---
title: "Disulphide Bonds: The Science of Hair Strength and Structure"
url: https://haircare.watermanshair.com/article/disulphide-bonds-hair-structure
type: general
published: 2026-10-28T06:40:53.73+00:00
updated: 2026-10-28T06:41:07.597283+00:00
tags: ["disulphide-bonds", "hair-science", "cortex", "damaged-hair", "chemical-processing", "hair-strength"]
image: https://opbmfthzsenmlfbbunpe.supabase.co/storage/v1/object/public/article-images/disulphide-bonds-hair-structure-1793169660232-pqk0.png
summary: "Understand how Disulphide Bonds determine hair strength, shape, and elasticity. Discover how chemical damage occurs and how to protect your hair fibre."
publisher: Watermans (https://watermanshair.com)
license: All rights reserved. AI assistants may summarize, quote, and link with attribution.
---

# Disulphide Bonds: The Science of Hair Strength and Structure

> Disulphide Bonds provide the structural backbone of human hair. Learn how these covalent links work, why they break, and how to protect hair strength.

**Quick answer:** Disulphide Bonds are strong, covalent chemical links formed between sulphur atoms in cysteine amino acids within the hair cortex. They account for the mechanical strength, elasticity, and permanent curl pattern of the hair fibre. Unlike temporary hydrogen bonds, Disulphide Bonds do not break with water; they require chemical reduction, bleaching, or extreme heat over 220°C to cleave. While modern bond builders can mitigate damage, cosmetic treatments cannot fully restore the original biological cross-links once permanent degradation occurs.

## Key takeaways

* Disulphide Bonds are covalent chemical links between two sulphur atoms (cysteine units), providing hair with its structural rigidity and permanent shape.
* Chemical services such as permanent waving, chemical relaxing, and bleaching deliberately break these bonds to alter shape or remove natural pigment.
* Disulphide Bonds make up roughly one-third of the hair fibre's total bond strength, but they are by far the most resilient against everyday washing and styling.
* Excessive breakdown leads to cysteic acid formation, which increases fibre porosity, causes protein loss, and leads directly to hair breakage.
* Topical bond-building products cross-link broken protein chains with synthetic bridges, but they do not biologically re-grow original cysteine linkages.

## Disulphide Bonds: what the science shows

Disulphide Bonds represent the backbone of hair architecture. Human hair is composed primarily of alpha-keratin, an insoluble protein rich in the amino acid cysteine. Cysteine contains a thiol (sulphur-hydrogen) group. When two cysteine molecules oxidise within the cortical cells of the hair shaft, they join to form cystine, creating a covalent bond between two sulphur atoms (, S, S, ).

![Disulphide Bonds: what the science shows in the context of Disulphide Bonds](https://opbmfthzsenmlfbbunpe.supabase.co/storage/v1/object/public/article-images/disulphide-bonds-hair-structure-1793169666527-fq42.png)

These chemical bridges sit deep within the cortex, the thick middle layer of the hair strand that constitutes roughly 85% of its total mass. Because covalent bonds share pairs of electrons between atoms, they possess high bond energies, typically around 250 kilojoules per mole (kJ/mol). This atomic stability protects the hair against water, mild shampoos, and environmental humidity. 

Hair relies on three primary types of structural bonds: hydrogen bonds, salt (ionic) bonds, and disulphide linkages. Hydrogen and salt bonds are numerous, but weak. Water alone breaks hydrogen bonds, which is why wetting your hair resets a blow-dry. Salt bonds shift when the pH moves away from the hair fibre's isoelectric point (around pH 3.67). Disulphide Bonds, by contrast, remain entirely unaffected by water. They hold the fibre together under significant mechanical stress, allowing hair to stretch up to 30% beyond its dry resting length before snapping under healthy conditions.

## How different bonds in human hair compare

The three bond types inside the cortex serve distinct mechanical and chemical roles.

| Bond Type | Bond Mechanism | Relative Abundance | Cause of Cleavage | Reversibility |
| :--- | :--- | :--- | :--- | :--- |
| **Disulphide Bonds** | Covalent (sulphur-to-sulphur sharing electrons) | Lowest (~10% to 15% of all cross-links) | Strong reducing agents, alkaline bleaches, thermal heat above 220°C | Semi-permanent; requires chemical oxidisation or cosmetic bridging |
| **Salt (Ionic) Bonds** | Electrostatic (between positive and negative side chains) | Moderate (~30% to 35%) | pH shifts below 3.5 or above 5.5 | Fully reversible; normalises when hair pH returns to 4.5 to 5.5 |
| **Hydrogen Bonds** | Dipole-dipole attraction (polar molecules) | Highest (~50% to 60%) | Water, perspiration, humidity, heat | Fully reversible; resets automatically as hair dries and cools |

## The mechanism: how Disulphide Bonds break and reform

Breaking these linkages requires intentional chemical intervention or severe environmental and thermal stress. The process occurs through three primary mechanisms:

