j. Soc. Cosmet. Chem., 36, 75-86 (January/February 1985) Factors affecting the kinetics of disulfide bond reduction in hair R. RANDALL WICKETT and BRUCE G. BARMAN, The Procter and Gamble Company, Miami Valley Laboratories, P.O. Box 39175, Cincinnati, OH 45247. Received October 31, 1984. Presented at the Annual Meeting of the Society of Cosmetic Chemists, New York, December 6-7, 1984. Synopsis The kinetics of hair disulfide bond reduction by dihydrolipoic acid (DHL), dithiothreitol (DTT), and 1,3 dithiopropanol (DTP) and their analogs have been investigated using a single-fiber technique. Structure- activity relationships are described, and it is shown that the overall rate of reduction may be altered by factors that affect the equilibrium constant of the reaction between the reducing agent and hair or the rate of diffusion of the reducing agent into the hair. DHL is many times faster in reducing hair than are its monothiol analogs correlating with the much higher equilibrium constant of the reaction between DHL and protein disulfide bonds. Reduction of hair with DHL leads to the formation of a moving boundary of reducing agent in the hair that can be clearly visualized by electron histochemistry. Structure activity relationships in analogs of DTT and DTP illustrate the effects of hydroxyl groups on reactivity and differences between the formation of five-membered dithiolane or six-membered dithiane rings on oxidation. INTRODUCTION A variety of hair care products from permanent waves to depilatories act by reducing hair disulfide bonds. The efficacy of these products may be affected by both the rate at which they react with hair disulfide bonds and the ease with which they penetrate into the hair structure. In a previous work on reduction kinetics in hair using a single-fiber tensile kinetics (SFTK) technique (1), dithiothreitol (DTT) and dihydrolipoic acid (DHL) were found to reduce hair much faster than did thioglycolic acid at pH 10 or below. It was postulated that this difference in kinetics is due to the much higher equilibrium con- stant between these dithiol reducing agents and hair disulfide bonds. Equilibrium constants are higher for these compounds because they form stable ring structures on oxidation (2). The equilibrium constant can affect overall reaction kinetics because the reaction leads to an increase in the diffusion constant of the reductant in the hair. This increase in diffusion constant can lead to the formation of a moving boundary of reducing agent in the hair that can be visualized by histochemical techniques. In this work we present a comparison of lipoic acid to its monothiol analog, 8-thiooc- 75

76 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tanoic acid, that clearly shows that it is formation of the cyclic dithiol that leads to the increase in the rate of reduction. We also present studies of reduction kinetics with homologous derivatives of dithiothreitol and dithiopropanol intended to help elucidate the relative importance of structural factors affecting diffusion into the hair. The SFTK method is ideally suited for such studies because quantitative comparisons can be made using small amounts of reducing agent. MATERIALS AND EQUIPMENT Human head hair from a single donor, who had never subjected her hair to chemical treatment, was used for all of the work described in this report. The hair was given a double lathering with Prell © Shampoo and thoroughly rinsed prior to use. Tensile measurements were made on an Instron © tensile tester interfaced to a Hewlett Packard 9825A © microcomputer. Measurements of hair diameter were made on an optical diameter gauging system made by the Diffracto © Corporation. Reducing agents that were not commercially available were synthesized in our laboratories by James Brown, PhD. THE SFTK METHOD The single-fiber kinetics method has been described in detail in a previous work. The method is based on stress relaxation caused by the breakage of disulfide bonds. Hair is extended to 102% of its original length in buffer under the pH and temperature conditions of the test and then rapidly stress relaxed using a strain cycling technique. The buffer is then replaced by a solution of the reducing agent of interest and reaction is followed by loss of tensile stress. Data may be analyzed quantitatively using either a pseudo-first-order model or a moving-boundary model of the reduction kinetics de- pending on the shape of the force-versus-time curve. The dithiol reducing agents studied in this work generally follow the moving-boundary model described by equation 1 below (1). 1. F(t) = F(0)exp(- (2/3) (KC/T)t3/2), where F(t) is the force at time t, F(0) is the force at time zero, C is the initial concentration of reducing agent, T is the root mean square of the two semi-diameters of the elliptical hair shaft, and K is the apparent rate constant. K is a combination of the rate of the reduction reaction and a factor related to the rate of diffusion of the reducing agent into the hair. RESULTS AND DISCUSSION COMPARISON OF DIHYDROLIPOIC ACID TO MONOTHIOL ACIDS Figure 1 shows SFTK curves for reactions of the sodium salts of DHL (dihydrolipoic acid), 8-thiooctanoic acid, and thioglycolic acid with different sections of the same hair at pH 9.0 using 0.1 M thiol (0.05 M DHL). Note that the rate of force reduction with DHL is much faster than with either 8-thiooctanoic acid or thioglycolic acid. The only difference in structure between DHL and 8-thiooctanoic acid is the presence of