Hair Structure and Hair Chemistry



    Hair Structure

  • We will begin by defining the hair.

  • Hair is composed primarily of proteins (88%).

  • These proteins are of a hard fibrous type known as keratin.

  • Keratin protein is comprised of what we call "polypeptide chains.”

  • The word, polypeptide, comes from the Greek word "poly" meaning many and "peptos" meaning digested or broken down.

  • In essence, if we break down protein, we have individual amino acids.

  • Many (poly) amino acids joined together form a "polypeptide chain".

  • Two amino acids are joined together by a "peptide bond", and the correct number of amino acids placed in their correct order will form a specific protein; i.e. keratin, insulin, collagen and so on.

  • The "alpha helix" is the descriptive term given to the polypeptide chain that forms the keratin protein found in human hair.

  • Its structure is a coiled coil.

  • The amino acids link together to form the coil and 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 amino acid extending to bond with the nitrogen atom of the next amino acid.


    • The A Helix Coil

  • In the organization of a single hair, three "alpha helices" are twisted together to form a "protofibril".

  • This is actually the first fibril structure of the hair.

  • Nine protofibrils are then bundled in a circle around two or more to form an eleven-stranded cable known as the "microfibril".

  • These microfibrils are embedded in an amphorous unorganized protein matrix of high sulfur content, and hundreds of such microfibrils are cemented into an irregular fibrous bundle called a "macrofibril".

  • These macrofibrils are grouped together to form the cortex (or the main body) layers of the hair fiber.

  • Packed dead cells surround these structures and are known as the cuticular layers of the hair.

  • In the center of these structures lies the medullary canal, which is actually apart of the excretory system and houses any foreign debris, heavy metals, synthetics and medications that are thrown off by the body and eventually released through the canal.

Bonding in Keratin Protein

  • When the hair is in its normal unstretched state.

  • It is referred to as A of alpha keratin.

  • The original configuration of the hair is held in place by the bonding found in the cortex layers of the hair.

  • As we stated earlier, keratin protein begins with an alpha helix building into protofibrils, microfibrils, macrofibrils, then cortex layers.

  • The bonds in the hair are located within each and every alpha helix.




    • The Hydrogen Bond
  • The first bond we will discuss is the hydrogen bond.

  • This bond is located between the coils of the alpha helix and is responsible for the ability of the hair to be stretched elasticity) and return back to its original shape.

  • The hydrogen bonds allow us to change the shape of the hair temporarily with the aid of water.

  • These bonds are electrolytically controlled and are the most readily broken down and the most readily reformed.

  • These bonds are responsible for approximately 35% of the strength of the hair and 50% of the hair's elasticity (some would argue up to 99.9% of the hair’s elasticity).

    • The Salt Bond
  • The salt bond is also an ionic (electrolytically controlled) bond formed by the electron transfer from the side chain of a basic amino group (an amino acid with an 00C- group) to the side chain of an acidic amino acid, i.e. NH3+. (This is two positive and negative charges attracting one another.)

  • This occurs in a position paralleled to the axis line of the rotation of the helix of the hair.

  • The salt bond is responsible for approximately 35% of the strength of the hair and 50% of the hair's elasticity.

    • The Cystine Bond
  • The cystine bond also known as the disulfide bond, sulfur bond, or just S bond is formed by cross-links between cystine residues (amino acids) of the main polypeptide chains.

  • This bond is perpendicular to the axis of the hair and between the polypeptide chains.

  • Because of its position in the hair, it is responsible for the hair's toughness or abrasion resistance. (It actually holds the hair fibers together.)

  • These cross-links are frequent in the hair fiber, with maximum of frequency of one cystine bond every four turns of the alpha helix.

  • This is what enables us to permanent wave the hair.

    • The Sugar Bond
  • The sugar bond is formed between the side chain of an amino acid having an OH group and an acidic amino group.

  • This bond is also formed perpendicular to the axis of the hair.

  • Because of its position, it gives the hair toughness but little strength (5%).

  • Some moisture is contributed to the hair as a by-product of this bonding.