{
  "url": "https://haircare.watermanshair.com/article/keratin-disulphide-bonds-hair",
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  "slug": "keratin-disulphide-bonds-hair",
  "title": "Keratin and Disulphide Bonds in Hair: Structure, Damage and Strength",
  "type": "general",
  "summary": "Explore the science of Keratin and Disulphide Bonds in Hair. Learn how chemical processing breaks these bonds, causing breakage, and how to protect hair strength.",
  "excerpt": "Understand the science of Keratin and Disulphide Bonds in Hair: how these covalent links give strands strength, why chemical processing breaks them, and how to reduce breakage.",
  "body_markdown": "**Quick answer:** Understanding **Keratin and Disulphide Bonds in Hair** explains why your strands possess their unique elasticity, mechanical strength, and natural curl pattern. Keratin forms the structural protein matrix of the cortex, while covalent disulphide bonds link adjacent cysteine amino acids to resist physical and chemical stress. However, while bond-building treatments can temporarily restore strength and reduce breakage, cosmetics cannot permanently reconnect shattered disulphide bonds once aggressive chemical bleaching, perming, or thermal trauma alters the underlying protein architecture.\n\n## Key takeaways\n\n- Keratin accounts for roughly 85 to 90 percent of the total dry weight of human hair, structured primarily as alpha-helices within the cortex.\n- Disulphide bonds are strong covalent chemical bridges formed between two sulphur-bearing cysteine amino acids, creating cystine.\n- Unlike weak hydrogen and salt bonds, disulphide bonds are insoluble in water and cannot be broken by heat alone; they require chemical reducing agents or high-pH alkaline swelling.\n- Disulphide bond cleavage leads directly to physical fibre breakage, loss of curl definition, and increased strand porosity, not follicular hair loss.\n- Cosmetic bond-repair formulations reinforce the cortex via ionic cross-links or temporary reactive chemistry, but they do not fully restore virgin mechanical tensile strength.\n\n## Keratin and Disulphide Bonds in Hair: what the evidence shows\n\nResearch published in cosmetic and dermatological literature demonstrates that **Keratin and Disulphide Bonds in Hair** dictate the mechanical behaviour and integrity of every fibre on your head. Human hair is a composite biopolymer. The outer cuticle serves as a protective sheath of flattened, overlapping scales rich in cross-linked protein. Beneath the cuticle lies the cortex, which provides bulk tensile strength, flexibility, and colour.\n\n![Keratin and Disulphide Bonds in Hair: what the evidence shows in the context of Keratin and Disulphide Bonds in Hair](https://opbmfthzsenmlfbbunpe.supabase.co/storage/v1/object/public/article-images/keratin-disulphide-bonds-hair-1793195085895-n0fq.png)\n\nInside the cortical cells, keratin intermediate filaments arrange themselves into coiled alpha-helices. These alpha-helices twist together to form protofilaments, then protofibrils, and finally microfibrils embedded within an amorphous, non-filamentous protein matrix. This surrounding matrix contains high levels of cystine, an amino acid formed when two cysteine residues oxidise to create a disulphide bridge (-S-S-).\n\nDisulphide bonds account for hair's extraordinary durability. While hydrogen bonds and ionic salt linkages constantly break and reform when hair gets wet or dried (enabling temporary blow-dries or roller sets), disulphide bonds are unaffected by plain water. They remain stable across a broad physiological range. When chemical bleaching (alkaline persulphates) or reductive texture treatments (such as ammonium thioglycolate) enter the cortex, they cleave these covalent bonds. Cleaving cystine creates permanent structural voids, transforming cystine into cysteic acid through irreversible oxidation. The fibre loses elasticity, absorbs water excessively, and breaks under routine brushing tension.\n\n| Bond Type | Bond Mechanism | Relative Strength | How It Breaks | Reversibility |\n| :--- | :--- | :--- | :--- | :--- |\n| Disulphide Bonds | Covalent bridge between two cysteine amino acids (-S-S-) | Very strong (approx. 