An overview on the chemical composition of hair.

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Chemical Composition of Hair
Human hair is a complex fiber made up of various morphologic components and different chemical species. The chemical composition of hair fiber includes essential functional elements like amino acids, keratin, melanin, and protein.

Though there are different chemical components present in the human hair, it is an “integrated” system. The different chemical components in human hair act together to maintain the normal flow of functions. Some of the important structural and functional chemical components and their significance for hair care are presented below

  • Protein
  • Water
  • Hair Lipids
  • Hair Pigments

  • Protein
    Protein is the most important element and is present throughout the hair from root to tip. Hair, from its growth under the skin of the scalp, is filled with a fibrous protein called keratin. Most of the keratinous proteins are present within the cortical cells, but significant and important proteins are present also within the cuticle which provides elasticity to the hair. Proteins are also present in the medulla of hair fibers but they are probably of little physicochemical significance.

    The protein of hair is made up of long chains of amino acids. The amino acids are joined to each other by chemical bonds called peptide bonds or end bonds. The joining of these small peptide bonds produces long chains of amino acids which are called polypeptide chains.

    The keratin protein found in hair is called "hard" keratin. It remains undiluted in water and is made up of eighteen amino acids among which the most important is cystine which provides strength to the hair. It is generally observed that male scalp hair or dark colored hair contains more cystine when compared to female or lighter colored hair.

    Other than cystine, the cuticle of human hair contains cysteic acid, proline, threonine, isoleucine, methionine, leucine, tyrosine, phenylalanine, and arginine in quite high proportions compared to the other layers of the hair shaft.

    Role of protein in the cortex
    The middle layer of the hair shaft is the cortex, the most important portion where protein plays an important role.

    Cortex is made up of millions of polypeptide chains which are cross-linked with each other by three different types of side bonds. These three side bonds that link the polypeptide chains of the hair are hydrogen, salt and disulfide bonds. Hydrogen bonds that are found in abundance in the polypeptide chains are a weak physical side bond. The disulfide bonds are stronger and while there are fewer of these bonds than hydrogen or salt bonds, they provide most of the hair strength and durability.

    Water content in the hair
    Water content is important in the chemical composition of hair. It promotes normal and healthy hair growth. The weight of the hair is 1.37 in accordance with metric measurements, but when it is impregnated with water the weight increases by 12-18%. The process of water absorption is very rapid, about 75% of the maximum of water that hair can take up is absorbed within 4 minutes of exposure of the hair to water.

    The water binding of amino and guanidino groups is responsible for the large percentage of water absorption capacity by keratin particularly at low humidity. The absorption of water gives the hair its required moisture content which is essential for healthy hair appearance.

    With increasing humidity more water is absorbed, producing a decrease in the energy binding of water already associated with the protein.

    Hair Lipids
    Hair lipids are composed of fatty acid, phytosphingosine, ceramide, cholesterol and cholesterol sulfate. In the hair structure, lipids are present in Inner Root Sheaths (IRS) and hair shaft lipids provide sheen to the hair and contribute towards its tensile properties.

    The lipid content of the hair is not constant but varies with age and other factors. There is a maximal increase in hair lipids after puberty in both the genders. After a women reaches middle age there is a decrease in lipid content but it is not so in the case of men.

    The lipid content of the hair also varies between the ethnic races. On average, black African-American, hair contains more lipid than Caucasian hair.

    Hair Pigments
    Melanin is the hair pigment which gives color to the skin and hair. They are produced by a group of specialized cells called melanocytes which are situated near the hair bulb. The melanin found in the cortex gets collected and formed into a bundle of pigment protein complexes called melanosomes. The size, type and distribution of the melanosomes will determine the natural color of the hair. Melanin can be of two types:
    • Eumelanin
    • Phaeomelanin
    Eumelanin is brown/black in color and is the most common type. This form of melanin gives the hair different hues ranging from black to brown. Phaeomelanin is the uncommon type and is red in color. It gives yellow, ginger and red shades to the hair.

    Other than these important elements, the chemical composition of hair also contains trace elements which are essential for its healthy functionality. Studies are going on to find out the facts about trace elements in hair as there is still quite some uncertainty about their correct position or functions.

    Understanding the structural and functional aspects of the hair helps us in knowing any wrong practices that we are inflicting on our hair. It also helps us to take care of hair in the best possible way.

    To know more about the effects of wrong practices on hair read the following:

    Hair shaft damage
    Severely Weathered Hair (Trichorrhexis nodosa)
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