Black Hair Structure
We will begin by defining the black hair structure. Black 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". Black hair has 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.
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