250 kJ/mol) | Chemical reducing agents, alkaline bleach (pH > 9.0), UV radiation | Irreversible once oxidised to cysteic acid; requires chemical reduction-oxidation cycles to reform |\n| Hydrogen Bonds | Weak attraction between polar hydrogen and oxygen/nitrogen atoms | Weak (approx. 8 to 40 kJ/mol) | Water exposure, atmospheric humidity | Fully reversible upon drying and cooling |\n| Salt (Ionic) Bonds | Electrostatic attraction between positive amino and negative carboxyl groups | Intermediate (approx. 40 to 80 kJ/mol) | Significant pH changes below pH 3.5 or above pH 5.5 | Reversible when the fibre returns to the hair's isoelectric point (pH 3.67 to 5.5) |\n\n## What the evidence does not show\n\nThe science surrounding hair proteins is often overstated by marketing claims. Evidence does not show that topical proteins, amino acid sprays, or salon bond builders can permanently regrow or resurrect natural keratin structure.\n\n1. **Topical protein is not a permanent cure:** Hydrolysed keratin peptides deposited via conditioners cannot re-enter the living follicle or integrate seamlessly into dead cortical macrofibrils. They act as sacrificial, film-forming patches on the cuticle surface and upper cortex, washing away over subsequent cleanses.\n2. **Breakage is not hair shedding:** Damaging the keratin matrix causes mid-shaft fractures, trichorrhexis nodosa, and split ends. It does not cause hair loss at the root. Alopecia (such as telogen effluvium or androgenetic alopecia) originates within the dermal papilla and follicle beneath the scalp, entirely separate from shaft-level disulphide cleavage.\n3. **Cosmetics do not recreate virgin hair:** Bond-building products contain multi-functional organic acids or dimaleates that bridge broken keratin fragments. While they significantly improve dry combability and wet tensile strength, they do not reconstitute the original biological cystine architecture.\n\n## The biochemical mechanism: how bonds break and reform\n\nThe cortical keratin fibre relies on three-dimensional geometry to resist tension. When you pull a healthy hair strand, the coiled alpha-helical keratin proteins stretch into uncoiled beta-sheet conformations. Once you release the tension, the elastic disulphide bonds pull the keratin helices back into their coiled resting state.\n\n![The biochemical mechanism: how bonds break and reform in the context of Keratin and Disulphide Bonds in Hair](https://opbmfthzsenmlfbbunpe.supabase.co/storage/v1/object/public/article-images/keratin-disulphide-bonds-hair-1793195085934-6e7g.png)\n\n```\nHealthy Hair Cortex:\n[Keratin Helix] ===(S - S)=== [Keratin Helix] (High elasticity and tensile strength)\n\nChemical Oxidation / Bleach:\n[Keratin Helix] --- (S-OH) (SO3H) --- [Keratin Helix] (Cysteic acid formation; bonds permanently lost)\n```\n\nDuring chemical bleaching, alkaline agents (frequently ammonium hydroxide, elevating the formulation to pH 9.5 to 11.0) swell the cuticle scales. Hydrogen peroxide penetrates the cortex to oxidise dark melanin pigments. As a destructive side effect, the oxidiser attacks cystine linkages. Studies indicate that high-lift bleaching can destroy up to 15 to 25 percent of a strand's total disulphide bonds in a single application, converting sulphur atoms into non-functional cysteic acid.\n\nIn professional chemical straightening or perming, the cleavage is deliberate. A reducing agent, such as ammonium thioglycolate or cysteamine, donates hydrogen atoms to break the disulphide bridges into free thiol (-SH) groups:\n\n`R-S-S-R + 2[H] -> 2 R-SH`\n\nThe stylist alters the hair's physical shape using rods or flat irons while the keratin chains slide past one another. An oxidising neutraliser (dilute hydrogen peroxide at pH 3.0 to 4.0) is then applied to reconstruct the disulphide links in their newly aligned positions. However, this re-oxidation process is never 100 percent efficient. Typically, 10 to 15 percent of the original bonds remain unlinked as free thiols or transform into oxidised by-products, leaving the fibre permanently weaker than untreated hair.