classifications, in Table 28-6.

Many small peptides are known to have physiological significance. Aspartame, a

common artificial sweetener, is a dipeptide. Glutathione, a tripeptide, acts as a scavenging

agent for harmful oxidizing agents believed to cause cancer. Enkephalins, which are natu-

rally occurring analgesics (pain relievers) that occur in the brain, are pentapeptides. The

antibiotics gramicidin-S and tyrocidine A are cyclic decapeptides (i.e., they contain ten

amino acids linked into a ring by peptide bonds) produced by a bacterium.

H 2 NCHCNHCHCOOCH 3

O

CH 2

COOH Ph

CH 2

H 2 NCHCH 2 CH 2 CNHCHCNHCH 2 COOH

O

COOH CH 2 SH

O

aspartame

(Asp-Phe-OCH 3 )

glutathione

(Gln-Cys-Gly)

28-9 Polypeptides and Proteins 1131

HNC COH OH

H

R

H 2 O

O

peptide

bond

H

HNC COH

H

R

O

H

HNC CN

H

R

O

C

O

C

H

H H R

Space-filling models of aspartame

(top) and glutathione (bottom).

Tyr-Gly-Gly-Phe-Leu

leucine enkephalin

Tyr-Gly-Gly-Phe-Met

methionine enkephalin