According to a rigid definition, an amino acid is an organic acid that possesses at least one amino group.1 Almost a limitless number of molecules with various functional groups fall under this definition, but in more general terms, the definition limits the number of choices to the small group of natural L-a-amino acids that make up proteins and some other naturally occurring compounds.
Most proteins are composed of approximately 20 types of amino acids in varying proportions. Some additional amino acids are only found in special proteins; for example, hydroxyproline occurs in collagen and gelatin. All amino acid constituents of proteins are a-amino acids, referring to their molecular structure wherein the amino group is attached to the same carbon atom as the carboxyl group. Amino acids with a b-, g- and d- structure, or even with a sulfonate acid group instead of a carboxyl group, are found in living organisms in forms of small peptides or as free amino acids. The term free here is used to describe the amino acids that are not embedded in proteins.2
For most consumers, the term amino acid is still not as familiar as protein or vitamin, yet amino acids have been used in cosmetics for a long time. Among the various applications, the most significant is the use of natural moisturizing factor (NMF) amino acids and hydrolyzed proteins, and the latter has been used in hair care applications for nearly 50 years.3 Several moisturizers for cosmetic applications containing amino acids were reported as early as 1983.4
NMF is a complex mixture of free amino acids and other low molecular weight, water-soluble compounds found in corneocytes.5 It is known to contribute to the maintenance of water balance in the stratum corneum. The main components of NMF are pyrollidone carboxylic acid (PCA), lactic acid and amino acids, and it is used mainly in skin care as a powerful moisturizer.
Hydrolyzed proteins often contain free amino acids, to some extent. They are used in skin care and also are known to contribute to conditioning and protection of hair. Hydrolyzed collagen and hydrolyzed keratin are well-known, although various hydrolyzed proteins of vegetable origin have been introduced in recent years.
The properties of hydrolyzed proteins are determined by their average molecular weight and amino acid composition. Thus, it should be quite useful to understand the properties of amino acids before applying them in hair care. In this paper, the interaction and benefits of amino acids for hair are discussed.
Amino Acids and Hair Interaction
Fundamentals: Two fundamental factors can be involved in the interaction of hair and amino acids in simple aqueous solutions: diffusion and electrical charge of the molecules.
Given the small molecular size of amino acids and their hydrophilicity, diffusion is considered to play a major role in the uptake of amino acids but the hydrophobic nature of human hair is apparently a barrier for the diffusion process. Thus it is supposed that damaged hair shows higher affinity for amino acids than the natural hair due to its lower hydrophobicity. In addition, the increased number of ionic groups on the damaged hair protein will also contribute to the higher affinity. Figure 1 shows the comparison of arginine uptake by natural and damaged hair. The damaged hair takes up more arginine than the natural hair.
Amino acids that possess additional carboxylic groups on their carbon backbone are called acidic amino acids, and those with additional basic groups are called basic amino acids. This classification is useful to understand the nature of the interaction. Arginine is a basic amino acid that possesses a guanidinium group. The acid dissociation constant of the guanidunium group is 9.04, so it bears a cationic charge at a pH below 9. Therefore, arginine has strong affinity for hair in a pH range of 4–9. Acidic or neutral amino acids have a negative or neutral net charge in a pH range of 3–7, thus they are hardly taken up by hair (see Figure 2).
Above all, the guanidinium group of arginine is known to have quite a high affinity for hair protein.6 Figure 3 shows the amount of arginine recovered from hair either by water or by acidic buffer solution (pH 3.5).
The arginine recovery differed between the two conditions, shown by the errors, and the difference increased as the pH of the arginine solution was increased. This difference represents the existence of a strong interaction between acidic groups on hair and arginine.
Applications: Understanding the interaction of hair and amino acids in cosmetics is much more complicated than the above described cases because cosmetic formulations are complex mixtures of chemicals. Consequently, the chemical interaction between amino acids and other ingredients has to be taken into consideration. For example, neutral glycine and acidic glutamic acid present in conditioner are also taken up by hair.
At lower concentrations, arginine uptake predominates, and at higher concentrations, the uptake of the other amino acids increases. This hints at the possibility of arginine being used as an anchor for the deposition of other ingredients having weaker affinity to hair. One example is illustrated in Figure 5.
When PCA is applied to hair as an arginine salt, the uptake is larger than that of sodium, lysine and histidine salts.
