Which of the following cannot be traced in proteins?

Proteins are made up of multiple peptide chains. Peptides are polymers made up of amino acids. In a peptide chain amino acids are joined together by peptide bonds. Amino acids are amphoteric compounds having both basic (amino) group as well as acidic (carboxylic) group. Hence the name amino acids. Amino acids are organic compounds made up of carbon, hydrogen, oxygen, nitrogen and sulphur. The amino, carboxylic and substituent groups are attached to the $\alpha$-carbon atom. Oxygen is present in the carboxylic (-COOH) group and nitrogen is present in the amino (-NH2) group. The side chain may also contain oxygen, nitrogen and sulphur in addition to carbon and hydrogen atoms.