The Three Different Types Of Side Bonds Are

The three different types of side bonds are hydrogen bonds, salt bonds, and disulfide bonds. These cross-linkages form the structural framework of the hair’s cortex, determining everything from elasticity and strength to texture and the hair’s ability to hold a style. Understanding these bonds is fundamental for cosmetologists, chemists, and anyone serious about hair health, because every chemical service—from perming and relaxing to coloring and daily heat styling—relies on manipulating these specific connections.

Introduction: The Architecture of Hair Strength

Human hair is primarily composed of keratin, a fibrous structural protein. Without these lateral connections, hair would lack the cohesion necessary to function as a durable fiber. Think of the polypeptide chains as the rungs of a ladder; the side bonds are the rungs connecting the two side rails. Still, while the peptide bonds (end bonds) link amino acids together in long polypeptide chains to form the backbone of the protein, it is the side bonds (cross bonds) that link these chains laterally to one another. There are three distinct categories of these cross-linkages, categorized by their chemical nature and relative strength: hydrogen bonds, salt bonds (ionic bonds), and disulfide bonds (covalent bonds) Simple, but easy to overlook..

1. Hydrogen Bonds: The Physical, Temporary Links

Hydrogen bonds are the most numerous but the weakest of the three side bonds. They account for approximately one-third of the hair’s total strength and fifty percent of its elasticity. Chemically, a hydrogen bond is an electrostatic attraction between a hydrogen atom (which carries a partial positive charge) covalently bonded to an electronegative atom (like oxygen or nitrogen) and another nearby electronegative atom Simple, but easy to overlook..

The Mechanism

In the hair cortex, these bonds form between the oxygen of a carbonyl group (C=O) on one polypeptide chain and the hydrogen of an amide group (N-H) on an adjacent chain. Because they are based on dipole-dipole attractions rather than shared electrons, they are classified as physical bonds rather than chemical bonds Less friction, more output..

Water and Heat: The Disruptors

The defining characteristic of hydrogen bonds is their extreme sensitivity to water and heat Small thing, real impact..

• Water: When hair gets wet, water molecules penetrate the cortex and insert themselves between the polypeptide chains. Water molecules are polar and effectively "compete" for the hydrogen bonding sites, breaking the existing bonds between keratin chains. This is why wet hair stretches significantly (high elasticity) and loses its styled shape.
• Heat: Thermal energy provides enough kinetic energy to overcome the weak electrostatic attraction. Applying heat (blow drying, flat ironing) breaks hydrogen bonds, allowing the chains to shift into a new configuration. As the hair cools, the bonds reform in the new position, "setting" the style.

Practical Implication

This physical reversibility is the basis of wet setting, blowouts, and heat styling. No chemical reaction occurs; the polypeptide chains simply slide past one another and re-lock via hydrogen bonding in a new arrangement. Still, because they reform easily with moisture from humidity, styles held solely by hydrogen bonds are temporary Nothing fancy..

2. Salt Bonds (Ionic Bonds): The pH-Dependent Links

Salt bonds, also known as ionic bonds or electrostatic bonds, also account for roughly one-third of the hair’s strength. Unlike hydrogen bonds, salt bonds are formed by the attraction between positively and negatively charged amino acid side chains (R-groups) on adjacent polypeptide chains And it works..

The Mechanism

Amino acids have side chains that can be acidic (carrying a negative charge, e.g., aspartic acid, glutamic acid) or basic (carrying a positive charge, e.g., lysine, arginine, histidine). When an acidic side chain (COO⁻) aligns near a basic side chain (NH₃⁺), a strong electrostatic attraction forms—a salt bridge It's one of those things that adds up..

pH Sensitivity: The Isoelectric Point

Salt bonds are highly dependent on the pH of the hair environment. Hair has an isoelectric point (pI) of approximately 4.5 to 5.5. At this slightly acidic pH, the number of positive and negative charges on the amino acid side chains is balanced, maximizing the number of salt bonds and giving hair its maximum strength and hardness.

