Q) (.) C: .0 0 .0 INTERACTION BETWEEN HAIR PROTEIN AND ORGANIC ACID 143 1.7 1.6 1.5 1.4 1.3 1.2 7000 6000 5000 4000 Wavenumber/ cm-1 Figure 2. NIR spectrum of chemically untreated Chinese hair in nitrogen atmosphere at 25 ° C after being dried under vacuum overnight. combination of 2 x amide I + amide III. The bands in the 4500-4200 cm- 1 region seem to be combinations of CH stretching and CH deformation for CH2 and CH 3 (9). Since the band of the first overtones of CH stretching for CH2 and CH 3 seems not to be changed by the interaction with acids, this intensity of the band was used for the correction of band intensity. The combination band of NH stretching and amide II was used for the following analysis. NIR SPECTRA OF THE TREATED HAIR Figure 3 shows NIR spectra in the 5 300 - 4700 cm - l regions for the untreated hair and that treated with the acid solution for 20, 40, and 60 min. In addition to the combi- nation band of NH stretching and amide II around 4890 cm - 1 , two other bands can be seen in this figure. The band at around 5170 cm - l is due to the combination of OH stretching and OH deformation of water (8), and the other small band around 5050 cm- 1 is due to the side-chain amide group (9,10,17). INTENSITY OF THE WATER BAND It can be seen that the intensity of the water band decreases with treatment by the acid solution. The trend of the decrease in water is clear in Figure 4, where the relative absorbance at 5170 cm -l is plotted against the treatment time, T. The relative absor- bance is the highest for the untreated hair. This means that the untreated hair contains

144 JOURNAL OF COSMETIC SCIENCE 0.10 0.08 0.06 C 0.04 0.02 -- 0 5300 5200 5100 5000 4900 4800 4700 Wavenumber/ cm- 1 Figure 3. NIR spectra in the 5300-4700 cm- 1 region corrected on the basis of the intensity of the CH stretching band around 5900 cm -l. Thin solid line: untreated hair. Dotted line: hair treated with MA/ BOE/ethanol solution for 20 min. Dashed line: treated for 40 min. thick solid line: treated for 60 min. some water despite being dried under vacuum overnight and measured in the dry condition. The absorbance of the water band decreases with increasing treatment time in the acid solution, suggesting that the bound water is replaced by substances in the acid solution. PEAK SHIFT OF PROTEIN BAND BY ACID In Figure 3, the peak of the NH stretching and amide II combination band (protein band) appears to be shifted toward lower wavenumbers by the acid treatment. To display it in more detail, the second derivatives of the spectra in the range of 49 5 0-4 7 5 0 cm - 1 are shown in Figure 5, showing clearly a peak shift toward lower wavenumbers with the acid treatment. In contrast, little shift was observed for the combination band in the case of the hair treated with an aqueous solution of HCl/BOE/ethanol (data not shown). Therefore, the peak shift should be caused by malic acid. For the stretching mode, it is known that the band shifts toward lower wavenumbers if the bond is affected by the hydrogen bond. On the contrary, the amide II band, NH in-plane deformation mode, may shift toward higher wavenumbers by the hydrogen bond. The effect of the hydrogen bond on the peak shift is, however, generally higher for stretching modes than for deformation modes. Therefore, the peak shift toward lower wavenumbers for the combination band of NH stretching and amide II means that the hydrogen bond is strengthened. A similar trend for the peak shift and that of set durability improvement was also found for bleached Japanese hair (6). Thus it seems that malic acid works in the same manner for damaged hair, although all kinds of damaged hair have not been investigated.