KINETICS OF HAIR DISULFIDE BOND REDUCTION 79 Figure 2B. Thioglycolate-treated hair stained with silver methenamine. Mag. 16200 X. COMPARISON OF DTT, DTP, AND DTB SFTK curves for dithiothreitol (DTT), dithiopropanol (DTP), and 1,4 dithio 2-butanol (DTB) under identical reaction conditions are shown in Figure 3. Rate constants ob- tained from fitting the data to equation 1 are shown in Table I. These rate constants are extremely sensitive to differences in the SFTK curves due to the dependence of the

80 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ß .:•., •,v :• .: _., -, Figure 2C. Lipoate-treated hair stained with silver methenamine. Mag. 16200 rate on the 3/2 power of time and thus tend to exaggerate differences between reducing agents. We define a dimensionless parameter K* as K* = 5000 X K3/2X sec X (M/ cm) 3/2. This parameter provides a more linear quantitative estimate of the differences between reducing agents and is also shown in Table I along with values of t(1/2), the time required to reduce the normalized tensile force to 0.5. We can use t(1/2) for direct comparison in this case because all three compounds were run on different sections of the same hair and there is no difference in T. (See equation 1). For cross comparisons