In this blog post, we’ll explore the principles behind how curly hair and perms form, along with changes in protein structure.
Hairstyle is one of the key factors many people consider when grooming their appearance. From washing, blow-drying, and styling our hair every day to getting it cut, dyed, or permed at the salon, we devote a great deal of attention to our hair. Among these treatments, perms offer the advantage of straightening curly hair or adding curls to straight hair, and they allow us to adjust the degree of curl to create a variety of hairstyles that suit different looks. In the past, perms were primarily done by women, but today they have become a popular treatment enjoyed by people of all ages and genders. So, what chemical principles allow us to get a perm?
To understand the chemistry of hair, we first need to examine the properties and structure of the protein that makes up hair. Hair is composed of a protein called keratin. Keratin is a representative structural protein found not only in hair but also in feathers, wool, fingernails, toenails, and horns. To perform these functions, keratin must have a very strong structure and, unlike most proteins, must also be insoluble in water. If keratin were a protein that dissolved easily in water, we would face the absurd situation of our hair, fingernails, and toenails being damaged every time we took a shower. So how does keratin form a fibrous structure that is both strong and insoluble in water?
Amino acids have a structure in which an amino group (NH₂), a carboxyl group (COOH), a hydrogen atom (H), and a side chain (R) are bonded to the central carbon atom. Each amino acid has a different type of side chain, and this side chain plays a crucial role in determining the properties of each amino acid. Proteins are formed when these amino acids are linked together; the bond formed in this process is called a peptide bond. Peptide bonds are formed through a dehydration condensation reaction, in which the amino group of one amino acid reacts with the carboxyl group of another, resulting in the release of a water molecule (H₂O).
This peptide backbone contains NH and CO groups, which form hydrogen bonds with complementary groups within the protein, thereby stabilizing the partial charges present in the peptide bonds. Typical examples of these arrangements include the alpha-helix (α-helix) and beta-sheet (β-sheet) structures. Among these, the alpha-helix structure refers to a coil-like structure wound spirally around a central axis. The amino acid chains that make up keratin also form this alpha-helix structure. The amino acids that make up keratin include many hydrophobic amino acids, such as alanine, isoleucine, valine, methionine, and phenylalanine, and their side chains are arranged toward the outside of the helical structure. Since hydrophobic side chains are unstable when in contact with water, keratin naturally becomes insoluble in water.
As a result, keratin molecules bond with each other to form a fibrous structure rather than bonding with water molecules. First, two alpha-helix structures wrap around each other to form a twisted structure, and the hydrophobic side chains on the surface of the keratin further stabilize this three-dimensional structure. The two coiled strands formed in this way are arranged end-to-end and then side-by-side, forming a strong fiber. This fibrous structure is further stabilized not only by interactions between the hydrophobic side chains but also by disulfide bonds connecting the coils.
Disulfide bonds are formed when the sulfur (S) and hydrogen (H) groups present in the side chain of cysteine—a type of amino acid—form a covalent bond with each other. During the process of two sulfur-hydrogen groups bonding, a hydrogen atom is released from each, and two sulfur atoms share an electron to form a sulfur-sulfur (S-S) bond. These disulfide bonds make keratin fibers stronger and stiffer, and it is precisely these bonds that are exploited in the core principle of perming.
Looking at the perming process, the hairstylist first wraps the hair around rods in the desired shape and then applies a perm solution with a distinctive odor. This solution contains a reducing agent that breaks the disulfide bonds inside the hair. The reducing agent breaks the existing bonds by converting the disulfide bonds (S-S) into two sulfur-hydrogen groups (S-H). Since this process is a reduction reaction that adds hydrogen to the disulfide bonds, the substance is called a reducing agent.
During this process, the hair is placed in an environment where the appropriate temperature and moisture levels are maintained. Heat combined with moisture temporarily weakens the hydrogen bonds that maintain the alpha-helix structure of keratin, making the hair more flexible. As a result, the keratin chains can shift into a new configuration, and the hair is prepared to rearrange its structure to match the shape of the rod.
After sufficient time has elapsed, the reducing agent is rinsed out, and a neutralizing agent is applied. The neutralizing agent re-oxidizes the reduced cysteine residues, causing them to form new disulfide bonds. Since the hair remains wrapped around the rod at this point, cysteine residues in different positions form new disulfide bonds. Ultimately, the bonds are fixed in the shape of the wrapped hair, completing the perm.
Finally, after washing the hair and allowing it to cool sufficiently, the hydrogen bonds that maintain the alpha-helix structure of keratin are reestablished. Hair that has undergone this process retains the shape fixed by the new disulfide bonds, resulting in naturally wavy permed hair.
Through the perm—a common procedure in our daily lives—we can see that it is possible to alter the structure of proteins by forming new bonds. Changes in protein structure can be easily observed not only in perms but also in various everyday phenomena, such as boiling eggs, grilling meat, and making tofu. As such, when a protein’s structure changes, its properties change as well, and these characteristics are widely utilized not only in food but also in various industrial sectors.
Today, with significant advances in our understanding of protein structure and the roles of amino acids, we have moved beyond simply altering existing bonds to designing desired amino acid sequences and precisely controlling the structure and function of proteins. These technologies are being utilized in various fields, including protein engineering, biopharmaceuticals, industrial enzymes, biomaterials, and eco-friendly materials. Recently, technologies for protein structure prediction and new drug development using artificial intelligence (AI) have also been advancing rapidly.
As such, changes in protein structure go beyond simply explaining the principles of perming. They are not only a crucial key to understanding the various biological phenomena that occur around us but have also established themselves as one of the core principles driving the future development of medicine, biotechnology, and industrial technology. Understanding the chemical principles hidden within a single strand of hair can be considered the first step toward grasping the astonishing potential of proteins as biological molecules.