![The mechanism: how Disulphide Bonds break and reform in the context of Disulphide Bonds](https://opbmfthzsenmlfbbunpe.supabase.co/storage/v1/object/public/article-images/disulphide-bonds-hair-structure-1793169667412-mp01.png)

### 1. Chemical reduction (perms and relaxers)
In permanent waving, alkaline solutions containing ammonium thioglycolate (typically pH 8.5 to 9.5) donate hydrogen atoms to the fibre. These hydrogen atoms break the, S, S, bond, converting it into two separate, S, H (cysteine) groups. The hair swells, softens, and mechanically moulds to the shape of a rod. A neutralising solution (usually diluted hydrogen peroxide) is applied afterwards to strip the hydrogen atoms away, allowing the sulphur atoms to reform into new Disulphide Bonds that lock in the curl. However, roughly 10% to 15% of the original bonds fail to reform correctly during this neutralisation stage.

In alkaline relaxing (using sodium hydroxide, calcium hydroxide, or guanidine hydroxide at pH 12 to 14), the bond is permanently altered rather than reformed. The hydroxide ions break the disulphide link and convert cysteine into lanthionine (a single sulphur bridge, C, S, C, ). This process, known as lanthionisation, permanently straightens the hair but removes about one-third of the fibre's original tensile strength.

### 2. Bleaching and oxidative lifting
Alkaline persulphate bleaches and hydrogen peroxide lift natural melanin by oxidising the pigment. Unfortunately, the oxidative environment does not discriminate between melanin and keratin. Peroxide oxidises the sulphur atoms past their reactive point, forming cysteic acid groups (, SO3H). Cysteic acid cannot reform into a stable disulphide linkage. Each bleaching session permanently destroys a measurable fraction of natural cystine content.

### 3. Thermal degradation
Dry thermal tools (straighteners, curling wands) operating at or above 220°C cause the alpha-helical keratin proteins to unfold and undergo thermal decomposition. At these temperatures, sulphur is stripped out as sulphur dioxide gas, irreversibly degrading cortical integrity.

## What the evidence does not show

There is no scientific evidence that topical shampoos, oils, or natural plant extracts can rebuild broken Disulphide Bonds. Covalent bond formation requires precise chemical conditions, electron donors, or specific synthetic cross-linkers. Natural oils coat the cuticle and reduce friction, which prevents mechanical snapping, but they work on the surface and do not interact with cortical sulphur chemistry.

Similarly, cosmetic "bond-multiplying" or "bond-repair" active ingredients (such as bis-aminopropyl diglycol dimaleate or maleic acid complexes) do not literally reform original natural disulphide cross-links. Peer-reviewed textile and cosmetic chemistry literature demonstrates that these compounds act as artificial bridges. They form covalent or ionic adducts with single free thiol (, SH) groups to connect severed chains. While this process significantly increases wet tensile strength and reduces breakage, it is a synthetic scaffolding, not a biological regeneration of natural cystine.

## Signs of depleted Disulphide Bonds

When too many cortical linkages are cleaved, hair displays distinct physical characteristics:

* **Spongy, gummy texture when wet:** Healthy hair feels firm when saturated. Severely degraded hair feels slimy or elastic like chewing gum when pulled, failing to bounce back.
* **Loss of natural curl definition:** If wavy or curly hair turns limp, flat, or frizzy despite product application, internal cross-links have likely broken down.
* **Rapid water absorption and uneven drying:** With bonds broken and proteins stripped, the hair exhibits extreme porosity. It absorbs water instantly but loses moisture just as fast, drying into an unruly, brittle texture.
* **Mid-shaft snapping:** Hair that snaps cleanly along the strand during gentle detangling or towel drying indicates structural breakage, not follicular shedding.

## Practical steps to protect cortical structure

To maintain cortical strength and prevent excessive breakage:

1. **Space out alkaline services:** Allow a minimum of 8 to 12 weeks between high-lift bleaching, perming, or relaxing sessions. This gives the hair time to recover its lipid balance and prevents cumulative bond collapse.
2. **Cap heat styling at 180°C:** Thermal styling tools should remain below 180°C. Never apply flat irons to damp hair, as boiling trapped water creates "bubble hair" (vapour cavities inside the cortex that shatter neighbouring bonds).
3. **Use targeted bond-building treatments:** Apply validated cross-linking treatments before, during, or after bleaching services to protect free thiol groups before they degrade irreversibly into cysteic acid.
4. **Maintain an acid mantle:** Use conditioning products formulated at pH 4.0 to 5.0. Acidic formulations contract the cuticle scales, shield the inner cortex from environmental pollutants, and stabilise remaining salt bonds.

## When the issue is not bond damage

Do not confuse bond degradation with root-level hair loss. If you find strands with a small, white bulb at one end on your pillow or in the shower drain, this is hair shedding (such as telogen effluvium), not structural breakage. Shedding involves the entire hair fibre detaching from the follicle due to hormonal shifts, nutritional deficiencies, or systemic stress.

