ABSTRACT
All proteins are made up from the same set of 20 standard amino acids. A typical amino acid has a primary amino group, a carboxyl group, a hydrogen atom and a side-chain (R group) attached to a central α-carbon atom. Proline is the exception to the rule in that it has a secondary amino group. All of the amino acids, except for glycine, have four different groups arranged tetrahedrally around the central Ca atom which is thus known as an asymmetric center or chiral center and has the property of chirality. The standard 20 amino acids differ only in the structure of the side-chain or 'R' group. They can be subdivided into smaller groupings on the basis of similarities in the properties of their side-chains. The aliphatic side-chains of alanine, valine, leucine , isoleucine and methionine are chemically unreactive, but hydrophobic in nature. The remaining amino acids all have polar, hydrophilic side-chains, some of which are charged at neutral pH.
