Amino acids and amino acid derivatives are mild and highly biodegradable raw materials for personal care products, and because they have unique functionality as cosmetic raw materials, more applications and expanded use are expected in the future, as Ajinomoto explains.

Amino acids are important substances for human as basic molecules constituting proteins and are also important for industrial substances because of the various physiological functions and physicochemical properties of each amino acid.

In recent years, with advances in production technology of biotechnology and an increase in production volumes, purified amino acids are becoming available at relatively low prices. Also, new findings of amino acids have been clarified through research activities. As a result, amino acids are widely used in various industries, including foods, supplements, medicines, feed, and personal care products.

In personal care products particularly, amino acids are known as the main components of natural moisturizing factor (NMF), which has the function of retaining water in the stratum corneum to maintain healthy skin. The constituent components of NMF are low molecular weight molecules such as amino acids, pyrrolidone carboxylic acid (and its salts), urea, mineral salts, and organic acids (and its salts), with the majority being amino acids and their derivatives.¹

Amino acids in the stratum corneum are primarily derived from profilaggrin, a protein produced in the granular layer of the skin. Profilaggrin is transferred to stratum corneum cells following epidermal turnover and then converted to filaggrin. It is further broken down into amino acids, some of which undergo metabolism and function as NMF within the stratum corneum.²

When the amount of NMF contained in the stratum corneum decreases, the moisturizing function of the stratum corneum decreases, hence the concept of replenishing NMF components to dry skin is helpful. In fact, when measuring the content of each NMF component in the stratum corneum, it has been reported that when NMF components are applied externally, amino acids and amino acid derivatives penetrate into the stratum corneum and improve moisture retention.³

In the personal care market, environmentally friendly natural and organic products, as well as social values such as ethics, have attracted a great deal of attention. Amino acids or amino acid derivatives, which are biological constituents and have high biodegradability, contribute to the development of a sustainable cosmetics industry.

Ajinomoto has a portfolio of personal care ingredients such as surfactants, wetting agents, emollients, oil gelling agents, functional powders etc. In this article, we will introduce humectants and functional powders.

Pyrrolidone carboxylate

Pyrrolidone carboxylic acid (PCA) has a structure in which glutamic acid is cyclized through dehydration condensation and is a humectant that is the main component of NMF. Since pyrrolidone carboxylic acid is obtained from glutamic acid produced by a fermentation method using a plant-derived sugar source, it can be said to be a fermentation-derived humectant.

As unneutralized PCA contains carboxylic acid, it is sometimes used as a pH adjuster in cosmetic formulations, but it is mainly used in the form of sodium or zinc salts with increased water solubility. Here, the character of the sodium salt and zinc salt of pyrrolidone carboxylic acid are described.

Pyrrolidone carboxylic acid sodium salt (PCA Na)

Sodium salt of pyrrolidone carboxylic acid has high solubility in water and can maintain a stable liquid state even at high concentrations, so it is mainly distributed as an aqueous solution as a cosmetic raw material (manufactured by Ajinomoto, product name: Ajidew™ NL-50). Figure 1 shows the structural formula of PCA Na.

Because PCA Na has high water solubility, it can be incorporated into various water-containing formulations and is commonly used as a humectant in emulsion systems such as milky lotions and creams, and cleansing formulations such as hair shampoo and body wash. Figure 2 shows the relationship between pyrrolidone carboxylic acid (PCA) and the amount of water in the stratum corneum.

After measuring skin capacitance at the inner side of the forearm, the stratum corneum was collected from the same site using tape stripping method, and PCA was extracted from the stratum corneum for quantitative analysis. There is a positive correlation between the amount of PCA in the stratum corneum and the amount of skin moisture.

Figure 3 shows the results of comparing the moisture absorption properties of PCA Na and other humectants. The measurement method is as follows. After measuring the initial moisture content of each sample using the Karl Fischer method, the sample was weighed in a weighing bottle (initial weight).

Next, the sample was left in a constant temperature and humidity chamber at 25°C and 65% RH, and the change in weight was recorded for 14 days. Subsequently, the weight was similarly recorded for 14 days under conditions of 25°C and 35% RH. The moisture content relative to the theoretical solid content was determined as the moisture content (%) using the following formula.

