Structural Biochemistry/Chemical Bonding/ Disulfide bonds
Introduction
A disulfide bond, also called an S-S bond, or disulfide bridge, is a covalent bond derived from two thiol groups. In biochemistry, the terminology R-S-S-R connectivity is commonly used to describe the overall linkages. The most common way of creating this bond is by the oxidation of sulfhydryl groups. (2 RSH → RS-SR + 2 H+ + 2 e-) This process of oxidation can produce stable protein dimers, polymers, or complexes, in which the sulfide bonds can help in protein folding.
Disulfide Bonds in Proteins
Disulfide bonds in protein membranes are found in both bacteria and eukaryotes. In proteins, these bonds form between the thiol groups of two cysteine amino acids. The cysteine amino acid group is the only amino acid capable of forming disulfide bonds, and thus can only do so with other cysteine groups. These bonds are responsible for the stabilizing the globular structure and are the strongest type of bond that a protein can possess and are one of the major forces responsible for holding proteins in their respective conformations, and therefore have an important role in protein folding and stability. The typical bond dissociation energy of a disulfide bond ranks at 60 kcal/mole and has a bond length of 2.05 Å. Fairly low energy is required to produce rotations about the S-S bonds, thus these rotations are common. At dihedral angles near 90°, the bonds tend to be more stable. However, the bonds become significantly better oxidants at angles approaching 0° and 180°. Disulfide bonds have been identified in the protein folding in E. Coli. They are used in many processes, including DNA replication.
Disulfide Bonds in cyclic peptides
Most cyclic peptide bonds are formed between disulfide bonds. As a result, the denaturation of cyclic peptides can often be attributed to the stability of disulfide bonds. In the study with the peptide 1 (cyclo(1,4)-Cys-Gly-Phe-Cys-Gly-OH), where it was conducted in buffer solutions between pH 1-11 at 70 degrees C. It was found that the most stability came from pH ~ 3 and a Vshape between pH ~1-~5. As the pH goes from neutral to basic, degradation was found between Gly2-Phe3, which is due to the breaking of disulfide bonds.
