The skunky smell of many old treatments used to “permanently” change the shape of straight hair into curls, was due to chemicals that broke the bonds between the sulfur atoms that span neighboring proteins in a hair shaft and keep the proteins from sliding past each other. Hydrogen bonds connect the various parts of each protein and also contribute to the overall stability of the hair. Since hydrogen bonds are weak, low energy bonds, simply heating can permit some deformation of hair structure. Moisture can also loosen up hair proteins by insinuating water molecules in place of more direct interactions between proteins.
Protein Structure and Function
Proteins are synthesized on ribosomes in cells as long chains of thousands of amino acids. There are twenty different amino acids, e.g. glutamate (a.k.a. MSG), leucine (supplemented to retain muscle protein during weight loss) or cysteine (disulfide bridges span two cysteines on the same or different proteins) that provide different chemical functions along the chains.
The quality of the amino acids in different regions cause those regions to twist into helices or flat sheets with extensive hydrogen bondsbiochemistry.suite101.com/article.cfm/hydrogen_bonds_form_biological_structures between the common structures that link the amino acid along the length of the chain. The helices and sheets fold together and bond (including disulfide bridges) to form the three dimensional shape of the functional protein.
Heat and Moisture Loosen Up Protein Hydrogen Bonds
Hydrogen bonds release only about as much energy as encountered in the kinetic energy of individual water molecules as they collide with each other. So, individual hydrogen bonds would be continually broken and reformed. The helices and sheets of a protein are relatively rigid, so several hydrogen bonds must be broken simultaneously for a protein to change its overall shape.
At higher temperatures, as provided with a curling iron or hair dryer, enough energy is imparted to the hair proteins to loosen up the proteins by breaking multiple bonds at the same time and permitting the hair protein, keratin, to stretch and slip past other keratin proteins. Cooling the protein in the new shape will permit new weak bonds and curls to form. Subsequently moistening the hair will let water molecules compete for the new bonds and permit a return to the original shape.
Permanent Waves Alter Disulfide Bridges
Cysteine amino acids terminate in an -SH group that can form a disulfide bond (-SS-)with a similar group on a cysteine in the same protein or in an adjacent protein. These disulfide bonds lend strength and a certain amount of rigidity to keratin proteins and to hair. The disulfide bonds also limit altering the overall shape of straight or curly hair. If these bonds are broken with special sulfhydryl reagents, however, then more substantial and lasting changes can be made.
Most of the permanent wave solutions contain sulfur molecules themselves, and that is why they have skunky smells. The hair proteins are stretched and contorted in new shapes after the disulfide bonds that hold them together in the shafts of hair have been broken (reduced). While in the new shape, the wave solutions are washed away and new disulfide bridges are permitted to form between new neighboring cysteines. This process is slow, and that is why washing the hair is avoided for a day or two.
Hair Styling Reveals Molecular Biology of Proteins
Hair is made of typical proteins and the modification of these protein structures by heat and moisture reveals the underlying structures of the proteins. Similar protein shape or conformation changes underlie many of the complex functions of cells.
References:
Alberts, B. et al. 2008. Molecular Biology of the Cell, 5th ed., Garland Science.
Campbell, N.E, et al. 2007. Biology, 8th ed., Benjamin Cummings.
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