State the basic structure of a 2-amino acid (or alpha amino acid)

The basic structure of a 2-amino acid is amine group external image amino.gif and carboxylic acid / carboxyl group external image carboxy.gif. These two groups of atoms are called functional groups. A functional group is a group of atoms that reacts in a chemical reaction. The R group refers to different group of atoms. Each R group gives rise to a specific amino acid. For example if the R group is a -CH3 or a methyl group, the amino acid is alanine.


Figure 1: general structure of an amino acid


Figure 2: amino acid alanine

There are 20 common 2-amino acids and each one differs in their R group. The simplest amino acid is glycine. It R group is a H atom.

2-amino acids contain an asymmetric or chiral carbon atom, a carbon atom that has four different groups attached to it. In 2-amino acids the central carbon atom is chiral. Molecules that have a chiral carbon atom can exits as mirror images of each other and the two different isomers are optically active with plane polarized light. Normal light consists of electromagnetic radiation which vibrates in all planes. When it is passed through a polarizing filter the waves only vibrate in one plane and the light is said to be polarized. The two mirror images of a 2-amino acid, known as enantiomers, both rotate the plane of polarized light. One enantiomer rotates it to the left and the other to the right. Apart from their behavior towards polarized light entantiomers have identical physical properties.

Figure: Mirror images of a generalized 2-amino acid showing the two enantiomers