Calculate the number of disulfide bond arrangements possible for each of the polypeptides listed below, assuming that each of the cysteine residues forms a disulfide bridge. Also assume that all disulfide bridges are intramolecular, not intermolecular.

a. a polypeptide with four cysteines

b. a polypeptide with six cysteines

c. a polypeptide with 8 cysteines

d. a polypeptide with 10 cysteines

a. a polypeptide with four cysteines has two disulfide bonds

b. a polypeptide with six cysteines has three disulfide bonds

c. a polypeptide with 8 cysteines has four disulfide bonds

d. a polypeptide with 10 cysteines has five disulfide bonds

Explanation:

A disulfide bond is formed by two sulphur atoms linked covalently. Since it specifically require two linked sulphur atoms, and cysteine contains just one sulphur atom, then a polypeptide with an even number of cysteine molecules will have disulfide bonds halved the number of cysteine molecules.

Therefore, a polypeptide with even numbers of cysteine molecules like:

4 has 2 disulfide bonds;

6 has 3 disulfide bonds:

8 has 4 disulfide bonds;

10 has 5 disulfide bonds