The physics behind curly hair

Across the animal kingdom, hair and fur are exceedingly common.

Although these alpacas are all covered in fur, the thickness, curliness, and length of the fur varies from animal to animal and from location to location within an individual animal. The size and shape of the hair follicle plays a role, but the molecular protein structure of the keratin fibers composing the hair or fur of an animal is the main culprit.

They come in many varieties of thicknesses, styles, and structures.

The emperor tamarin monkey is notable for its facial hair, which typically curls downward in a set of long, low-looping arcs, although there are exceptional examples that possess alternative features. Its hairstyle is determined by the bonds that occur in the protein structure of its keratin.

Despite these varied properties, hairs are generally similar.

Various thicknesses of skin are found on varying locations on the human body, and often correlate with the level of hairiness of that region. Your armpits have particularly thin skin, whereas your knees have relatively thick skin. Skin thickness also varies by age, health, and other factors.

For all animals, it’s composed of keratin: a protein-based structure.

This diagram shows the structure and the various structural elements of the keratin found in hair. Keratin, despite its complexity, is just a protein at its core.

All proteins, in turn, contain amino acids as building blocks.

This diagram shows the surface-charge densities (left) and structural organization (right) of the protein structures for the light-harvesting complex 2 and 3 molecules (top and bottom) used as antenna proteins in photosynthesis. Note that all proteins are composed of a chain of amino acids.

Only 20-22 amino acids are biologically available for protein construction.

This chart shows the structure of the 21 proteinogenic α-amino acids found in eukaryotes, grouped according to the pKa values of their side chains and charges carried at pH typical to the human body: 7.4.

Genetics encodes the underlying structure of an animal’s hair.

This image shows the standard RNA codon table, where each of the 64 possible three-base-pair codons involving U, C, A, and G bases are shown. These codons encode amino acids, as well as the information to begin (⇒) or end (Stop) encoding a particular protein out of those amino acids. Note the important feature of redundancy of the table, as there are only typically 20 amino acids for 64 codons. DNA typically encodes 20 amino acids as well, with thymine replacing uracil.

All forms of keratin possess the same secondary structure: an alpha helix.

Although the primary structure of proteins is determined by its amino acid sequence, the way those proteins then fold leads to more complex secondary, tertiary, and quaternary structures. A protein’s secondary structure is determined by the types of hydrogen bonds that occur between the various amino acids, with the two main shapes being beta-sheets (left) and alpha-helixes (right).

An individual hair’s curliness, however, depends on amino acid bonds within the keratin.

This eight-panel image of the same woman shows different stages of getting ready. On the bottom row, the leftmost image shows wet hair, the image to the right of it shows the same hair after being dried with a blow dryer, the next image shows the hair after being thermally straightened. The primary difference in the hair’s appearance is due to the presence and distribution of hydrogen bonds.

Hydrogen bonds reflect how atoms near a protein’s boundary attract and repel.

This drawing illustrates the interactions of water molecules with one another. Water is a V-shaped, highly polar molecule, possessing a negatively charged side (where the oxygen atom is) and positively charged ends where the hydrogens are. Neighboring water molecules interact with one another by way of hydrogen bonds, depicted with dotted lines in this drawing.

These bonds continuously break and reform, representing impermanent structures.

This image shows a series of hydrogen-bonded naphthalene tetracarboxylic diimide (NTCDI) islands that are held together in a two-dimensional assembly by hydrogen bonds. The image at right is the same as the image at left, except with an overlay of the NTCDI shown and the contrast enhanced.

Salt bridges occur when ions are involved in hydrogen bonding.

Unlike hydrogen bonds, which occur between neutral atoms that have different electronegativities, salt bonds (or salt bridges) represent electrostatic interactions between ions, such as a positive ammonium ion (here, a part of lysine) and a negative carboxylate ion (here, a part of glutamic acid) at a bonding distance of fewer than 4 ångströms.

They, too, are easily broken, unrelated to permanent “curling” features.

There are three main types of bonds that occur within proteins to determine their tertiary and quaternary structures: hydrogen bonds, which occur between neutral, electronegative or electropositive atoms within a molecule, salt bridges, which represent bonds between ions and atoms, or disulfide bonds, which are sulfur-sulfur bonds that arise from the cysteine amino acids within a protein.

One key amino acid determines curliness: cysteine.

Cysteine, one of the 20 amino acids found in humans, is one of only two amino acids (along with methionine) to contain a sulfur atom, and the only one that terminates its R-group chain in sulfur. Two cysteine molecules whose sulfur atoms link together form a dimer known as cystine.

Possessing a terminal sulfur atom, two cysteines can bond together, forming cystine.

When two cysteine amino acids, shown at top, have the sulfur groups within them bind and link together, that disulfide linkage creates cystine, with the disulfide bond fundamentally changing the structure of the larger molecule containing both member amino acids.

These disulfide linkages primarily determine one’s hairs’ curliness.

Hair that contains regular disulfide linkages within individual strands will curl naturally, with the tightness of the curl determined by the spacing of the disulfide linkages within the intramolecular amino acids.

Relaxing one’s hair involves breaking these disulfide bonds, straightening one’s hair.

Chemically relaxing one’s hair involves the use of a strong alkali, ammonium thioglycolate, or formaldehyde to break the disulfide bonds that form cystine. This can be performed under either high or low heat, with the latter technique known as soft bonding.

Perming one’s hair, alternatively, forms or reforms cystine, holding keratin together in a new, curlier configuration.

After applying hydrogen peroxide to either naturally straight or chemically straightened hair, curling the hair allows disulfide linkages to form between sulfur-containing cysteine chains, allowing a perm with wavy, curly, or kinky hair depending on the tightness of the curls. Large curlers, as shown here, typically produce wavy hair.

Macroscopically, hair’s curliness is rooted in its molecular structure.

Whether someone’s hair is naturally curly or straight (or wavy or kinky) depends primarily on the amino acid structure of their hair, the amount and location of cysteine amino acids present, and how or whether those cysteines form disulfide linkages between the sulfur atoms within them.

Mostly Mute Monday tells a scientific story in images, visuals, and no more than 200 words.