Hair is made up of around 80% alpha-keratin, a collection of multi-layered, flattened cells connected by rope like filaments that provide the structure and strength of the hair fibre. Alpha-keratin synthesis begins near the adhesion point of the base of the hair fibre to the cell membrane. Two polypeptide chains (made up of alanine, leucine, arginine and cysteine) twist together to form a structure called a coiled curl dimer, these coiled curl dimers are bonded together with disulphide bonds that can only occur between the cysteine residues.
Two coiled curl dimers align with each other, using disulphide bonds, forming a protofilament. Through further disulphide bonding, alignment of two protofilaments allow the formation of a protofibril. Four protofibrils then combine to form an intermediate filament unit, this is the basic alpha-keratin subunit.
The interfilamentous units are embedded in a matrix formed from coiled curl fibres that are dominant in either cysteine, glycine, tyrosine or phenylalanine. The different amino acid content matrixes account for the variability in strength of the alpha-keratin protein.
Coiled curl fibres high in cysteine will provide the strongest base for an interfilamentous protein. Properly bound filaments build a thick and smooth hair fibre with high tensile strength resistant to breakage and damage. Units that are higher in cysteine will form strong, water-repellent fibres less vulnerable to the detrimental effects of shampoos, heated styling tools and chemical treatments.