Keratin: The Chemistry of Hair and Nails

Your hair, fingernails and the outer layer of your skin are made mostly of one family of proteins: keratin. So are the horns of a rhinoceros, the claws of a cat, the scales of a lizard and the feathers of a bird. Keratin is tough, waterproof and insoluble — and yet a hair straightener, a perm lotion or a bottle of bleach can change it within minutes. Understanding a few bonds in keratin explains nearly everything that happens in a hair salon.

What hair is made of

A single hair is a long fibre that grows from a follicle in the skin. The living part is at the root; the part you see is made of dead, hardened cells packed with keratin. A hair is roughly:

• about 65–95 % protein, almost all of it keratin
• a few per cent water (more in humid air)
• small amounts of lipids, pigment (melanin) and trace minerals

Under a microscope, a hair has three layers:

1. Cuticle: overlapping flat cells like roof tiles on the outside. They protect the hair and reflect light — smooth, flat cuticles make hair shiny.
2. Cortex: the thick middle, packed with bundles of keratin fibres. This gives hair its strength, elasticity and colour.
3. Medulla: a soft core, sometimes missing in fine hair.

Keratin’s structure: helices, coils and ropes

Keratin is a protein, so it’s a long chain of amino acids joined by peptide bonds (see proteins explained). The type in human hair and nails is called α-keratin, because its chains are coiled into α-helices — spiral shapes held in place by hydrogen bonds between every fourth amino acid along the chain.

From there, keratin builds upward like a rope:

1. Two α-helices twist around each other to form a coiled coil.
2. Pairs of coiled coils join into larger units, which join end to end and side by side.
3. These build up into intermediate filaments about 10 nm wide.
4. Thousands of filaments bundle into macrofibrils, which fill the cells of the cortex.

This hierarchy of twisted fibres is why a single hair is surprisingly strong — a healthy hair can support a weight on the order of 100 grams before breaking.

(Bird feathers and reptile scales use a related but different form called β-keratin, based on flat β-sheets rather than helices, which makes them harder and less stretchy.)

Three kinds of bonds between chains

What makes keratin so useful — and what lets us restyle it — is the set of bonds that hold neighbouring chains together. There are three to remember:

Bond	Strength	Broken by
Hydrogen bonds	Weak	Water, heat
Ionic bonds (salt bridges)	Weak–moderate	Changes in pH
Disulfide bridges	Strong (covalent)	Reducing agents (e.g. perm lotions)

Disulfide bridges: the sulfur secret

Keratin is unusually rich in the amino acid cysteine, which has a thiol group, –SH, on its side chain. When two cysteines on neighbouring chains come close, their sulfur atoms can be oxidised to form a covalent disulfide bridge:

–CH₂–SH + HS–CH₂– → –CH₂–S–S–CH₂– (+ 2H⁺ + 2e⁻)

Hair keratin contains so much cysteine that sulfur makes up around 4–5 % of its mass. That’s where the distinctive smell of burning hair comes from: heating breaks down the sulfur-containing groups, releasing smelly sulfur compounds. (See sulfur for more about the element.)

The more disulfide bridges, the harder the keratin. Fingernails and toenails have more cross-links than hair, which is why they’re rigid rather than flexible.

Wet hair and heat styling: hydrogen bonds

Why does hair stretch more when wet? Water molecules slip between the keratin chains and form their own hydrogen bonds with the protein, replacing many of the hydrogen bonds between chains. The helices can then stretch and slide more easily. Wet hair can stretch by about half its length before breaking, much more than dry hair — which is also why it’s easier to damage when wet.

Blow-drying, curling tongs and straighteners use the same idea:

1. Heat (and water) break many hydrogen bonds.
2. The hair is held in a new shape — straight or curled.
3. As it cools and dries, new hydrogen bonds form, locking the new shape in.

Because hydrogen bonds are weak and reform easily, the effect is temporary. On a humid day, water from the air seeps in, breaks the new hydrogen bonds, and the hair drifts back towards its natural shape. That’s the chemistry of “frizz”. Very high heat from straighteners also damages the cuticle and can break down keratin permanently.

Perms and chemical straightening: disulfide bridges

To change hair shape permanently, you have to break and remake the strong disulfide bridges. A perm does this in two steps:

1. Reduce: a lotion containing a reducing agent, typically a thioglycolate salt, breaks the disulfide bridges into separate –SH groups. The hair becomes soft and can be wrapped around rollers (for curls) or combed straight.
2. Oxidise: a “neutraliser”, usually dilute hydrogen peroxide, oxidises the –SH groups back into disulfide bridges — but now between different pairs of cysteines, matching the new shape.

This is a textbook example of redox chemistry in everyday life. The new shape grows out with new hair from the root, but the treated section keeps it until it’s cut off. The rotten-egg-like smell in salons comes from the sulfur-containing thiol compounds in the lotion.

pH and ionic bonds

Keratin contains amino acids with acidic and basic side chains, which form ionic bonds with each other. Their charges depend on pH. Hair is most stable at a slightly acidic pH, around 4.5–5.5. Strongly alkaline products — such as some relaxers and perm lotions — make the cuticle swell and open, letting chemicals into the cortex but also causing damage. Conditioners and “acid rinses” tend to be mildly acidic, which helps the cuticle scales lie flat, making hair smoother and shinier (see the pH scale explained).

Colour: melanin, bleach and dye

Natural hair colour comes from melanin pigments in the cortex: eumelanin (brown–black) and pheomelanin (red–yellow). The mix and amount give every shade from black to blonde (see melanin: the chemistry of skin colour).

• Bleaching uses hydrogen peroxide in alkaline conditions (often with ammonia). The alkali opens the cuticle, and the peroxide oxidises melanin into colourless products. It also oxidises some disulfide bridges into sulfonic acid groups (–SO₃H), which can’t reform — one reason bleached hair is weaker.
• Permanent dyes use small, colourless molecules that penetrate the cortex and then react with each other (with peroxide as the oxidant) to form large coloured molecules. Once formed, they’re too big to wash out.
• Temporary dyes are large coloured molecules that only coat the outside of the hair and wash out.

Nails and skin

Fingernails grow at around 3 mm per month and are made of hard keratin, densely cross-linked with disulfide bridges. The outer layer of skin (the stratum corneum) contains softer keratins mixed with lipids, forming a barrier that keeps water in and microbes out. Calluses form where friction triggers extra keratin production.

Common misconceptions

• “Hair is alive.” Only the follicle is alive; the visible shaft is dead, keratin-filled cells. That’s why cutting it doesn’t hurt.
• “Conditioner repairs split ends.” It can smooth and glue them temporarily, but broken keratin doesn’t heal.
• “Straighteners permanently change hair.” Heat alone mainly rearranges hydrogen bonds, which are temporary — though heat damage is permanent.
• “Shaving makes hair grow back thicker.” Shaving cuts each hair straight across, leaving a blunt tip that feels coarse, but it doesn’t change the follicle.

Key takeaways

• Hair, nails and outer skin are built mainly from α-keratin, coiled into helices that bundle into strong fibres.
• Hydrogen bonds (weak) are broken by water and heat — the basis of temporary styling and frizz.
• Disulfide bridges (strong, covalent) between cysteines give toughness; perms break and remake them by reduction and oxidation.
• pH affects ionic bonds and the cuticle; bleach oxidises melanin.

For another structural protein, compare collagen; for the bonds themselves, see protein structure levels.