J. C osmet. Sci. J 5 7, 13 9-151 (March/ April 2006) Study of the interaction between hair protein and organic acid that improves hair-set durability by near-infrared spectroscopy TAKASHI ITOU, MASAYOSHI NOJIRI, YOSHIKAZU OOTSUKA, and KOICHI NAKAMURA, Kao Corporation, Hair Care Research Laboratories, 1-3, Bunka 2-chome, Sumida-ku, Tokyo 131-8501 (T.I., M.N., K.N.), and Kao Corporation, Analytical Research Center, 1334 Minato, Wakayama City, Wakayama 640-8580. (Y.0.), Japan. Accepted for publication October 31, 200 5. Synopsis In this study, hydrogen bonds around hair proteins were analyzed by near-infrared spectroscopy to reveal the mechanism of improving hair-set durability by treatment with a specific organic acid. The improvement of set durability was confirmed by measurement on single hair fibers, suggesting that improvement is not because of the surface adhesion increase but because of the internal changes in the hair. Through analysis by two-dimensional near-infrared correlation spectroscopy, it was found that a combina- tion band of stretching NH and amide II is deconvoluted into three bands interacting with different hydrogen bonds. From the assignment of the three bands, the behavior of the organic acid in the hair was clarified as follows: it adsorbs at the site where water originally binds, even in extremely dry conditions, prevents water penetration, 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. INTRODUCTION It is well known that hair damage is a cause of lusterless appearance, rough feel, and mechanical breakdown. According to recent studies ( 1-3 ), hair damage by bleaching or dyeing, along with repeated daily hair care processes, including hair drying with a hot dryer, generates several types of light-scattering origins in the internal structure of the hair, such as splitting of cuticle layers, the generation of micropores in the cortex, and the generation of a porous medulla. The pores inside hair account for its dull appearance. For a technology to suppress the pores, it has been reported that a combi- nation of specific organic acids and solvents are effective (3 ,4). In further research to confirm the change in appearance by the organic acid and solvent on various hairs, we 139

140 JOURNAL OF COSMETIC SCIENCE noticed that hair-set durability was also improved by the same treatment, as well as hair luster. Drying a wet hair, held in a given shape, with a hot dryer or a curling iron makes the hair set. This process is usually called water set. It is well known, however, that the created hair style is easily broken down with time, especially quickly in high-humidity conditions. This is due to the exchange of hydrogen bonds in the hair fiber, according to the movement of water molecules (5 ). Through treatment with an aqueous solution comprising malic acid (MA) and benzyl- oxyethanol (BOE), the setting durability of the hair was improved. This improvement was observed in the test using single hair fibers (6), suggesting that the effect is not on the basis of a surface-adhesion increase but a change in the internal property of hair. Consequently, the treatment makes it possible for hair to be set with a natural feel. This setting method is completely different from fixing by setting polymers or adhesive oils, which often are the causes of hair roughness or stickiness. Since water set is closely related to the interaction of proteins with water, near-infrared (NIR) spectroscopy is one of the best methods (7) to analyze hydrogen bonds and has been used by other researchers. The NIR region corresponds to overtone and combina- tion bands of infrared bands. The absorbance intensity of these bands is relatively weaker than that of normal modes in infrared spectroscopy, but the low absorbance leads to a high rate of transmission and gives internal information on the hair. The NIR spectrum ' is complex in general, but many bands for various materials, including proteins, have been assigned (8). For wool and human hair fibers, spectra in the NIR region have been measured and the main bands assigned (9-11). By measurements with NIR spectroscopy (6), it has also been suggested that the mecha- nism of improvement of set durability by organic acid is due to the strengthening of hydrogen bond interaction around hair proteins. For further analysis, generalized two- dimensional (2D) correlation spectroscopy was applied in this study. This method, proposed by Noda (12), has been applied extensively to analyze the IR and NIR spectra of proteins (13-16). In this method, a perturbation is applied to a system and various spectra are measured under sequential conditions. The measured fluctuations of spectral signals are then transformed into 2D spectra by use of a correlation method. The 2D correlation spectra can deconvolute overlapped bands, enabling us to know the correlations between bands due to different structures, and it provides information about the specific order of structural changes under perturbation. Any perturbation- mechanical, electrical, chemical, thermal, etc.-may be used, and temperature, concen- tration, or pH are well selected as variables. In this study, we selected the treatment time on hairs as the variable. EXPERIMENT AL HAIR SAMPLES Chemically untreated Chinese hair washed with a plain shampoo was used. The average diameter, measured by a micrometer, was 62 µm. For the acid treatment, hair fibers were immersed in an aqueous solution comprising MA (4%)/BOE (10%)/ethanol (15%) at 40 ° C for 20, 40, or 60 minutes, washed with water