Benefits of Amino Acids
Arginine is employed as an alkalizer in oxidative coloring agents and bleaching agents to reduce the irritative odor of ammonia and to develop milder products for the hair and scalp.6 Arginine prevents the decrease of tensile strength and hair surface hydrophobicity that are caused by oxidative coloring. It is also reported to prevent undesirable effects of hydrogen peroxide on hair proteins and hair surface lipids.7 During the coloring process, a considerable amount of the arginine contained in coloring agents is taken up by hair. This residual arginine confers a moist feel to hair and prevents color loss during the shampooing process.
A layer of fatty acids covalently linked to the surface of hair cuticle is responsible for the hydrophobic nature of natural hair. It also provides some benefits such as the low friction, smoothness and combability to the hair.8 Oxidative processes cause a decrease in hair surface hydrophobicity, resulting in the lack of smoothness. Cationic surfactants and silicones are often employed to improve the hydrophobicity.
Figure 6 shows surface hydrophobicity of bleached hair treated with hair conditioners. The average contact angle for natural hair was 100 degrees and a four-time bleaching treatment brought it down to 65 degrees. After treatment with a steartrimonium chloride conditioner (STAC), the contact angle increased slightly. The addition of 1.5% w/w of L-alanine resulted in a significant increase of the contact angle to a value, similar to that obtained with a distearyldimonium chloride conditioner (DSDAC).
Several amino acids have been reported to increase the tensile strength of hair in a dry state.9 This reinforcement is mainly achieved by ionic and hydrogen bonds, so the effect should diminish in a wet state; however, some amino acids have proven to be effective even in a wet state. Figure 7 shows the tensile strength of bleached hair measured in water. A significant increase was observed when phenylalanine and histidine were applied.
Improvement of Luster
Formation of medulla, or air-filled structures sometimes found at the center of hair shafts, and voids, hollow structures formed among cuticlar or cortical cells, by chemical treatment and grooming is known to result in a lusterless appearance in Asian hair.10 This finding is interesting because it indicates that not only the hair surface but also its internal structure is responsible for luster. Hair luster improved by treating bleached Asian hair with an amino acid mixture (see Figure 8).
A decrease of the light scattering lacunal structure is observed under a microscope, suggesting the amino acids penetrate and fill the void spaces to make hair shine from the inside. The moisturizing effect of amino acids is hypothesized to complement this function.
As mentioned, understanding the chemistry and interaction between amino acids and hair can provide a base for the development of hair care formulations containing hydrolyzed protein. However, it also should be noted that using specifically chosen, purified amino acids instead of hydrolyzed protein apparently has some advantages, such as: the possibility of avoiding odor and color problems; formation of stable products of high quality; minimizing the risk of allergic reactions; and the possibility of designing custom blends. Amino acids have been known and used as moisturizers for a long time, but because of their diverse chemical structure, they have the potential to exert a variety of functions in hair care preparations. This paper provides only a quick introduction for the novice formulator.
1. SH Pine, JB Hendrickson, DJ Cram and GS Hammond, Amino Acids, Peptides and Proteins, in Organic Chemistry 4th edition, London: McGraw-Hill (1981) ch 16–1
2. JP Greenstein and M Winitz, Nomenclature, Structure, and Occurrence of Amino Acids, in Chemistry of the Amino Acids, New York: John Wiley & Sons. Inc. (1961), ch 1
3. I Bonadeo and GL Variati, Affinity of hair for protein derivatives, Cosmet Toil 92 45–51 (1977)
4. M Takehara, Recent applications of amino acids for cosmetics: Interactions and synergistic effects of amino acids, Cosmet Toil 98 51–56 (1983)
5. CR Harding et al, Effect of Natural Moisturizing Factor and Lactic acid Isomers in Skin Function, in Dry Skin and Moisturizers: Chemistry and Function, M. Lodén and HI Maibach, eds, New York: CRC press (2000) ch 19, pp 229–267
6. M Arai, T Suzuki, Y Kaneko and M Miyake, Properties of aggregates of amide guanidine type cationic surfactant with 1-hexadecanol absorbed on hair, Proc Int Conference on Colloid Surface Sci. (2000)
7. E Oshimura and M Ino, Effects of arginine on hair damage via oxidative coloring process, J Cosmet Sci 55 suppl S155–S170 (2004)
8. JA Swift, Human hair cuticle: Biologically conspired to the owner’s advantage, J Cosmet Sci 50 23–47(1999)
9. K Hashimoto and Y Nakama, The features of recent damage hair in Japanese and development of shampoos and conditioners for the damaged hair, Fragrance J 10 29–36 (2003) (in Japanese)
10. S Nagase, S Shibuichi, K Ando, E Kariya and N Satoh, Influence of internal structures of hair fiber on hair appearance. I. Light scattering from the porous structure of the medulla of human hair, J Cosmet Sci 53 89–100 (2002)