KINETICS OF HAIR DISULFIDE BOND REDUCTION 77 1.0- 0.8- 0.6-- 0.4-- 0.2-- 0.0 h•oglycolate Dihydrolipoate ! I I I I i 0 10 20 30 40 50 60 Time (minutes) Figure 1. SFTK curves for dihydrolipoate, 8-thiooctanoate, and thioglycolate. the thiol group on the six position of DHL. The presence of this thiol group leads to the formation of a five-membered dithiane ring structure on oxidation. Even though the five-membered ring may be more strained than the six-membered dithiolane ring formed when DTT is oxidized (2,3), it still stabilizes the oxidized product. This leads to a higher equilibrium constant for reaction between DHL and disulfide bonds than for disulfide bond reaction with typical monothiol compounds. (Differences between five- and six-membered ring formation will be discussed further in the next section.) The equilibrium constant of the reaction can affect the overall reaction kinetics because the reaction of a reducing agent with hair disulfide bonds causes an increase in the diffusion constant of the reducing agent. Thus reducing agents with high equilibrium constants are very efficient in enhancing their own penetration and reduce hair along a sharp reaction front. In previous work (1) the presence of this sharp front was indicated by a kinetic model that fits the data and by light microscopy studies using dye pene- tration or reduction of iodinated fibers (4). The reducing agent itself can be visualized by electron microscopy using a modification of the histochemical technique of Swift (5). Control hair and hair treated for 5 minutes with either 0.5 M thioglycolate or 0.15 M dihydrolipoate at pH 9.3 was rinsed, fixed with 1% osmium tetroxide, thin sectioned, placed on 200-mesh gold grids, and stained with silver methenamine at 40øC for 3 hours. Electron micrographs at 16,200X are shown in Figure 2. This technique results in staining of SH groups and shows the extent to which the reducing agent has penetrated the hair. Lipoate diffusion is clearly seen to be of the moving- boundary type with all of the reducing agent behind the diffusion front, while thio- glycolate is seen to be distributed with a gradient from the outside inwards.

78 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2A. Control hair stained with silver methenamine. Mag. 16200 X. The results in Figures 1 and 2 are strong evidence that it is the high equilibrium constant between DHL and cystine, resulting from the ability of DHL to form a dithiane ring on oxidation, that causes it to reduce hair at a much faster rate than monothiol compounds with otherwise similar structures.