Chemistry, Third edition

344 18 · COMMON CLASSES OF ORGANIC COMPOUNDS

Peptides

Peptides are formed when the amino group of one amino acid interacts with the

carboxyl group of another. Water is eliminated in the reaction:

OOOO

|| || || ||

H 2 NCHCOHH 2 NCH 2 COHH 2 NCHC—NHCH 2 COHH 2 O

||

CH 3 CH 3

alanine glycine alanylglycine

The group –CONH–that is formed in such compounds is a peptide linkageor

peptide bond.Dipeptidesandtripeptidesare derived from the combinations of two

and three amino acids respectively. Polypeptidesis a term that describes peptides of

molecular mass up to 10 000 u. Molecules larger than this are known as proteins.

Polypeptides and proteins can also be thought of as polyamides made from amino

acids. The name of a peptide is constructed from the names of the amino acids units

as they appear left-to-right, starting with the amino acid fragment that has a free

• NH 2 group:

O

||

H 2 NCH 2 C—NHCH 2 COOH

glycylglycine

(Gly-Gly, a dipeptide)

OO

|| ||

H 2 NCH 2 C—NHCHC—NHCHCOOH

||

CH 3 CH 2 C 6 H 5

glycylalanylphenylalanine

(Gly-Ala-Phe, a tripeptide)

Proteins are found in all living cells. They make up a large part of an animal’s

body and are found, for example, in skin, blood, enzymes and hormones. A single

protein molecule contains many, many amino acid units joined by peptide linkages.

Proteins are divided into two groups:

1.Fibrous proteins, in which the molecules are long, tough and thread-like. They make

up keratin in nails, wool, hair and feathers. These proteins are insoluble in water.

2.Globular proteins consist of molecules that are folded into compact shapes. They

are found, for example, in enzymes and hormones, and in haemoglobin which

transports oxygen in the body.

Structure of proteins

The amino acids found in proteins are -aminocarboxylic acids. The prefixαrefers

to the fact that the amino group is attached to the same carbon atom bonded to the

carboxylic acid group:

Peptides

(i)Write the structural

formula of the dipeptide

glycylalanine (Gly-Ala).

(ii)How many different

tripeptides can be made

from different

combinations of the

three amino acids Gly,

Ala and Cys?

Exercise 18O