148 JOURNAL OF COSMETIC SCIENCE V 1 C V 2 750 I' I' ,. ,. .,. I' 800 () 850 .................. ,. I' 900 I' ._ ____________________�::_::_::_::__::_:::_::_.._··-··-·-------+-=509:._::..: 4950 4900 4850 4800 Wavenumber/ cm- 1 4750 T""" I E (.) ..._ L... Q) ..c E :::J C: Q) ro � Figure 7. Asynchronous 2D NIR correlation spectrum in the 495 0-4 7 5 0 cm- 1 region constructed from treatment-time-dependent spectral changes of hair. Dotted lines through the tops of the cross peaks show components A, B, and C of the protein band. Shaded peaks are negative. in the synchronous correlation map are positive, indicating that the NIR spectral in- tensities of the water band and band A are either increasing or decreasing together. By taking into account the finding that the spectral intensity of the water band decreases with the treatment time, as shown in Figure 4, it can be said that band A decreases with the treatment time together with the decrease in the water band. As a result, band A is assigned to the NH protein linked by a hydrogen bond to water. The other two bands cannot be clearly assigned by the data of these measurements, but band B is the most abundant and can be identified as the protein NH group interacting with other protein residues. Finally, band C seems to arise from the protein NH group interacting with the anionic carboxylic group of MA, because band C relates to the strongest hydrogen bond (18).

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