5.2 Keratin Chemistry, Peptide Bonds & the Three Chemical Side Bonds

5.2 Keratin Chemistry, Peptide Bonds & the Three Chemical Side Bonds

4. Keratinization & Chemical Composition

Living hair cells originate within the hair bulb immediately above the dermal papilla. Through active cellular division (mitosis), newly generated cells continuously push older cells upward within the follicle. As these cells migrate toward the scalp surface, they undergo keratinization:

• Cells become densely packed with fibrous keratin protein.
• Cells gradually dehydrate, lose their active cellular nuclei and organelles, and flatten.
• By the time the hair strand emerges past the epidermal surface of the scalp, the cells are completely dead, hardened protein fibers.

The Elemental COHNS Composition

Human hair is composed primarily of keratin, a specialized structural protein containing five basic chemical elements known by the acronym COHNS:

Amino Acids and Peptide Bonds (End Bonds)

Proteins are long polymer chains built from twenty-one physiological amino acids. Amino acids join end-to-end like railroad cars via strong chemical bonds known as peptide bonds (or end bonds). A long chain of amino acids joined together by peptide bonds forms a polypeptide chain.

Critical Exam Rule: Cosmetologists must NEVER break peptide bonds. Peptide bonds form the primary longitudinal backbone of the hair fiber. If chemical over-processing, excessive caustic alkalis, or extreme thermal heat breaks peptide bonds, the polypeptide chain breaks apart. The hair shaft loses all structural integrity, resulting in severe dissolution, melting, or complete chemical breakage.

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5. The Three Chemical Side Bonds: Hydrogen, Salt & Disulfide

While peptide bonds join amino acids end-to-end longitudinally, the parallel polypeptide chains within the cortex are linked crosswise like the rungs of a ladder by three distinct types of side bonds. These side bonds are responsible for hair elasticity, structural memory, and mechanical resistance.

                  Polypeptide Chain A              Polypeptide Chain B
                           │                                │
                           ├─── [ Hydrogen Bond ] ──────────┤  (Weak physical; 1/3 strength)
                           │    (Broken by water/heat)      │
                           │                                │
                           ├─── [ Salt Bond ] ──────────────┤  (Weak physical; 1/3 strength)
                           │    (Broken by pH shifts)       │
                           │                                │
                           └─── [ Disulfide Bond ] ─────────┘  (Strong chemical; 1/3 strength)
                                (Broken by reducers/alkalis)

1. Hydrogen Bonds (Weak Physical Side Bonds)

• Nature & Strength: A hydrogen bond is a weak, physical, cross-linking side bond formed between adjacent positive hydrogen atoms and negative oxygen atoms on neighboring polypeptide chains. Although individually weak, hydrogen bonds exist in vast quantities, accounting for approximately one-third (33%) of the hair's overall cross-linking strength.
• Action: Hydrogen bonds are easily broken by the application of water or thermal heat. When hair is shampooed or wetted, hydrogen bonds break, allowing the hair to be stretched and wrapped around rollers or molded into finger waves. As the hair dries or cools, these hydrogen bonds reform into the new physical configuration. This principle governs all wet styling, blow drying, and curling iron work.

2. Salt Bonds (Weak Physical Side Bonds)

• Nature & Strength: A salt bond is also a weak, physical, cross-linking side bond formed by the ionic electrical attraction between opposing positive and negative charges of acidic and basic amino acid side chains. Salt bonds account for approximately one-third (33%) of the hair's total tensile strength.
• Action: Salt bonds are severed by changes in pH. Exposure to strong alkaline solutions (such as bleaching lighteners, perm lotions, or relaxers) or strong acidic solutions breaks salt bonds by altering electrical charges. Salt bonds automatically reform when the hair shaft is returned to its normal physiological pH level (pH 4.5–5.5) using balancing neutralizers or conditioning rinses.

3. Disulfide Bonds (Strong Chemical Side Bonds)

• Nature & Strength: A disulfide bond is a strong, covalent, chemical cross-linking bond formed between the sulfur atoms of two adjacent cysteine amino acids, joining them to create cystine. While fewer in number than hydrogen or salt bonds, disulfide bonds account for one-third (33%) of the hair's strength and provide its permanent structural memory and curl configuration.
• Action: Disulfide bonds cannot be broken by water, mechanical tension, or mild heat. They can only be severed by chemical reducing agents (such as ammonium thioglycolate or cysteamine in permanent waving and thio relaxers) or by strong alkaline hydroxides. In permanent waving, reducing agents break disulfide bonds so the cortex can take the shape of the perm rod; chemical oxidation (neutralization with hydrogen peroxide) then reforms the disulfide bonds in their new curled geometry.
• Lanthionization: When hair is treated with hydroxide chemical relaxers (sodium, potassium, lithium, or guanidine hydroxide), the hydroxide ion permanently removes one sulfur atom from a disulfide bond, converting it into a lanthionine bond. Lanthionine bonds can never be reformed. Disulfide bonds broken by hydroxide relaxers are permanently altered.

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6. Exam Scenario Breakdowns & Technical Traps

Exam Trap: Peptide Bonds vs. Side Bonds Examination questions frequently ask which bonds are altered during routine salon wet styling or permanent waving. Never confuse end bonds (peptide bonds) with side bonds:

• Side bonds (hydrogen, salt, disulfide) are routinely broken and reformed during styling, perming, and relaxing.
• End bonds (peptide bonds) join amino acids together longitudinally; breaking peptide bonds destroys the hair protein and causes irreversible chemical melting and catastrophic breakage.

Exam Trap: Lanthionization Chemistry If an exam scenario asks whether a neutralizer can reform disulfide bonds after a client receives a sodium hydroxide relaxer, the answer is NO. Hydroxide relaxers do not break disulfide bonds reversibly; they permanently convert cystine into lanthionine (lanthionization). Neutralizers used after hydroxide relaxers are neutralizing shampoos designed strictly to lower pH and remove chemical residue, not oxidizing agents that reform bonds.