220 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II Reaction Rate Constants for Hair Fibers Reduced With 1 M Cysteamine at pH 7.0, 7.6, 8.0, or 8.5, 23øC Treatment k x 103 (s- l) Mean, SD pH 7.0 2.60, 0.35 pH 7.6 3.29, 0.34 pH 8.0 4.61, 1.56 pH 8.5 4.01, 0.90 Nine fibers per treatment. analyses. The results of amine acid analysis (Figure 2a) indicate that under experimen- tal conditions a maximal degree of reduction was achieved in five minutes. This was equivalent to reduction of 74% of cystine bonds (Figure 3). Further increases in length of reduction time only served to allow the reaction to go to completion (30 minutes = 92% of cystine bonds reduced). Cysteamine. Disulfide bond reduction by a 1 M cysteamine solution at pH 7.6 was studied in order to have 10% of the active reducing species present in solution (pKsH = 8.6). The results of amine acid analysis (Figure 2b) indicate that approximately 62% of the disulfide bonds were reduced in 30 minutes. These results indicate that reduction for 30 minutes by a 1 M cysteamine solution at pH 7.6 produces similar increases in the percent cystine cleaved as 1 M ATG solution at pH 9.4 in five minutes (Figure 3). Comparing the percentage of disulfide bonds reduced by ATG and cysteamine at iden- tical reduction times indicates that a greater percentage of the cystine bonds in the hair fiber were reduced by ATG than by cysteamine. Correlation of SFTK measurement data with amino acid analysis data. In order to compare the effects of reduction by ATG with crysteamine, two conditions were chosen where each solution had an equivalent percentage of active thiel species, RS-, present at the respective pH. These conditions were 1 M ATG solution at pH 9.4 (pKsH = 10.4) and 1 M cysteamine solution at pH 7.6, 23øC (PKsH = 8.6). A logarithm plot of the mean reaction rate constants was prepared, and the half-life (t•/2) for each condition was identified (Figure 4). The half-life was defined as the time required for the concentration of the disulfide bonds remaining to decrease to 1/2 of the original value [S- S] o. For a first-order reaction, the half-life can be determined from the following equation: t•/2 = 0.693/k (6) where k is the reaction rate constant that was experimentally determined. Comparison of the two conditions from the SFTK measurements indicates that the rate of reduction of the stress-supporting bonds in the hair fiber by ATG at pH 9.4 was faster than the rate of reduction by cysteamine at pH 7.6. This trend was supported by comparison of the half-life calculated for each condition. For comparison of the effects of reduction by ATG at pH 9.4 vs cysteamine at pH 7.6, using amine acid analysis, a logarithm plot of the disappearance of the disulfide bond, LN [(C t - Cf)/(C o - Cf)], vs time was prepared (Figure 5). The mean reaction rate constant for each of the two conditions was determined and the half-life for each condition was identified. The results indicate that the rate of reduction of the disulfide

REDUCTION OF HUMAN HAIR 221 1400 ß ,-',- 1200 1 ( 2a ) • 1øøø t •8øø t õ 6oo t •, 4oo t ,• •øø 1 0 ! • ! 2'5 3O 0 I 2 3 5 10 15 20 Time ( minutes ) • SCMC • CysMPA 1600 (2b) 1400 "" 1200 I• SCMC • CysMPA '• 1000 , I "-' 800 - I õ 600- • oot i ii l 0 • i i i 0 5 10 15 20 2•5 30 Time ( minutes ) Figure 2. Evaluation of reduction of cystine in single-head virgin hair by 1M ammonium thioglycolate, pH 9.4, 23øC (2a) and ! M cysteamine, pH 7.6, 23øC (2b), at various time lengths using amino acid analysis, indicating the increasing amounts of disulfide bonds cleaved with time as shown by increase in SCMC derivative and decrease in cystine-MPA derivative. V], SCMC derivative of cysteine (n = 4) I, cystine- MPA derivative of cystine (n = 4). bonds in the whole-hair fiber by ATG at pH 9.4 is faster than the rate of reduction by cysteamine at pH 7.6 (both solutions having an equivalent percentage of active RS- species present at their respective pH). By combining the results from both the amino acid analysis and the SFTK measure- ments (Figure 6), a correlation of the effects of reduction was obtained. Correlation of these results indicates that disulfide bond reduction produced by ATG proceeds at a faster rate than by cysteamine. The overall rate of reduction of both the whole-fiber and