6.2 Chemical Composition, Side Bonds and Hair Pigment

Chemical Composition, Side Bonds and Hair Pigment

Exam Focus: The side bond table is the single most examined item in trichology. Know which bonds break with water, which with heat, which with pH change, and which only with chemicals — because that classification is exactly what distinguishes a temporary set from a permanent wave, and it recurs in Chapters 9, 10 and 11.

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1. Chemical Composition & The COHNS Elements

Human hair is composed of a tough, insoluble fibrous protein called keratin. Living cells originate in the hair bulb, absorb amino acids from the dermal papilla, and move upward through the follicle. As they ascend, they undergo keratinization—they fill with keratin protein, dehydrate, lose their nuclei, and die, emerging as the non-living keratin fibers of the hair shaft.

The COHNS Elements

Keratin protein is synthesized from five fundamental chemical elements known by the acronym COHNS:

Peptide Bonds & Polypeptide Chains

• Amino Acids: Units of structure joined together like pop-beads to build proteins.
• Peptide Bonds (End Bonds): Strong chemical bonds that link the amino end of one amino acid to the carboxyl end of an adjacent amino acid. Long chains of amino acids linked by peptide bonds are called polypeptide chains.
• Critical Rule: Cosmetologists must never break peptide bonds during salon services. If end bonds are broken by extreme over-processing or caustic chemicals, the polypeptide chain collapses, causing permanent chemical breakage and dissolving of the hair fiber.

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2. Side Bonds of the Cortex

The cortex consists of millions of polypeptide chains cross-linked together by three types of side bonds. These side bonds account for the hair's overall strength, elasticity, and responsiveness to wet and chemical styling:

┌─────────────────────────────────────────────────────────────────────────────┐
│                        CORTEX SIDE BOND COMPARISON                          │
├─────────────────────────────────────────────────────────────────────────────┤
│ 1. HYDROGEN BOND (Physical)  │ Broken by: Water / Heat   │ Reset: Drying/Cooling │
│ 2. SALT BOND (Physical)      │ Broken by: pH Changes     │ Reset: pH Normalized  │
│ 3. DISULFIDE BOND (Chemical) │ Broken by: Perms/Relaxers │ Reset: Neutralizer    │
└─────────────────────────────────────────────────────────────────────────────┘

1. Hydrogen Bonds

• Nature: Weak, physical, cross-link side bonds formed between neighboring hydrogen and oxygen/nitrogen atoms.
• Action: Easily broken by water (wetting the hair) or thermal heat (flat irons, curling irons, blow dryers). As wet hair dries or heated hair cools, the hydrogen bonds reform in the new mechanical shape.
• Strength: Individually weak, but because they exist in massive quantities, they account for one-third (33%) of the hair's total cross-link strength.

2. Salt Bonds

• Nature: Weak, physical, ionic side bonds formed between positive and negative electrical charges on adjacent polypeptide chains.
• Action: Easily broken by changes in pH (alkaline or acidic solutions). When hair is returned to its normal physiological pH range of 4.5 to 5.5, salt bonds reform automatically.
• Strength: Account for one-third (33%) of the hair's total structural strength.

3. Disulfide Bonds

• Nature: Strong, covalent chemical side bonds joining the sulfur atoms of two adjacent cysteine amino acids to form a single cystine molecule.
• Action: Cannot be broken by heat or water. Disulfide bonds are broken exclusively by chemical texture agents (reducing agents like ammonium thioglycolate or alkaline hydroxide relaxers) and high-lift lighteners. In permanent waving, reduction breaks approximately one-third of disulfide bonds, which are then reformed into their new curled configuration by an oxidizing neutralizer (hydrogen peroxide). Hydroxide relaxers permanently convert disulfide bonds into lanthionine bonds (a process called lanthionization), which can never be reformed.
• Strength: Account for one-third (33%) of the hair's total tensile strength.

3. Putting the Side Bonds to Work

Two inferences that recur through the rest of the guide:

• A style that a shampoo removes was held by hydrogen bonds. This is why humidity ruins a set and why a client who wants curl that survives rain needs a chemical service, not a better set.
• A hydroxide relaxer removes a sulphur atom permanently through lanthionization. The bond is not merely broken and re-formed elsewhere; it is converted into a lanthionine bond that cannot be reversed. That is why hydroxide and thio services are permanently incompatible on the same hair, as Chapter 10 details.

4. Melanin and What It Predicts

All natural hair color is the result of pigment granules called melanin deposited within the cortex by specialized cells called melanocytes in the hair bulb:

• Eumelanin: Provides dark brown and black pigment tones to hair.
• Pheomelanin: Provides lighter, warm tones ranging from yellow and red to strawberry blonde.
• Natural Hair Color: A genetic combination of eumelanin and pheomelanin ratios in the cortex.
• Gray / White Hair (Canities): Occurs when melanocytes gradually stop producing melanin pigment. The hair shaft contains only colorless keratin protein and tiny optical air pockets that reflect light, creating a white or silver appearance.

Why pigment prediction matters before you open a colour tube

Underlying pigment is not a matter of opinion. When melanin is oxidised by a lightener or a permanent colour, it is diffused in a predictable order — black, brown, red, orange, gold, yellow, pale yellow. That sequence is why:

• a client at natural level 4 who wants a level 8 blonde will pass through orange on the way, and needs either a longer lift or a toner to neutralise it;
• naturally red-toned hair takes longer to lift past its warm stages, because pheomelanin is more resistant to oxidation than eumelanin;
• the level of the natural hair, not the level on the swatch, determines the underlying pigment that will be exposed.

Chapter 11 works this through in detail with the level system and the law of colour. The point to fix here is anatomical: the pigment is in the cortex, and any service that changes it must first get through the cuticle.