• Alkaline Solutions (High pH): Hydroxide ions (OH⁻) neutralize the positive charges (NH₃⁺ becomes NH₂). The attraction vanishes, the bonds break, and the hair swells and softens. This is the principle behind alkaline perms and relaxers.
• Strongly Acidic Solutions (Low pH): Excess hydrogen ions (H⁺) neutralize the negative charges (COO⁻ becomes COOH). The bonds break again, causing the hair to contract and harden (acid hardening).

Practical Implication

Maintaining the acid mantle (pH 4.5–5.5) is critical for preserving salt bonds. Shampoos and conditioners formulated at this pH help keep the cuticle closed and the ionic cross-links intact. Chemical services intentionally raise the pH to break salt bonds, allowing the hair to swell so that reducing agents can reach the deeper disulfide bonds.

3. Disulfide Bonds: The Permanent, Chemical Anchors

Disulfide bonds (cystine bonds) are the strongest and most permanent of the side bonds. While fewer in number than hydrogen or salt bonds, they provide the primary structural integrity, abrasion resistance, and permanent shape of the hair fiber. They account for the final one-third of hair’s strength but are the only bonds that survive wetting, drying, and thermal styling without breaking Not complicated — just consistent..

The Chemistry of Cystine

Disulfide bonds form exclusively between two molecules of the sulfur-containing amino acid cysteine. When two cysteine residues on adjacent polypeptide chains come into close proximity, an oxidation reaction occurs. The hydrogen atoms on the sulfur atoms (–SH groups) are removed, and the two sulfur atoms link together to form a disulfide bridge (–S–S–). $ 2 \text{ Cysteine } (–SH) \xrightarrow{\text{Oxidation}} \text{ Cystine } (–S–S–) + 2H^+ + 2e^- $

It's a covalent bond—a sharing of electrons between atoms. It is roughly 10 to 15 times stronger than a hydrogen or salt bond The details matter here..

Chemical Alteration: Reduction and Oxidation

Because they are covalent, disulfide bonds cannot be broken by water, heat, or pH changes alone. They require a chemical reaction:

1. Reduction (Breaking): A reducing agent (thiol compounds like ammonium thioglycolate, glyceryl monothioglycolate, or cysteine hydrochloride) donates hydrogen atoms to the disulfide bridge. This cleaves the S–S bond, converting cystine back into two cysteine residues (–SH). The polypeptide chains are now free to move. This is the "processing" phase of a perm or relaxer.
2. Oxidation (Reforming): An oxidizing agent (usually hydrogen peroxide or sodium bromate) removes the hydrogen atoms from the cysteine –SH groups, forcing the sulfur atoms to re-bond into new disulfide bridges. This "neutralization" phase locks the hair into its new shape (curly or straight).

The "Lanthionine" Warning

In hydroxide relaxers (lye or no-lye), the extremely high pH breaks disulfide bonds via a different mechanism (beta-elimination), converting cystine into lanthionine. This bond cannot be reformed. This is why hydroxide relaxers are permanent and hair relaxed with them can never be permed later—the chemical architecture has been irreversibly altered.

Comparative Summary: The Hierarchy of Bonds

| Feature | Hydrogen Bonds | Salt (Ionic) Bonds | Disulfide

Completing the Comparative Summary

*Strength values are approximate and expressed as the energy required to break a single bond in a typical hair protein environment The details matter here..

How Everyday Hair Care Interacts with These Bonds

1. Cleansing and Conditioning

   • Most shampoos are formulated near the hair’s isoelectric point (pH ≈ 4.5–5.5). At this pH, hydrogen bonds are abundant while salt bridges remain stable, allowing the cuticle to lie flat and the cortex to retain its natural alignment.
   • Conditioners often contain cationic surfactants and polymeric film‑formers that preferentially interact with the negatively charged hair surface, reinforcing salt bridges and creating a protective coating that mitigates humidity‑induced hydrogen‑bond disruption.

2. Heat Styling

   • Temperatures above 150 °C can temporarily break hydrogen bonds, causing the hair to lose its temporary shape (e.g., a blown‑dry straight style). The effect is reversible once the hair cools, because the bonds re‑form as the temperature drops.
   • Disulfide bonds, however, remain intact under the same heat, which is why a flat‑iron can straighten curly hair without permanently breaking the covalent links; the shape change is due to temporary rearrangement of weaker bonds, not covalent alteration.

