362 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS involving experimentation with humans. Our finding that the in vitro method shows good correlation with the water bath immersion procedure indicates that probably a method like the one using keratin could, after further experimentation, be developed as a preliminary laboratory screen for evaluating the water resistance of sunscreen prod- ucts. REFERENCES (1) F. Bottari, E. Nannipieri, M. F. Saettone, M. F. Serafini, and D. Vitale, Substantivity of sunscreen a study on the interaction of four alkyl-4-aminobenzoates with keratin, J. Soc. Cosmet. Chem,, 29, 353-363 (1978). (2) O. J. Lorenzetti, J. Boltralik, E. Bushy, and B. Fortenberry, The influence of protein vehicles on the penetrability of sunscreens,_]. Soc. Cosmet. Chem., 26, 593 (1975). (3) I. Willis and A.M. Kligman, Aminobenzoic acid and its esters, Arch. Dermatol., 102, 405 (1970). (4) B. Cumpelik, Substantivity of sunscreen, Cosmet. Toiletries, 91, 61 (1976). (5) U. Hoppe, Neue hautaffine lichtschutzsubstanzen, J. Soc. Cosmet. Chem., 24, 317 (1973). (6) L. P•rez, G. Taucher, and O. Cori, Phitochemistry, 19, 183-187 (1980). (7) F. Kohout and J. Norwwod, Interpretation of research data analysis of variance, Am. J. Hosp. Pharm., 38, 96-104 (1981). (8) D. Schwartz, Mgthodes Statistiques, Deuxieme edition (Editions M•dicales Flaremarion, Paris, 1963). (9) R. Sayre, P. Poh Agin, D. Desrochers, and E. Marlowe, Sunscreen testing methods: In vitro predic- tions of effectiveness, J. Soc. Cosmet. Chem., 31, 133-143 (1980). (10) S. J. Katz, Relative effectiveness of selected sunscreens, Arch. Derm., 102, 466-468 (1970). (11) N.J. Van Abb•, The substantivity of cosmetic ingredients to the skin, hair and teeth, J. Soc. Cosmet. Chem., 25, 23 (1974). (12) D. E. Wurster and R. E. Dempski, Adsorption of lipid-soluble substances by human keratin, J. Am. Pharm. Ass. Sci. Ed., 49, 305 (1960). (13) R. J. Scheuplein, Mechanism of percutaneous adsorption, J. Invest. Dermatol., 45, 334 (1969). (14) N. J. Van Abb• and B. Cumpelik, J. Soc. Cosmet. Chem., 25, 23 (1974).

J. Soc. Cosmet. Chem., 36, 363-371 (September/October 1985) The disulfide interchange reaction of hydrolyzed hair keratin and bis-dinitrophenyl cystine in concentrated hydrochloric acid ELAINE L. PESCATORE and MAW-SHENG WU, The Gillette Company, Personal Care Division, Gillette Park, Boston, MA 02106. Received March 7, 1985. Synopsis The disulfide exchange reaction of cystine from hair keratin hydrolysates and bis-dinitrophenyl-cystine in concentrated hydrochloric acid was investigated. The reaction rate constants for the forward and the reverse reaction were found to be 12.8 and 5.0 hr -•, respectively. The equilibrium constant was 2.56. Using this reaction, the cystine contents in hydrolysates from intact and peroxide-bleached Caucasian hair were determined. The corresponding liquid retentions for the hair samples were also measured. A corre- lation between the decrease in cystine content due to the bleaching (from 647 • mole/g for intact hair to 450 • mole/g for 5 times bleached hair) and the increase in the liquid retention (from 38 to 53%) was demonstrated. INTRODUCTION The disulfide bonds of cystine in hair keratin contribute significantly to the physical and chemical properties of hair fibers. These bonds can be ruptured in a variety of ways including bleaching, waving, or exposure to UV radiation. Hair fibers undergoing these reactions will change their properties. An estimate of the number of disulfide bonds broken by these treatments would indicate the extent of these reactions with the hair. A disulfide interchange reaction in a concentrated acidic medium was demonstrated by Sanger (1,2) using bis-dinitroprophenyl cystine (DNP-S-S-DNP) and cystine (Cy-S-S- Cy) as model compounds k• Cy-S-S-Cy + DNP-S-S-DNP • 2 Cy-S-S-DNP (eq. 1) k2 (A) (B) (C) The mechanism of this reaction was deduced by Benesch and Benesch (3) and can be represented by the following scheme: INITIATION: RSSR + H + • RS + + RSH EXCHANGE: RS + q- R'SSR' • RSSR' q- R'S + INHIBITION: R'S + q- RSH • RSSR' q- H + 363