KINETICS OF HAIR DISULFIDE BOND REDUCTION 85 Table III Derivatives of Dithiopropanol Compound R K K* DTP H 3.3 x 10 -5 5.1 DPOM CH 3 3.1 X 10 -5 4.9 DTP-E 1 CH2CH 2 - OH 2.3 X 10- 5 4.0 DTP-E3 (CH2-CH2-O)34 9.4 X 10 -6 2.2 DPOV (CH2)5CO24 1.2 X 10 ¸ 2.6 DPOC CH 2 -- CO2 H 8.5 X 10- 6 2.1 DPOT CH2-- Q -CO2 H 3.3 X 10 -6 1.1 The units ofK arecmM-•sec-•5 None of these derivatives is as fast as DTP. While the methoxy derivative of DTT, DBOM, reduces hair much faster than DTT, it appears that removal of the single hydroxyl group in DTP is compensated for by the added bulk of the methyl group and that the difference in rate between DTP and DPOM is not significant. The four additional CH2 groups in DPOV do not decrease the reaction rate compared to DPOC. Perhaps their added bulk is compensated for by an increase in the hair/water partition coefficient, due to the decreased polarity of DPOV, resulting in equivalent rates of diffusion into the hair. The presence of the benzene ring in DPOT does result in a significantly slower rate of reduction. Ethoxylation of DTP to DTP-E1 leads to an approximately 20% decrease in K*, and ethoxylation to DTP-E3 leads to an approximately 60% decrease in K*, corresponding to a 20% decrease per ethoxylate group. We do not know if this correlation holds at higher levels of ethoxylation. CONCLUSIONS 1. Dithiol compounds that form five- or six-membered rings on oxidation reduce hair much faster than do monothiol compounds of similar structure. This difference is probably due to the much higher equilibrium constant for the reaction between cyclizing dithiols and disulfide bonds. 2. Histochemistry confirms the "moving-boundary" mechanism for dithiol reductants. 3. Formation of a six-membered ring on oxidation is slightly preferred over five- membered ring formation. 4. Polar substituents capable of hydrogen bond formation can dramatically slow dif- fusion into hair. 5. The addition of up to four CH 2 groups into the hydrocarbon chain of a reducing agent may have little effect on the overall reduction rate, while addition of a benzene ring can lower the rate significantly. REFERENCES (1) R. R. Wickett, Kinetic studies of hair reduction using a single fiber technique, J. Soc. Cosmet. Chem., 34, 301-316 (1983).

86 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (2) W. W. Cleland, Dithiothreitol: A new protective reagent of SH groups, Biochemistry, 3, 480-482 (1960). (3) U. Schmidt, P. Grafen, and H. W. Goedde, Chemistry and biochemistry of lipoic acid, Angew. Chem. Internat., Edit/Vol. 4, 846--856 (1965). (4) K. W. Herrmann, Hair keratin reduction and swelling in mercaptan solutions. Trans. Faraday Soc., 59, 1663 (1963). (5) J. A. Swift, Electron histochemistry of cystine containing proteins in thin transverse sections of human hair, J. Royal Microsc. Soc., 88, 449-460 (1968).