Application for Organic Nitrogen compounds - Role of Protein in Hair

Protein is made up of polypeptides which is a polymer composed of amino acids. In other words, amino acids are building blocks of proteins. There are currently 22 natural occurring amino acids.

There are generally two classes of proteins, namely the fibrous proteins and globular proteins. Fibrous proteins are structural proteins which are filament-like and elongated in shape and confer mechanical properties on the proteins due to the secondary structure that the amino acids sequence favour. Globular proteins are folded into compact structures and it is typically used for synthesis, transport and catabolism.

 Disulfide bond formed between two cysteine molecules

The major protein found in hair and fingernails is alpha-keratin which is a fibrous protein. Its secondary structure is predominantly alpha-helices. The protein strands within hair and other alpha-keratins are crosslinked to some extent by covalent bonds between cysteine residues to form disulfide bonds. The chemical composition of the cysteine residues in alpha-keratin affects its macromolecular structure and function. For example, the characteristics of hair (straight, curly etc) is based on the disulfide bonds present in the hair protein. The more such disulfide bonds there are between the strands, the more rigid the protein becomes as a whole.

Coil Structure of the alpha-keratin in hair

Credits: http://www.pearsonhighered.com/mathews/ch06/c06fp.htm

The disulfide bonds are very strong covalent bonds which cannot be broken by simply heating. In order to curl straight hair, the disulphide bonds have to be broken chemically by reduction, form the desired shape by curling and form new disulfide bonds by oxidation to maintain the new shape.