3. Chemical Texturising

   • Perming: A reducing agent (commonly ammonium thioglycolate) delivers electrons to the disulfide bridges, cleaving them into free thiol groups. The hair is then wrapped around rods, allowing the cortex to assume a new shape while the weaker hydrogen and salt bonds re‑orient. Oxidation with hydrogen peroxide re‑creates the –S–S– linkages, “locking” the new configuration.
   • Relaxing: In a lye‑based relaxer, the high pH triggers β‑elimination, converting cystine into lanthionine, a non‑reversible bond. The hair’s architecture is permanently altered, which is why previously relaxed strands cannot be curled again without further chemical modification.

4. Color and Bleaching

   • Oxidative hair dyes rely on hydrogen peroxide to open the cuticle and generate reactive oxygen species that penetrate the cortex. While the primary target is melanin, the same oxidative environment can transiently affect disulfide bonds, especially in previously chemically treated hair. Proper post‑color conditioning helps re‑establish hydrogen and salt bridges, preserving elasticity.

The Consequences of Bond Disruption

• Excessive Reduction (over‑processing with strong reducers, frequent perming, or repeated relaxer applications) depletes the cysteine reservoir, leading to a “brittle” cortex. The hair’s ability to retain moisture diminishes, and the strand is prone to split ends and breakage.
• Irreversible Lanthionine Formation (as in harsh hydroxide relaxers) eliminates the possibility of re‑establishing the original disulfide network. The hair becomes more fragile because the new bond cannot be reinforced through ordinary styling; it must be rebuilt with specialized protein‑rich treatments.
• Environmental Stressors such as high humidity or frequent washing can repeatedly break hydrogen bonds, causing the hair to lose its styled shape and appear frizzy. While this does not damage the disulfide framework, it can exacerbate wear on the weaker bonds, making the fiber more susceptible to mechanical stress.

Strategies for Preserving Bond Integrity

1. pH‑Balanced Products – Using shampoos and conditioners that stay within the 4–6 pH window helps keep hydrogen and salt bonds stable, reducing unnecessary bond fatigue.
2. Controlled Heat – Applying heat protectants that create a thin barrier on the hair surface lessens the direct impact of high temperatures on hydrogen bonds, preserving the style longer with less structural strain.
3. Targeted Bond‑Rebuilding Treatments – Modern “bond‑repair” formulas contain low‑molecular‑weight cysteine precursors, hydrolyzed keratin, or stabilized disulfide‑forming agents that can re‑link broken disulfide bridges after a chemical service. While they cannot reverse lanthionine formation, they can replenish cysteine availability and reinforce existing disulfide networks.
4. Gentle Handling – Minimising friction (e.g., using silk pillowcases, wide‑tooth combs) reduces mechanical stress that can snap weakened hydrogen or salt bonds, especially in hair that has undergone multiple chemical processes.

Conclusion

Hair’s resilience is a layered architecture built upon three distinct types of side bonds. In practice, hydrogen bonds provide a fleeting, moisture‑dependent scaffolding that can be reshaped with heat or humidity. Salt (ionic) bonds add a modest but crucial stabilising force, particularly in textured hair, and are readily modulated by pH changes. Disulfide bonds, forged through covalent cysteine linkages, deliver the enduring shape and strength that define a haircut’s final silhouette; they are the only bonds that survive wetting, drying, and most forms of thermal styling, yet they demand specific redox chemistry to break or reform Small thing, real impact..

Understanding this hierarchy enables a more informed approach to hair care. On the flip side, by selecting products that respect the natural pH of the scalp and hair, limiting unnecessary heat exposure, and employing strategic chemical treatments, one can preserve the weaker bonds while safeguarding the strong disulfide network. When damage does occur—whether through over‑processing, harsh relaxers, or relentless environmental stress—the remedy lies in carefully timed bond‑rebuilding interventions and gentle, pH‑balanced maintenance. In sum, the health and style of hair hinge on a delicate balance among hydrogen, salt, and disulfide bonds, each playing a unique yet complementary role in the overall integrity of the fiber And it works..