Proteins are found in virtually every living system. They are the enzymes that are the driving force for our biological processes. They are the main components found in hair, skin, tissue and bone, and they provide the active basis for our immune response. Proteins are macromolecules (or polymers) composed of amino acids linked together by covalent peptide bonds. There are 20 different common amino acids, and they can be found in many different combinations (known as sequences) in protein molecules. Both the types of amino acids present as well as the amino acid sequence determine the final properties of the protein, and as you can imagine the possibilities are endless. The recipe for each highly-specialized macromolecule is contained within the DNA for all living organisms.
Proteins have four levels of structure that aid them in performance of the function for which they are designed.
- The primary structure is the amino acid sequence. This is the fundamental “building block” of the protein.
- The secondary structure arises as the molecule grows in size and begins to twist or fold into an alpha-helix, beta sheet, or other less defined “turn” structures.
- The tertiary structure is the structure that develops when side chains on a protein molecule are attracted to one another and assemble together to give the molecule a distinctive shape
- The quaternary structure is the final structure composed of multiple assembled protein molecules that form a complex.
The primary structure is formed via covalent bonding, while the other three structures are due to hydrogen bonding and hydrophobic interactions. Proteins and the study of them is a lifetime pursuit, but hopefully this brief background will establish the idea that proteins are quite remarkable in their design and function.
Proteins in Hair Care Products
Proteins have been used in cosmetics applications probably since the development of vanity in the human race. The beneficial properties of these natural substances were readily recognized and utilized, as illustrated in the tales of Cleopatra and her infamous milk baths.
Proteins adsorb readily onto the surface of skin and hair, forming moisture-retentive films. The films act to smooth and flatten the hair cuticle, which makes the hair shiny and more-easily detangled. These films can also provide some protection from the environment and pollutants. Proteins are generally hygroscopic, meaning they attract water molecules from the air, so they also act as humectants. Proteins added to bleaching or perming solutions have been found to significantly reduce damage to the cuticle, and their addition to dyeing solutions has been found to improve dye uptake into the hair while minimizing damage as well.
Most proteins used in personal care products have been hydrolyzed, a chemical method of breaking down the large structure of the protein into a smaller fragment of the primary structure (either a polypeptide or in some cases the amino acids themselves). These smaller polypeptides are more water soluble and thus more easily mixed into a formulation and also more readily absorb into the cortex of the hair.
Hydrolyzed proteins penetrate the cuticle and absorb into the cortex of the hair. Research has shown that as much as 30-50% of the protein found in shampoos is absorbed and retained by the hair. The percentage is even higher in conditioning products due to the absence of cleansing surfactants. This protein absorption has been found to increase the strength and elasticity of hair fibers. Also, the more damaged the hair, the greater the extent of absorption and retention. The high level of protein-retention by the hair may lead to buildup problems for some people, which can manifest as dry or brittle hair. This effect is more pronounced when a person has healthy hair that has had little exposure to thermal or chemical treatments. The best way to minimize or avoid this problem is to use protein-containing products sparingly if you notice build-up problems.
Some amino acids found in many proteins are positively-charged, which causes them to be attracted to negative substrates such as hair and skin. Proteins and polypeptides can also be chemically modified (quaternized) much like other polymers to have a greater number of positive charges on them to make them more substantive to hair. A few examples of these types of molecules are soydimonium hydroxypropyl hydrolyzed wheat protein, lauryldimonium hydroxypropyl hydrolyzed wheat protein and cocodimonium hydroxypropyl hydrolyzed wheat protein. These modified polypeptides are excellent conditioning agents and static reducers.
Some developments have been made by DuPont in devising genetic engineering techniques to produce a spider silk protein in its intact form (non-hydrolyzed) that is water soluble. They have made claims in their patents that this whole protein forms far-superior films on the hair and provides many excellent benefits. As technology in this area of biomaterials and genetic engineering develops further, we can hope to see more contributions of this sort to the field.
In summary, proteins are extraordinarily complex natural materials that can be of great benefit to the hair when applied in shampoos, chemical treatments, conditioners and styling products. On the exterior they provide moisture-retention, humectant properties, smoothing and detangling, and shine. As they penetrate the interior of the hair, they add strength and elasticity and act to “patch” weak spots. They are retained by the hair in high percentages, so some users may find it beneficial to rotate protein-containing products with ones without proteins. Many consumers have also found that using a very moisturizing conditioner paired with a protein product in their routine gives added benefit, probably due to the protein acting to seal in the extra moisture. As always, everyone’s hair is different as is their perception of what makes their hair feel and look nice, so it is always best to find what works best for you through experimentation
|Some common proteins found in hair care products|
|Protein||Major Amino Acids (generally many more amino acids are present)|
|Keratin||Proline, lysine, cysteine (a sulfur-containing amino acid)|
|Silk||Glycine and alanine|
|Soy||Glutamic acid, aspartic acid|
|Rice||Glutamic acid, aspartic acid, arginine|
|Milk||Glutamic acid, proline (contains all eight of the “essential” amino acids)|
|Wheat||Arginine, Leucine, Methionine|