Amino acids covalently bond to each other through peptide bonds. A peptide bond links the carboxyl group of one amino acid to the amino group of the other. Peptides are small chains of amino acids ranging from two to fifty monomers. A peptide bond forms through a condensation reaction that releases a water molecule. When an amino group and a carboxyl group link together, it results in the formation of an amide group. The peptide bond has a rigid planar structure and shows some characteristics of a double bond. This is because the double bond on the carbonyl can act as a resonance structure with the carbon-nitrogen peptide bond. This resonance leads to the decrease in the length of the single bond, an increase in the length of the double bond, and inhibits rotation of the groups around the peptide bond. Peptide bonds can exist in cis and trans conformations. In the cis conformation, the alpha carbons are on the same side of the peptide bond, and in the trans conformation, they are on opposite sides of the bond. Peptide bonds mostly occur in the trans conformation except when proline contributes its amino group to bond formation. Amino acids linked through peptide bonds have an N-terminal and a C-terminal. The N-terminal is the end where the amino group is not involved in the formation of a peptide bond, and the C-terminal is the end where the carboxyl group is not involved in the formation of a peptide bond. The sequence of amino acids in a peptide or protein always starts with the N-terminal and ends with the C-terminal. When two amino acids link together through a peptide bond, they are known as dipeptides. When three and four amino acids link together through peptide bonds, they are known as tripeptides and tetrapeptides, respectively. And when several amino acids link together through peptide bonds, they are known as polypeptides. Peptide bonds can be broken down rapidly through hydrolysis using chemical catalysts, such as acids or enzymes known as proteases. Breaking of peptide bonds involves the addition of a water molecule. Alternatively, these bonds can break spontaneously through a slow process in the presence of water.
