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

222 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS lOO i • ,,I, ß 8o 6O 40 0 5 10 15 20 25 3o Time (minutes) Figure 3. Comparison of the percentage of cystine reduced in single-head virgin hair by 1 M ammonium thioglycolate, pH 9.4, and 1 M cysteamine, pH 7.6, 23øC, at various time lengths. O, 1 M ATG, pH 9.4 (n = 4) V, 1 M cysteamine, pH 7.6 (n = 4). 0 -7, I I I I 0 200 400 600 800 1000 Time (seconds) Figure 4. SFTK measurements: comparison of the reduction of stress-supporting disulfide bonds in single- head virgin hair by 1 M ammonium thioglycolate, pH 9.4, with 1 M cysteamine, pH 7.6, 23øC, using the mean reaction rate constant for each condition and first half-life. O, 1 M ATG, pH 9.4, slope = --7.44 X 10 -3 s-• ,, 1 M cysteamine, pH 7.6, slope = --3.29 X 10 -3 s -• stress-supporting disulfide bonds by ATG was faster than by cysteamine under condi- tions yielding equal concentrations of RS-. As shown in Figure 6, the results from amino acid analysis correlate well with the results