Structure of Amino Acids

Proteins are polymers - long chain molecules made up of many repeating units called amino acids. Amino acids all have similar structures (shown on the right) with two functional groups, an amino (-NH₂) group and a carboxylic acid (-COOH) group. The only difference between different amino acids is the R group.

There are twenty naturally occurring amino acids which are classified into four groups according to the properties of the -R group. These four groups, along with example amino acids, are shown in the table below.

Group	Example	Amino Acid Structure
Nonpolar	alanine
Polar (neutral)	serine
Polar (acidic)	glutamic acid
Polar (basic)	lysine

 

Acid/Base Properties of Amino Acids

Amino acids are unique in that they possess both an acidic and a basic functional group on the same molecule. In fact, amino acids do not exist in the form shown in the above figures. The carboxylic acid donates its proton to the amino group and a zwitterion is formed (shown on the right).

As you might imagine, the actual form of the amino acid will depend on the pH of its environment. At low pH both the acid and amino groups will be protonated and at high pH both the acid and amino groups will be deprotonated. The pH at which the zwitterionic form predominates is called the isoelectric point. This is summarized in the figure below.

low pH (acidic)		isoelectric point		high pH (basic)

 

Chirality of Amino Acids

All amino acids (except glycine where R = H) are chiral molecules. The twenty naturally occurring amino acids are all of the L form. The above figures can be considered Fischer projections with the R and carboxylic acid groups coming out of the plane of the paper.