82 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Rate Constants and Half Times for DTT, DTB, and DTP Compound K K* t(1/2) (min.) DTT 6.8 x 10 6 1.8 23.5 DTP 3.3 x 10 -5 5.1 9.8 DTB 4.7 X 10 -5 6.5 7.0 0.05 M, pH 10.0, 22øC. The units of K are sec-• M • 5 CM-• 5 reaction between cystine and DTB. The DTB-cystine reaction would be expected to have a somewhat higher equilibrium constant than the DTP-cystine reaction, because the five-membered dithiane ring formed by DTP on oxidation is strained compared to the six-membered dithiolane ring formed by DTB. However, this difference is not large enough to produce major changes in the reduction rate, as DTB only reduces hair about 25% faster than DTP. 0. 0.6 0.4 \ \ \ \ \ \ \ \ DTB ' ' ...... '"'• DBOC I I I I 0 10 20 30 40 50 60 DBOM \ - DTT Time (minutes) Figure 4. SFTK curves for analogues of dJthiothreJtol (HS- CH 2 -CH- CH- CH 2 xo.o, 22øc. I I R• R 2 DTB -- 1,4-dithio-2-butanol. R• = H, R2 = OH. DBOM = 2,3-dimethoxydithiothreitol. R• = OCH•, R 2 = OCH•. DTT -- dithiothreitol. R• = OH, R 2 = OH. DBOC = 2,3-diacetoxydithiothreitol. R• --- OCO2H, R 2 -- OCO2H. -SH) 0.5 M, pH

KINETICS OF HAIR DISULFIDE BOND REDUCTION 83 Table II Values of K* for DTT Derivatives (HS - CH 2 - CH - CH 2 - CH - CH 2 - SH) I OR OR Compound R K K* DBOC CO2H 3.8 x 10 7 0.26 DTT H 6.8 X 10 -6 1.8 DBOM CH• 1.6 X 10 5 3.2 DERIVATIVES OF DITHIOTHREITOL Two diether derivatives of DTT, dimethoxy DTT (DBOM) and diacetoxy DTT (DBOC), are compared with DTT and DTB in Figure 4. DBOM and DTB are signif- icantly faster than DTT in reducing hair, while DBOC is much slower. Values of K • obtained for these derivatives are given in Table II. 1.0- 0.8- 0.6- 0.4- 0.2- 0.0 Figure 5A. SFTK curves for derivatives of dithiopropanol (HS-CH2 10.0, 22øC. \x. ,. \\",. • \ x. \DP-E3 I i I i 0 10 2o 3o 4o Time (minutes) - CH - I ! 50 60 -SH). 0.5 M, pH DP-E3 R = (CH2CH 2- O)2- CH2CH2-- OH. DP-E1 R = CH2CH2OH. DPOM R = CH 3. DTP R = H.