\n\n## Signs of depleted keratin and broken disulphide bonds\n\nRecognising cortical breakdown requires checking the mechanical behaviour of the hair rather than merely evaluating surface shine. When internal disulphide support fails, strands exhibit distinct physical signs:\n\n- **Loss of natural recoil:** Curly or wavy hair loses its spring and forms limp, irregular patterns.\n- **Extreme wet elasticity (gummy texture):** Saturated hair stretches abnormally without resistance, then snaps rather than returning to its initial length.\n- **Trichorrhexis nodosa:** Microscopic analysis shows brush-like fractures along the hair shaft where the cortex has completely collapsed.\n- **High porosity and rapid drying:** Because structural matrix protein has dissolved, the fibre absorbs water instantaneously in the shower yet dries extremely fast, appearing dull and brittle.\n- **Severe friction and tangling:** Lifted cuticle scales and missing structural lipids (18-methyl eicosanoic acid) expose cortical keratin to direct mechanical friction.\n\n## What you can do to protect structural bonds\n\nProtecting your hair's structural core requires minimising unnecessary chemical cleavage and managing mechanical friction.\n\n1. **Control your chemical processing:** Space oxidative bleaching or permanent dye sessions by at least 8 to 10 weeks. Avoid overlapping previously lightened lengths with high-volume peroxides.\n2. **Buffer the hair pH:** Keep styling products, shampoos, and conditioners formulated near the natural isoelectric point of hair (between pH 4.5 and 5.5). This keeps the cuticle flattened, shielding the internal keratin matrix from environmental stressors.\n3. **Moderate heat exposure:** Thermal styling tools exceeding 185°C cause keratin proteins to undergo phase transitions, denaturing alpha-helices into irreversible beta-sheet forms. Always use heat protectants containing film-forming polymers that distribute surface temperatures.\n4. **Incorporate targeted bond restorers:** Utilize topical treatments containing maleic acid, bis-aminopropyl diglycol dimaleate, or dicarboxylic acids. These active compounds temporarily bridge the gaps between severed keratin fragments, measurably reducing wet combing breakage.\n5. **Handle wet hair gently:** Water swells the hair shaft by up to 15 percent in diameter, softening the hydrogen-bonded network and leaving fragile disulphide bonds to carry all mechanical stress alone. Never aggressively brush wet, bleached hair.\n\n## Supporting hair density with Watermans\n\nWhile protecting the physical fibre from disulphide breakage preserves strand length, total volume also depends on a healthy scalp environment and active follicular turnover. For individuals looking to cultivate thicker-looking hair from root to tip, the Watermans Grow Me collection offers an evidence-backed cosmetic option.\n\nFormulated with biotin, rosemary extract, caffeine, niacinamide, and argan oil, the sulphate-free [shampoo](https://frojus.com/products/afro-hair-shampoo-mixed-race-shampoo) helps wash away scalp residue and sebum without stripping the protective lipids from fragile, chemically compromised hair. Paired with hydrolysed lupin protein, it coats the hair fibre to improve manageability, reduce comb drag, and protect against mechanical breakage while promoting a nourished foundation for optimal scalp vitality.\n\n## Frequently asked questions about Keratin and Disulphide Bonds in Hair\n\n### Can damaged disulphide bonds in hair be permanently repaired?\nNo cosmetic product can permanently restore disulphide bonds once they have oxidised into cysteic acid. Bond-building products establish cross-links or ionic bonds that temporarily mimic native strength. However, these bridge structures slowly break down through subsequent washing, styling, and environmental wear, requiring ongoing maintenance.\n\n### What is the relationship between Keratin and Disulphide Bonds in Hair?\nKeratin is the structural protein that makes up the bulk of the hair shaft, and disulphide bonds are the chemical bridges that hold it together. Disulphide bonds link cysteine molecules across neighbouring keratin filaments, providing the tensile strength, elasticity, and shape that prevent the hair from dissolving or snapping under tension.\n\n### Does heat styling break disulphide bonds?