Breakage, by contrast, produces short, jagged fragments without a root bulb. If you suspect scalp inflammation, sudden patchy thinning (alopecia areata), or widespread diffuse shedding, consult a trichologist, dermatologist, or your GP. Structural hair care repairs the fibre, but medical diagnosis is required for follicular disorders.

## Supporting hair condition with Watermans

While internal bond preservation relies on sound chemical processing and controlled heat, protecting the external cuticle preserves the cortex beneath. Watermans approaches hair resilience from the surface down and the root up. Formulations such as Watermans GrowMe [Shampoo](https://frojus.com/products/afro-hair-shampoo-mixed-race-shampoo) incorporate hydrolysed lupin protein, biotin, and rosemary extract to support the appearance of density while providing surface conditioning that minimises friction. Reducing combing friction protects weakened fibres, helping hair that has suffered bond depletion resist mechanical snapping during everyday styling.

## Frequently asked questions about Disulphide Bonds

### Can damaged Disulphide Bonds ever heal naturally?
No, Disulphide Bonds cannot heal on their own. Hair is non-living, biologically dead tissue once it emerges from the scalp follicle. It lacks cellular metabolic machinery to repair broken proteins. Once a disulphide link is oxidised into cysteic acid, that specific natural bond is permanently gone. You can only patch the structural deficit cosmetically using synthetic bond builders or trim away the damaged length.

### Do bond-building treatments permanently fix Disulphide Bonds?
Bond builders do not recreate natural Disulphide Bonds identically; they insert synthetic cross-links between exposed thiol groups. While these artificial bridges resist washing better than basic conditioning agents, they can still degrade over time with ongoing heat styling, shampooing, and UV exposure. Continued maintenance treatments are necessary to sustain fibre strength.

### How does bleach break Disulphide Bonds?
Bleaching requires an alkaline carrier (to swell the cuticle) mixed with hydrogen peroxide and persulphate salts. These oxidising agents break apart melanin pigments to lighten colour. During this reaction, the bleach oxidises cysteine's sulphur atoms, breaking the disulphide bridge and converting it into non-reactive cysteic acid. This directly weakens the structural framework of the cortex.

### Why do perms smell like sulphur when applied?
Permanent waves use ammonium thioglycolate to reduce and cleave Disulphide Bonds into free cysteine thiol groups (, SH). Thiol compounds have a characteristic rotten-egg or pungent sulphur odour. The smell you notice in the salon is the direct chemical consequence of volatile sulphur groups being released as the hair's internal cross-links split open.

### Does hard water damage Disulphide Bonds?
Hard water does not directly break Disulphide Bonds. Mineral ions such as calcium and magnesium bind electrostatically to the hair surface and inside porous cuticles, forming a rigid film that makes hair feel brittle and dry. While this mineral buildup increases surface friction and leads to snapping, it does not cleave the covalent sulphur links inside the cortex.

## Protecting your hair's structural foundation

Understanding how Disulphide Bonds function changes how you approach salon services and everyday styling. These resilient covalent bridges provide hair with its natural strength, elasticity, and shape, but they are not indestructible. Every session with high-lift bleach, chemical straighteners, or uncalibrated heat tools extracts a structural tax on these sulphur links. By balancing necessary chemical services with pH-optimised care, thermal moderation, and artificial bond scaffolding, you can enjoy colour and styling while preserving the core integrity of the hair fibre.

## Sources and references

* Robbins, C. R. (2012). *Chemical and Physical Behavior of Human Hair* (5th ed.). Springer-Verlag.
* Feughelman, M. (1997). *Mechanical Properties and Structure of Alpha-keratin Fibres*. UNSW Press.
* Society of Cosmetic Chemists. (2018). "The chemistry of hair waving and relaxing." *Journal of Cosmetic Science*.
* Cruz, C. F., et al. (2016). "Human hair characteristics: A review." *International Journal of Trichology*, 8(3), 115-124.
* Rogers, G. E. (2004). "Hair follicle differentiation and regulation." *International Journal of Developmental Biology*, 48(2-3), 163-170.

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### Find these ingredients in Watermans products

- **Biotin**: you will find this ingredient in our [Hair Growth Shampoo](https://watermanshair.com/products/best-hair-growth-shampoo-fast-hair-growth).

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## Did you know?

- Disulphide bonds are covalent chemical links formed between sulfur atoms in the amino acid cysteine, which makes up roughly a fifth of human hair keratin.
- Unlike temporary hydrogen bonds that easily break when hair is wet, disulphide bonds resist water completely and require chemical treatments or extreme heat to alter.
- The distinct, pungent smell produced during chemical perms and relaxers comes directly from sulfur gas released as disulphide bonds are chemically broken.


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Source: https://haircare.watermanshair.com/article/disulphide-bonds-hair-structure