Over the course of the next few weeks I will be running a series of posts on the chemical and physical properties of hair, covering a variety of subjects including the penetration of materials into the hair, the pH of hair and scalp, the trace elements in hair and so forth. Continue reading for my first post which covers the chemical composition of hair.
The properties of hair are primarily based upon the structure of hair. There is a strong need to understand the physical and chemical properties of hair in order to style and groom the hair on a daily basis. This understanding would lead us to treat hair with chemicals and other styling aids in a more intelligent way and thus alleviate damage done to hair during these grooming practices. Therefore, in this chapter we would discuss the chemical composition of hair first followed by other physical properties of hair such as moisture, elasticity, combing and brushing, static charge, porosity, thermal stability, shine, and pH of hair.
The hair protein is called keratin and contains carbon, hydrogen, oxygen, nitrogen and sulfur. The basic units of all proteins are called amino acids. There are 20 amino acids in nature. Theses amino acids are like the alphabets of all proteins that exist in nature. The structure of an amino acid is shown in Figure 2.1.
Figure 2.1: The chemical structure of an amino acid where – R Group changes in all amino acids.
When two amino acids combine or condense together by forming a peptide bond and release of a water molecule, it is called a peptide, as shown in Figure 2.2.
Figure 2.2: The formation of a peptide from the condensation of two amino acids and a water molecule. A peptide bond is formed between an –NH2 group of one amino acid and –COOH group of another amino acid.
When many amino acids combine like the above arrangement and form many peptide bonds, this new compound is called a polypeptide, as ‘Poly’ means many in Greek language. The chemical structure of a polypeptide is shown Figure 2.3.
Figure 2.3: The structure of a polypeptide. The green color bonds are peptide bonds.
One of the most important amino acids present in hair is cysteine and it forms a cystine bond in conjunction with another molecule of cysteine. The structure of cysteine as an amino acid is shown in Figure 2.4.
Figure 2.4: The chemical structure of a molecule of Cysteine amino acid.
When two molecules of cysteine amino acid combine at – R groups, the newly formed molecule is called Cystine. This cystine molecule is a crosslink in the hair and plays a major role in the strength of hair fibers. Also, all of the permanent changes in hair are attributed to changes in Cystine crosslink. The formation of cystine molecules from two molecules of cysteine is shown in Figure 2.5.
Figure 2.5: The formation of a cystine crosslink from two molecules of Cysteine amino acid. This crosslink is shown as a red connecting line between two sulfur atoms of cysteine molecules. It is also called a disulfide bond.
When two polypeptides containing cysteine residue come face to face, they form crosslinks at sulfur atoms that are also called disulfide links between two polypeptides. This is the case in human hair and wool. The figure 2.6 shows the formation of disulfide/cyctine crosslinks in two polypeptides facing each other.
The Figure 2.6 shows two types of major bonds in the crosslinked Polypeptides, that is, cystine/disulfide bonds and peptide bonds. There are two other important bonds present in the hair too and they are hydrogen bonds and salt linkages. These bonds are not as permanent and can be broken easily. For example, the hydrogen bonds are present in dry hair but can be broken upon wetting the hair. This is one reason why wet hair fibers are weaker than dry hair fibers in their tensile strength. The hydrogen bonds are weak attractive forces between the —NH site of one peptide with the —CO site of the other peptide. The other weak links known as slat linkages are present between the acidic and basic residues of the amino acid. The acidic residues (— R groups) have negative charge and basic residues (— R' groups) have positive charge. Therefore, the salt linkages are formed between the negative and the positive sites present on the polypeptides. The Figure 2.7 shows hydrogen bonds and salt linkages as well. Therefore, in Figure 2.7, all four important bonds are shown. To summarise, these four bonds are peptide bonds, cystine/disulfide bonds, salt linkages and hydrogen bonds. The first two bonds are very permanent and the second two bonds are weak bonds and easily breakable.
Figure 2.7: The peptide bonds, cystine/disulfide bonds, hydrogen bonds, and salt linkages are shown between two polypeptide chains. They exist in Keratin proteins.
The keratin proteins shown in Figure 2.7 undergo many chemical reactions such as hair straightening (relaxing), permanent waving, hair bleaching, and straightening via crosslinking with formaldehyde, Glyoxylic Acid, Glyoxylol Carbocysteine, and Ethylene Diamine, etc.
Source: LJ Wolfram. (1981). The Reactivity of Human Hair. A Review. In Hair Research, Ed: Orfanos, Montagna, Stuttgen: Springer-Verlag: Berlin Heidelberg, p. 479.