Amino acids are the basic building blocks of proteins. Fundamentally, amino acids are joined together by peptide bonds to form the basic structure of proteins. However, owing to the many ‘side groups’ that are part of the amino acids other sorts of bonds may form between the amino acid units. These additional bonds twist and turn the protein into convoluted shapes that are unique to the protein and essential to its ability to perform certain functions within the human body. Although we are studying only about 20 amino acids(depending upon the alkyl group attached to the a-carbon , there are about six more found in the body. Many others are also known from a variety of sources.

The Structure of Amino Acids
Amino acids are carbon compounds that contain two functional groups: an amino group (NH2) and a carboxylic acid group (COOH). A side chain attached to the compound gives each amino acid a unique set of characteristics.

Zwitterion

Amino acid physical properties indicate a "salt-like" behavior. Amino acids are crystalline solids with relatively high melting points. In solution, the amino acid molecule appears to have a charge which changes with pH. An intramolecular neutralization reaction leads to a salt-like ion called a zwitterion. The accepted practice is to show the amino acids in the zwitterion form.

(1) The carboxyl group can lose a hydrogen ion to become negatively charged.

(2) The amine group can accept a hydrogen ion to become positively charged