Hair Bonds: The Four Forces That Hold Keratin Together

Why is hair both strong and flexible, and why does it eventually return to its original state even after being styled with a flat iron or permed? The secret lies in the various types of chemical bonds that hold together the protein inside hair, keratin. Depending on their type, these bonds can break and reform, playing a crucial role in determining the shape and texture of hair. Today, we will take a closer look at the four major bonds that connect keratin.

｜What Are Chemical Bonds in Hair?

In skincare, the concept of “amino acids → peptides → proteins” frequently appears. When amino acids link together, they form peptides, and when peptides gather, they form proteins. The same principle applies to hair. When amino acids join hand in hand and line up, they become peptides, and multiple peptides then come together to form a single keratin protein. These keratin proteins further bundle together like strands in a rope to create what we recognize as a single hair fiber.

At each of these stages, chemical bonds act as adhesives that connect the structures. How densely and evenly these bonds are maintained determines the strength and shape of the hair. Keratin is supported like a ladder, with various types of bonds connected both vertically and horizontally to hold the structure together.

｜Peptide Bond

A peptide bond is the bond that links amino acids in a straight sequence to form one long chain. It serves as the most fundamental framework of hair structure, like the vertical rails of a ladder, and becomes the foundation upon which other horizontal bonds can form. Although it is a strong bond that does not break easily, peptide bonds can be compromised over time under severe thermal or mechanical stress.

｜Disulfide Bond

A disulfide bond is a strong bond formed when two molecules of cystine, a major amino acid in hair, are linked together. Disulfide bonds connect keratin chains horizontally, binding loosely scattered strands into a tight bundle, like tying individual threads into a single rope. These bonds are key determinants of the shape and elasticity of hair. The perm treatments performed at salons are a representative example of utilizing the process in which disulfide bonds are temporarily broken and then reconnected. Depending on how these bonds are rearranged during the treatment, the degree of curliness and the strength of the curl can differ. As disulfide bonds become increasingly damaged, hair stretches more easily, breaks more readily, and loses its ability to maintain its shape.

｜Hydrogen Bond

 A hydrogen bond is much weaker than the two bonds described above, but this type of bond has a significant impact on the hair’s everyday changes. Hydrogen bonds are highly sensitive to water and heat, so they repeatedly break and reform simply through washing and drying the hair. For example, hair becomes frizzy on a rainy day, and curls can be created with a hair dryer while blow-drying, because hydrogen bonds are easily broken and then reformed.

｜Ionic Bond

Amino acids, the basic building blocks that make up hair proteins, possess a force called “charge,” which allows them to attract or repel each other. When amino acids with opposite charges come into proximity—similar to the N and S poles of a magnet—an ionic bond is formed. However, ionic bonds are sensitive to changes in pH, so it is greatly affected by chemical treatments such as hair dyes or bleaching agents that change the hair’s pH.

Understanding Hair Bonds, Understanding Hair Care

The bonds within hair are not visible in everyday life, but they are key factors that determine the texture and shape we feel. Therefore, simply understanding these scientific principles makes it clear why hair shows certain reactions, and the direction of the care it needs becomes more definite. This understanding naturally leads to criteria for care and management that suit each individual. Use products that are designed to provide solutions to hair needs, based on fundamental science. They will help your hair maintain stable beauty every day.