9.1 Keratin Chemistry, Side Bonds, & Disulfide Restructuring

Keratin Chemistry, Side Bonds, & Disulfide Restructuring

High-Yield Exam Focus: Chemical texture services alter the natural curl pattern of the hair by chemically restructuring the side bonds within the cortex. Cosmetology licensing examinations heavily emphasize the three types of side bonds (hydrogen, salt, and disulfide), the logarithmic nature of the pH scale, and the chemical reduction reaction that breaks disulfide cross-links without compromising polypeptide end bonds.

All chemical services—whether permanent waving, chemical hair relaxing, or curl reforming—rely on a profound understanding of trichology, organic chemistry, and molecular physics. To change the structure of human hair without destroying its structural integrity, a licensed cosmetologist must master the chemical architecture of keratin protein.

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1. Chemical Composition & Architectural Hierarchy of Hair

Human hair is composed of roughly 90% keratin protein, a fibrous structural protein that provides strength, elasticity, and resilience. Keratin is synthesized from complex combinations of five elemental building blocks known as the COHNS elements:

• Carbon (C): 51%
• Oxygen (O): 21%
• Nitrogen (N): 17%
• Hydrogen (H): 6%
• Sulfur (S): 5%

Amino Acids and Peptide (End) Bonds

Amino acids are organic compounds made of carbon, oxygen, hydrogen, and nitrogen. In the hair follicle, amino acids link together end-to-end like boxcars on a train through chemical connections called peptide bonds (also known as end bonds):

• Polypeptide Chains: When hundreds or thousands of amino acids join via peptide bonds, they form long, spiral-shaped chains termed polypeptide chains.
• Helix Structure: Polypeptide chains coil into a spring-like helical configuration (protofibrils and microfibrils), which bundle together to form the cortex of the hair fiber.
• The Golden Rule of Chemical Texturizing: Peptide bonds must NEVER be broken during a salon service. If chemical solutions break peptide bonds, the polypeptide chains disintegrate, resulting in complete dissolution, irreversible fiber breakdown, and severe hair loss.

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2. The Three Types of Side Bonds in the Cortex

The hair shaft consists of three primary layers: the protective outer cuticle, the thick middle cortex containing pigment and fibrous strength, and the central medulla (which may be absent in fine hair). The cortex accounts for roughly 80% to 90% of the hair's overall weight and houses millions of parallel polypeptide chains.

These polypeptide chains are cross-linked by three distinct types of side bonds that provide elasticity and tensile resistance. Each bond type accounts for approximately one-third of the hair's total structural strength:

Hydrogen Bonds

Hydrogen bonds are weak physical side bonds resulting from the attraction between opposite electrical charges (a slightly positive hydrogen atom attracted to a slightly negative oxygen or nitrogen atom on an adjacent chain). Because they are physical bonds, they are easily broken by wetting the hair with water or applying thermal styling heat. When the hair dries or cools into a new shape, hydrogen bonds reform in that temporary position. Although individually weak, their sheer abundance accounts for one-third of the hair's total strength.

Salt Bonds

Salt bonds are weak ionic physical side bonds formed by the attraction between positive amino acid groups and negative acidic groups on adjacent polypeptide chains. Salt bonds depend strictly on pH balance. They are easily broken by shifts in pH—either strong acidic solutions or alkaline chemicals. When the hair is restored to its natural, acidic pH range of 4.5 to 5.5, salt bonds naturally reform. Like hydrogen bonds, they account for approximately one-third of the hair's structural resistance.

Disulfide Bonds

Disulfide bonds are strong, covalent chemical side bonds formed between two sulfur atoms of adjacent cysteine amino acids. When two cysteine molecules bond together via their sulfur atoms, they create a single amino acid called cystine:

• Resistance: Unlike physical bonds, disulfide bonds cannot be broken by water, atmospheric moisture, or normal heat styling.
• Chemical Modification: Disulfide bonds can only be altered by chemical reducing agents (such as thioglycolic acid, ammonium thioglycolate, or bisulfites) or destructive extreme thermal heat (above 450°F / 232°C).
• Significance: Disulfide bonds account for one-third of hair strength and 100% of the permanent change achieved during permanent waving and chemical relaxing services.

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3. The pH Scale & Cuticle Penetration in Chemical Services

To alter cortex disulfide bonds, texturizing chemicals must first bypass the hair's outer cuticle, which consists of shingle-like, overlapping scales rich in hydrophobic lipids.

Understanding the Logarithmic pH Scale

The term pH (potential hydrogen) represents the concentration of hydrogen ions in an aqueous solution on a scale from 0 to 14:

• Acidic: 0.0 to 6.9 (high concentration of hydrogen ions $H^+$)
• Neutral: 7.0 (pure distilled water; equal balance of $H^+$ and $OH^-$ ions)
• Alkaline: 7.1 to 14.0 (high concentration of hydroxide ions $OH^-$)
• Natural Hair & Scalp: Hair maintains a natural acid mantle with a pH between 4.5 and 5.5.

Exam Math Concept: The pH scale is logarithmic. Each whole unit change represents a tenfold (10×) change in ion concentration. A pH of 6.5 is 10 times more alkaline than 5.5. A pH of 7.5 is 100 times ($10 \times 10$) more alkaline than 5.5. A traditional alkaline permanent wave with a pH of 9.5 is 10,000 times ($10^4$) more alkaline than natural hair!

Action of Alkaline Solutions on Hair Structure

Chemical texturizers must possess an alkaline pH to:

1. Softens and Swells: Alkaline solutions soften and physically expand the tight keratin protein of the hair cuticle.
2. Lifts Cuticle Scales: Cuticle scales flare outward, creating macroscopic gaps between the plates.
3. Cortex Penetration: The active chemical reducing agent diffuses past the cuticle directly into the cortex where disulfide cross-links reside.

If a solution is too alkaline, or left on porous hair too long, the cuticle swells past its elastic limit, causing permanent cuticle erosion, frizz, porosity damage, and cortex breakage.

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4. The Chemistry of Reduction: Breaking Disulfide Bonds

Once inside the cortex, permanent wave lotion initiates a specific chemical reaction termed reduction.

Reduction vs. Oxidation

In classical chemistry, oxidation and reduction always occur simultaneously:

• Reduction: The chemical addition of hydrogen ($+H$) or the chemical removal of oxygen ($-O$).
• Oxidation: The chemical addition of oxygen ($+O$) or the chemical removal of hydrogen ($-H$).

The Permanent Wave Reduction Reaction

The active chemical agent in permanent waving is a reducing agent. The reducing agent supplies free hydrogen atoms to the hair's disulfide cross-links:

$\text{R-S-S-R (Cystine Disulfide Bond)} + 2[\text{H}] \longrightarrow \text{R-S-H} + \text{H-S-R (Two Cysteine Mercaptan Groups)}$

1. Two hydrogen atoms from the reducing agent attach to the two sulfur atoms in the disulfide bond.
2. The covalent bond linking the sulfur atoms breaks apart.
3. The single disulfide cross-link splits into two independent sulfhydryl (mercaptan) groups.
4. With cross-links detached, the parallel polypeptide chains can slide freely past one another, adopting the curved physical shape dictated by the perm rod.