• Moisture content (%) = (Change weight – theoretical solid content) / (theoretical solid content) x 100
• Theoretical solid content (g) = initial weight x (1-moisture content)

As a result, PCA Na showed higher hygroscopicity under high humidity and higher moisture retention under low humidity compared to glycerin and sodium hyaluronate. This shows that PCA Na is a humectant with high moisturizing properties.

Next, using PCA Na and glycerin, we show the change in skin capacitance when an occlusive patch is applied to human skin (Figure 4, left). The quantitative values of PCA Na and glycerin in the stratum corneum were collected from the same site using the tape stripping method are also shown (Figure 4, right).

The skin capacitance value was measured after rinsing the inner part of the forearm with water and acclimating for 30 minutes under constant temperature and humidity conditions and was used as the initial value. Next, 450uL of the evaluation sample of each concentration was applied as an occluded patch for one hour.

After that, the test site was gently wiped and allowed to acclimatize for 30 minutes under the same constant temperature and humidity conditions, after which skin capacitance was measured and the stratum corneum was collected using the tape-tripping method. PCA and glycerin in the collected stratum corneum were quantified by HPCE and HPLC respectively.

From the left panel of Figure 4, PCA Na shows a higher change in skin capacitance than glycerin, indicating that it has superior moisturizing properties. From the right panel of Figure 4, it was found that PCA Na was absorbed more into the stratum corneum than glycerin.

All these effects were PCA concentration dependent. From the above, PCA Na has moisturizing performance equivalent to or higher than that of glycerin, a typical humectant.

The data presented so far was an evaluation using a simple aqueous solution of each humectant and measured the moisturizing performance of the humectant itself. However, actual cosmetics are multi-component systems containing many raw materials and such complex systems are required to exhibit sufficient moisturizing effects. The following shows the moisturizing properties when PCA Na is blended into a face mask formulation similar to an actual product.

The value after pre-rinsing with water was taken as the initial value. After that, the face was washed with a commercially available foam face wash, and after 20 minutes of acclimatization in a constant temperature and humidity room under controlled conditions.

A face mask soaked with 10g of internal solution with/without PCA Na was applied to each side of the face respectively for each face for five minutes. After wiping off the excess liquid, the stratum corneum water content and trans epidermal water loss (TEWL) were measured five to 30 minutes later.

As a result, the moisture content of the stratum corneum, face masks containing 1% PCA Na recovered faster from the decrease in moisture content of the stratum corneum caused by washing with facial cleansers than products without PCA Na. Regarding TEWL, it was also found that the face mask containing 1% PCA Na quickly lowered TEWL which had increased after washing. Therefore, it was found that PCA Na exhibits moisturizing properties even in a complex formulation system.

Pyrrolidone carboxylic acid zinc salt (PCA Zn)

Zinc pyrrolidone carboxylate (trade name AJIDEW™ ZN-100) is a skin conditioning agent obtained by neutralizing pyrrolidone carboxylic acid with zinc. Zinc compounds are generally known to have skin astringent effects and sebum secretion suppressing effects, and PCA Zn can be expected to have similar effects.

Figure 7 shows the wrinkle improvement effect of continuous use of a PCA Zn containing lotion. After washing their faces twice a day (morning and evening), subjects continued to apply the test formulation around their eyes for three months.

Photos were taken under normal light after 15 minutes of acclimatization under constant temperature and humidity conditions (temperature 20°C ± 2°C, relative humidity 50% ± 5%) without any application after the last face wash. The subjects were 22 women in 30s to 50s with wrinkle grades 1 to 3.

Based on the anti-wrinkle product evaluation guidelines of the Japan Cosmetics Society, wrinkle grades 0 (no wrinkle) to 7 (very deep wrinkles) were evaluated visually by an experienced evaluator. Figures 7 and 8 show changes in wrinkle grade during continuous use and a photo of the appearance after three months of continuous use. It was found that continuous use of PCA Zn improved the appearance of wrinkles.

