Hair is comprised of up to 95% keratin protein. A protein is comprised of a chain of amino acids bonded together in a specific sequence to form a specific protein.
Hair has 18-23 different amino acids with cystine accounting
for as much as 4% of the protein which in turn will account for
up to 90% of the hairs overall strength. 2 amino acids join
together through a "peptide bond".
The colour of the hair depends on 2 other proteins found in hair
most commonly known as melanin. Melanin accounts for 1% of the
hairs composition. Eumelanin will cause hair to be brown-black
where phaeomelanin will account for blonde-red. The absence of either type produces white/gray hair.
The alpha helix is the descriptive term given to the polypeptide chain that forms the keratin protein or human
hair. It is a coiled structure where the amino acids link together to form the coil. There are approximately 3-6
amino acids per turn of the helix "coil". Each amino acid is connected together by a "peptide bond".
The peptide bond is located between the carbon atom of one of the amino acid extending bond with the
nitrogen atom of the next amino acid.

The cystine bond also known as the disulfide bond, sulfur bond, or just the S bond is formed by crosslinks between the cystine amino acid of the main polypeptide chain. This bond is perpendicular to the axis of the hair and between the polypeptide chains. Cystine is rich with sulfur which plays a crucial role in bond formation. It is produced by the keratinocytes. These cells are located in the bottom of the derma papilla
(DP). Due to its position in the hair, it is responsible for the hair's strength or abrasion resistance. It actually holds hair fibers together. These crosslinks are frequent in the hair fiber, with a maximum of frequency of one cystine bond every four turns of the alpha helix. It is this bond that holds your hair together and gives rise to the hairs overall strength accounting for up to 90% of the strength.
It is essential to break this bond in order to change your hairs appearance. Whether you are adding or removing colour, perming or straightening your hair this bond must be broken. Unfortunately, not replacing this bond will account for your hair feeling dry and damaged.
