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.
These cross-link between different chains or between parts of the same chain. The resulting disulfide is called cystine. Intracellular proteins usually lack disulfide bonds, whereas extracellular proteins often contain several. The amino acid sequence of more than 2000 proteins are know and all have unique sequence related to their function. Amino acid sequences are important for several reasons.