898 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS us briefly consider what are, in terms of interchain bonding, the con- sequences of charge rearrangement attendant upon bleaching. The displacement of carboxylic groups from their salt linkages with posi- tively charged amino groups by cysteic acid residues greatly stabilizes this linkage. This is simply due to the fact that cysteic acid residues remain ionized at low pH values at which the carboxylate groups, even those in the intact fiber, would protonate, leading to destruction of the electro- static bond. Thus, breakdown of the covalent disulfide bond is com- pensated for somewhat by formation of a more stable electrostatic cross link. In slightly bleached fibers only a fraction of the cystine under- goes the oxidative breakdown, and only a few of the carboxylic •oups are displaced [TOm their salt links and just as many new, stronger bonds of the same type are formed. In an extensively bleached fiber, the dis- placement is virtually complete and theoretically, the strength of the fiber should be only slightly affected by increasing acidity. Some experi- mental support [or this view is derived from a brief study of the effect of pH on the mechanical performance of bleached and reduced fibers. In both cases the fraction of broken disulfide bonds was close to 40%. We have chosen to use reduced fibers rather than intact fibers as our controls because the importance of charge rearrangement has to be viewed against backgrounds of similarly disorganized structures. The following test procedure was employed: Both bleached and reduced hair were soaked in 0.01N HC1 for 6 hours at 25øC and then rinsed with fresh changes of deionized water until no more acid was released by the keratin. The fibers were then dried, mounted on tabs, and stretched 5% in deionized water (calibration step). They were then released, kept in deionized H20 for 12 hours, and transferred for an additional 12 hours to buffer solutions, in which they were restretched again. The force to attain the yield point was calculated in both cases and the ratio Yield force in buffer Yield force-calibration denoted as the yield index (Fig. 6). The results conform satisfactorily to the pattern expected on the basis of our theoretical considerations. Thus, the bleached fibers exhibit a region of maximal mechanical sta- bility between pH 3 and 5 where the displaced carboxylic groups are undissociated and those ionized are bound in the salt links. An increase in pH above 5 leads to the ionization of free carboxyls, the fiber hydra- tion increases and so does the ease of its deformation. This be- havior is sharply contrasted by that of reduced fibers. With no free

HAIR BLEACHING 809 carboxyl side chains to ionize, the region of their mechanical stabili'ty stays almost unchanged up to pH 8. This is not so under acidic con- ditions where reduced fibers show a precipitous fall in yield force.. It is obvious that the combination of the disulfide bond breakdown and elimi- nation of electrostatic cross links have a disastrous effect on mechanical performance of the fiber. Although bleached hair also shows some weak- ening (apparently a sizeable fraction of acid-labile, salt links is still present), the stabilizing effect of new electrostatic bonds, involving •he cysteic acid residues and the charged basic groups of arginine and lysine is prominently evident. ACKNOWLEDGEMENTS The authors are indebted to the following people for their assistance in providing some of the data presented in this paper. The electron micrographic examination of melanin was carried out by Mr. A.. Dano of the Gillette Safety Razor Research Laboratories in Boston. Dr. Kokoschka of the National Bureau of Standards examined samples of melanin in a Varian X-band esr spectrometer and Dr. R. K. Brown of Wayne State University determined the molecular weight of solubilized melanin by thin-layer gel chromatography. : (Received March 10', 197,0) REFERENCES (1) Schrotter, A. V., Wasser stoffhyperoxyd als cosmeticum, Chern. Ber., 7, 980 (1874). (2) Weber, I.E., Hydrogen peroxide bleaching, J. Soc. Dyers Colour., 39, 209 (1923). (3) Trotman, S. R., Conditions governing the bleaching of wool with hydrogen peroxide, Ibid., 42, 154 (1926). (4) Smolens, H. G., Hydrogen peroxide bleaching of wool, silk and cotton under chemical control, .4ruer. Dyest. Rep., 18, 123 (1929). (5) Holmes, J. F., Modern methods of wool bleaching, Text. Color., 55, 250 (1933). (6) Smith, A., and Harris, M., Oxidation of wool: effect of hydrogen peroxide on wool, J. Res. Nat. Bur. Stand., 16, 301 (1936). (7) Smith, A., and Harris, M., Oxidation of wool: photochemical oxidation, Ibid., 17, 97 (1936). (8) Wilson, N. C., The scientific aspects of bleaching, Text. J. Aust., 15, 496 (1941). (9) Elod, E., The structure of the wool fiber, Melliand Textilber., 23, 313' (1942). (10) Funatsu, M., Chemical studies on hair. Mechanism of the reaction of concentrated hydrogen peroxide with hair, Nippon Nogei Kabaku Kaishi, 32, 175 (1958) C. A., 52, 12000 (1958). (11) Laxer, G., and Whewell, C. S., The measurement of damage produced by treatment of wool with solutions of hydrogen peroxide, J. Soc. Dyers Colour., 68, 256 (1952). (12) Zahn, H., Chemical changes of wool by washing, steaming and bleaching, Text.- Rundsch., 19, 573 (1964). _