12.1 Introduction
In nature three kinds of polymers occur: (i) polysaccharides, (ii) proteins and (iii) nucleic acids. This section discusses proteins and peptides that are polymers of α-amino acids linked together by amide bonds. Scheme 1 shows representation for an α-amino acid and peptide. The repeating units in peptide is called amino acid residue. Proteins are polypeptides that are made up of 40-100 amino acids.

Scheme 1

12.2 a-Amino Acids

The structures of the 20 most common naturally occurring amine acids are shown in Scheme 2. They differ only in the side chain attached to the α-carbon. Among them, ten are essential amino

Scheme 2

acids (Scheme 3). These amino acids are to be obtained from diets because we either can not synthesize them at all or can not synthesize them in adequate amounts.