Amino acid mix combining fermentation-derived amino acids

As mentioned above, it has been reported that amino acids and amino acid derivatives permeate into the stratum corneum and improve moisturizing properties when NMF components are applied externally.³

Amino acid mixtures can also be useful raw materials for personal care products. NMF compositions contain more than ten types of amino acids, and some amino acids can take time to dissolve when preparing cosmetic formulations, so aqueous solutions of amino acid mixes (Prodew™ series) have been commercialized.

The main amino acids and amino acid derivatives of the Prodew series are derived from fermentation and are used for skin care (600), hair care (500), and makeup products (700) depending on the application and concept. Amino acid composition has been optimized for each application.

Figure 10 shows the effect of improving the shine of hair strands when applying 500. Hair of a Japanese woman that was bleached four times was soaked in 100g of an aqueous solution containing 2 wt.% or 5 wt.% of the product at 35°C for ten minutes, and then rinsed in ion-exchanged water at 35°C for one minute. It was visually confirmed that the shine of the hair strands soaked in the amino acid mix was improved, and the gloss index, measured using a goniophotometer, was also improved.

Next, we introduce Prodew 700 for makeup products. There is a strong need of a long-lasting effect to enjoy makeup products stress-free. On the other hand, when focusing on the skin, makeup cosmetics cover the bare skin with various mixtures (oils, surfactants, powders) for a long time, so some consumers feel that makeup cosmetics burden their skin condition.

Many consumers remove their makeup when they get home, and makeup cosmetics are generally not thought to actively improve skin conditions. Therefore, we have developed an amino acid mix that both improves the long-lasting effect and improves the condition of the skin when using makeup.

Figure 11 shows the changes in skin capacitance and trans epidermal water loss (TEWL) on bare skin after repeated use of BB cream containing the product. The evaluation was carried out over a four-week period during the winter season, and after morning skin care, the subject applied the test sample of BB cream to their right or left cheek and followed their daily routine.

The skin condition after continuous use was measured after waking up, pre-rinsing the bare skin with warm water at 35°C, acclimating it for 20 minutes under constant temperature and humidity conditions of 23°C and 40% RH, using a Corneometer CM825 (Courage + Khazaka).

When applying a BB cream containing 2% amino acid complex (50% aqueous solution), it was confirmed that the skin capacitance of bare skin improved and TEWL decreased after four weeks of continuous use compared to a product that did not contain the amino acid complex (50% aqueous solution).

Lauroyl lysine based composite powder with high biodegradability

In recent years, as sustainability and environmental initiatives have increased, there has been a growing movement in various countries to ban or reduce the use of polymeric plastics due to concerns about their environmental impact. Countries in North America, Europe and Asia have already tightened regulations on rinse-off personal care products that use microplastic beads (MPB), and they are being replaced by alternative products.

However, unlike rinse-off products, it is technically difficult to develop MPB substitutes that maintain the texture and feeling during use in skin care and makeup cosmetics. Therefore, its use has not been regulated in Japan or other major countries. Lauroyl lysine, which is an amino acid-based organic powder, is known as a highly slippery powder having plate-like crystals and is used in many makeup cosmetics. By taking advantage of the characteristics of lauroyl lysine, we are developing MPB substitutes with excellent texture and functionality (Amihope SB series).

Figure 12 shows a SEM image of the developed lauroyl lysine composite powder. Silica-based SB-101 has an excellent SPF boost effect, starch-based SB-102 has excellent soft-focus properties (Figure 13), and both powders exhibit good biodegradability.

Conclusion

This article introduced some amino acids mixtures and amino acid derivatives. Amino acids and amino acid derivatives are mild and highly biodegradable raw materials for personal care products, and because they have unique functionality as cosmetic raw materials, more applications and expanded use are expected in the future.

The widespread use of sustainable amino acids and amino acid derived raw materials in personal care products is expected to contribute to the development of a sustainable cosmetics industry with low environmental impact.

References

1. Spier HW, Pascher P. Der Hautarzt. 1956; 7, 55-60

2. Society of Cosmetic Chemists of Japan. Natural moisturizing factor. https://www.sccj-ifscc.com/library/glossary_detail/1126

3. Bio Industry. 22 (9), 31-37, 2005-09