INTERACTION BETWEEN HAIR PROTEIN AND ORGANIC ACID 149 (c) (a) N-H- ----- O=C .· C=O--.s, H�:N· . N\ rt··· 0 -....+-o�q .. A ... =o-----·�-N· B Wet . ,, .. ,.q,,. �---' Dry l Acid treatment N-H ---------- O�G . 0 Wet . . · . � • H ---N . _.,., ..... - ......, -- J .-- 7H•,;o O O=C CC=O ---------·H-N Dry (d) Figure 8. Schema to explain water set of hair and its improvement by treatment with an organic acid. (a, b) Untreated hair in dry and high-humidity conditions, respectively. (c, d) Organic acid treated hair in dry and high-humidity conditions, respectively, Shaded objects represent parts of internal hair proteins. Dotted and dashed lines represent hydrogen bonds. An arrow in (b) means that proteins can move with each other by the breakdown of hydrogen bond linkage between proteins. A MODEL TO EXPLAIN THE IMPROVEMENT OF HAIR-SET DURABILITY BY ORGANIC ACID Figure 8 illustrates a scheme to explain how organic acid works to improve hair-set durability. This figure does not represent a specific part of the internal hair structure, but represents the hydrophilic region that contributes to the water set and its relaxation by the permeation of water molecules, followed by the exchange of hydrogen bonds. This part is not identified yet, but may be in the cell membrane complex, endocuticle, or the space between globular keratin-associated proteins in cortical cells. Figure 8a shows hydrogen bonds of the untreated hair in dry condition as dotted and dashed lines. Even in the dry condition, some water molecules remain and bind to the hair proteins. There are many hydrogen bonds between the proteins, retaining the water set. In high-humidity conditions, water molecules steadily permeate into the hair (Fig- ure 86). The hydrogen bonds between proteins are then easily replaced by those of

150 JOURNAL OF COSMETIC SCIENCE protein-water because they are not so strong. In the case of acid-treated hairs, an organic acid penetrating into the same area occupies the position where water originally existed (Figure Sc). Consequently, the band of water and band A of the protein band in the NIR spectra decrease simultaneously as described above. The anionic carboxyl group of the organic acid makes stronger hydrogen bonds with protein NH than protein-water (A) and protein-protein (B) hydrogen bonds. Even under high humidity, water mol- ecules also permeate, but the strong hydrogen bonds with the organic acid are not easily replaced by water, and they prevent proteins from moving. As a result, the hair shape is maintained. It is known that treatment with some acids decreases the water uptake of wool (19) and that naphthalenesulfonic acid improves hair-set durability (20). It is seen from Figure 4 that MA/BOE/ethanol treatment also reduces the water uptake of hair. The present NIR results suggest, however, a new, additional, mechanism of set durability improvement in the strengthening of hydrogen bonds by carboxyl groups of acids such as malic acid. CONCLUSIONS Hair-set durability against high humidity is improved by treatment with malic acid. From the fact that the improvement was confirmed even for single hair fibers, it is concluded that this improvement is due to internal changes in the hair fiber. This makes natural setting possible, and not due to the common technologies based on adhesion or fixing by oils or polymers. By the analysis of 2D NIR correlation spectroscopy, the behavior of organic acid was determined. It adsorbs at the site where water originally binds, prevents water penetra- tion, and makes strong and stable hydrogen bonds with hair proteins. The formation of such strong and stable hydrogen bonds suppresses the exchange of hydrogen bonds that is the cause of the breakage of set durability. ACKNOWLEDGMENTS The authors thank Dr. Daisuke Adachi for his kind offer of software for the generalized 2D correlation analysis. The authors are also grateful to Professor Yukihiro Ozaki (Kwansei-Gakuin University) for his kind and valuable discussions and advice on NIR measurement and 2D correlation analysis. The authors also thank Dr. Naohisa Kure, Director of Hair Care Research Laboratories, Kao Corporation, for helpful discussions and guidance. REFERENCES (1) S. Nagase, S. Shibuichi, K. Ando, E. Kariya, and N. Sarah, Influence of internal structures of hair fiber on hair appearance. I. Light scattering from the porous structure of the medulla of human hair,]. Cosmet. Sci., 53, 89-100 (2002). (2) S. Nagase, N. Satoh, and K. Nakamura, Influence of internal structure of hair fiber on hair appearance. II. Consideration of the visual perception mechanism of hair appearance,]. Cosmet. Sci., 53, 387--402 (2002). (3) M. Okamoto, R. Yakawa, A. Mamada, S. Inoue, S. Nagase, S. Shibuichi, E. Kariya, and N. Satoh,