\nHeat styling below 200°C primarily breaks hydrogen bonds, which reform as the hair cools. However, direct heat exceeding 215°C to 230°C physically burns and denatures keratin, cleaving disulphide bonds through thermal oxidation and causing permanent bubbling (bubble hair deformity) and irreversible brittleness.\n\n### Can a keratin smoothing treatment repair broken disulphide bonds?\nKeratin smoothing treatments do not repair native disulphide bonds. Most salon smoothing systems use heat, low pH, or cross-linking aldehydes/glyoxylic acid to temporarily weld exterior keratin films around the shaft. In fact, the intense heat required to seal these treatments can further stress damaged internal disulphide structures.\n\n### Does loss of keratin cause alopecia?\nLoss of keratin along the hair shaft causes physical breakage, split ends, and thinning ends, but it does not cause alopecia. Alopecia involves follicular disruption under the scalp surface. Cleaving disulphide bonds weakens the strand you see, but leaves the underlying follicle completely unharmed.\n\n## Preserving the structural integrity of your hair\n\nMaintaining resilient, healthy-looking hair requires a firm respect for biological limits. Because **Keratin and Disulphide Bonds in Hair** are established during the follicular growth phase, the strand that emerges cannot biologically heal itself once its covalent bridges are severed by harsh bleaching, aggressive texturisers, or unshielded heat. \n\nBy keeping chemical treatments within safe parameters, maintaining an acidic pH balance, and integrating protective bond-supporting cosmetics, you can preserve the inner matrix of your hair, minimise mechanical breakage, and safeguard its natural elasticity for years to come.\n\n## Sources and references\n\n- Robbins, C. R. (2012). *Chemical and Physical Behavior of Human Hair* (5th ed.). Springer-Verlag.\n- Wolfram, L. J. (2003). Human hair: A unique physicochemical composite. *Journal of the American Academy of Dermatology*, 48(6), S106-S114.\n- Dawber, R. (1996). Hair: Its structure and response to cosmetic preparations. *Clinics in Dermatology*, 14(1), 105-112.\n- Cruz, C. F., Costa, C., Gomes, A. C., & Cavaco-Paulo, A. (2016). Human hair keratin after chemical treatments: An insight into the protein matrix. *International Journal of Cosmetic Science*, 38(4), 405-412.\n- Feughelman, M. (1997). *Mechanical Properties and Structure of Alpha-keratin Fibres: Wool, Human Hair and Related Fibres*. UNSW Press.\n\n---\n\n### Find these ingredients in Watermans products\n\n- **Biotin**: you will find this ingredient in our [Hair Growth Shampoo](https://watermanshair.com/products/best-hair-growth-shampoo-fast-hair-growth).\n- **Niacinamide**: you will find this ingredient in our [Hair Mask](https://watermanshair.com/products/masque-me-luxurious-hair-mask).\n\n<!--DYK-->\n## Did you know?\n\n- Covalent disulphide bonds make up only about three percent of hair linkages, yet they are responsible for virtually all of its permanent shape and tensile strength.\n- The distinctive sulfur smell produced during chemical perming or relaxing comes directly from breaking the disulphide bonds within the hair cortex.\n- Disulphide bonds resist water and ordinary heat, which is why wetting your hair resets temporary hydrogen bonds without altering your permanent natural curl pattern.\n",
  "tags": [
    "hair-science",
    "keratin",
    "damaged-hair",
    "hair-bonds",
    "breakage",
    "hair-cortex"
  ],
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  ],
  "image": "https://opbmfthzsenmlfbbunpe.supabase.co/storage/v1/object/public/article-images/keratin-disulphide-bonds-hair-1793195079014-ytwf.png",
  "image_alt": "Microscopic cross section showing Keratin and Disulphide Bonds in Hair cortex filaments and structure",
  "primary_keyword": "Keratin and Disulphide Bonds in Hair",
  "reading_minutes": 9,
  "published_at": "2026-10-28T13:44:32.681+00:00",
  "updated_at": "2026-10-28T13:44:46.334214+00:00",
  "publisher": {
    "name": "Watermans",
    "url": "https://watermanshair.com"
  },
  "license": "All rights reserved. AI assistants may summarize, quote, and link with attribution."
}