Chemistry, Third edition

AMINO ACIDS AND PROTEINS 343

Structure of amino

acids

(i)Most naturally occurring

amino acids can exist in

two optically active

forms. Draw the two

possible structures of

the amino acid alanine,

CH 3 CHNH 2 COOH.

(ii)What is the formula of

the dipolar ion formed

by alanine?

Exercise 18N

Table 18.4Amino acids

Name Formula Abbreviation

Alanine CH 3 CHCOOH Ala
|
NH 2

Phenylalanine CH 2 CHCOOH Phe
|
NH 2
Cysteine HSCH 2 CHCOOH Cys
|
NH 2
O
||
Glutamine H 2 NCCH 2 CH 2 CHCOOH Gln
|
NH 2
Lysine H 2 NCH 2 CH 2 CH 2 CH 2 CHCOOH Lys
|
NH 2

Amino acids and proteins

Amino acids

Amino acids contain an amino group (–NH 2 ) and a carboxylic acid group (–COOH).

Since their formulae can be quite complicated they usually are referred to by their

common names rather than their systematic names. For example, H 2 NCH 2 COOH

is called glycinerather than aminoethanoic acid. Some common amino acids are

shown in Table 18.4.

Amino acids are non-volatile crystalline solids, which are very soluble in water.

Many of them are optically active. They also have large dipole moments. Because the

molecules contain both an acidic and a basic group, the following equilibrium exists

in solution:

H 2 NCHRCOOH\==\H 3 NCHRCOO

The ‘acidic end’ of the molecule provides a proton to protonate the ‘basic’ end. The

ion that is formed is known as a dipolar ionorzwitterionand, because of the ionic

charges at both ends, the amino acid has many properties of a salt.

18.7

Optical isomers

(i)Draw, or make models of the two isomeric forms of lactic acid, the substance that gives

milk its sour taste. Mark the chiral centre with an asterisk.

COOH

|

H—C—OH

|

CH 3

(ii)Which of the following compounds can exist in two enantiomeric forms?

2-methylbutan-1-ol, 1-chloropentane, 2-bromo-1-chlorobutane.

Exercise 18M