Many amino acids jointed by a peptide bonds are called a polypeptide which is unbranched. A polypeptide chain consist of a regularly repeating part, called the main chain, and a variable part, comprising the distinctive side chains. The average molecular weight of most amino acids is about 110 and therefore most polypeptides would have a molecular weight of about 5,500 to 220,000. The mass of proteins is expressed in units of daltons. A stretched out or randomly arranged polypeptide chain is without biological activity.
Function arises from conformation.
The peptide unit is rigid and planar. There is no freedom of rotation about the bond between the carbonyl carbon atom and the nitrogen atom of peptide unit because of the partial double-bond character. In contrast, the link between the alpha carbon and the peptide nitrogen atom is a pure single bond. There is a large degree of rotational freedom about these bonds on either side of rigid peptide unit. Some proteins contain disulfide bonds.