DIMETHYL SULFOXIDE AND SKIN Table V Effect of Formaldehyde and Trichloroacetic Acid on Work Index of Guinea Pig Stratum Corneum" 137 Work Index No. of Work Index No. of (Solvcnt• Expcri- (Final Water • Experi- Reagent •, Water ] ments •,Initial Water,/ ments Comments 5% TCA 1.45 q- 0.12b 4 1.10 q- 0.26 3 c Reversible 10% formaldehyde 1.44 q- 0.39 4 1.56 q- 0.42 4 Irreversible The skin from one animal was used for the TCA results and the skin from another animal was used for the formaldehyde results. Mean and 95% confidence level (17). Reduction in number of experiments resulted from mechanical breakage of strips. groups may be similar to a mechanism suggested for trifluoroacetic acid (TFA). Near-infrared studies and electrical conductance measurements on model amide compounds and synthetic polypeptides have indicated that TFA is capable of protonating the amide groups with the subsequent formation of ion pairs (22). Aggregation of adjoining ion pairs within the insoluble skin matrix could produce a strengthening effect. In ad- dition, the precipitation of soluble components within the same matrix could also contribute to the increased strength. An irreversible effect was observed on strips treated with 10% for- maldehyde (Table V). Here the cross-linking ability of the reagent is revealed in a very definite manner. Ammonium Thioglycolate The effect of ammonium thioglycolate on stratum corneum was ex- amined since this reagent is capable of reducing disulfide bonds in wool and hair keratin. The maximum potency of this reagent occurs at pH _ 9. To eval- uate the changes in the work index due to the pH, five strips were treated with 0.1M ammonium chloride adjusted to pH 9. There was no sig- nificant change in the work index in ammonium chloride-treated strips at pH 9. Thus, it appears that the considerable weakening of stratum corneum in 0.1M ammonium thioglycolate can be attributed to the ac- tion of that reagent on the cystine cross linkages in the stratum corneum. The subsequent 24-hour relaxation process in which no attempt was made to exclude dissolved atmospheric oxygen resulted in a partial re- versibility of the cystine reduction process. The results are presented in Table VI.

138 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table VI Effect of Ammonium Thioglycolate on Work Index of Guinea Pig Stratum Corneum a Reagent Work Index No. of Work Index No. of Solvent• Experi- (FinalWater• Expcri- Water / ments •In• •ater/ mcnts Comments 0.1M NH4C1 pH 9 1.06 q- 0.08 b 4 1.04 q- 0.29 0.1M ammonium 0.48 q- 0.12 5 0.85 4- 0.11 thioglycolate No change in work index Partly reversible All experiments performed on the skin from a single animal. Mean and 95% confidence level (17). Reduction in number of experiments resulted from mechanical breakage of strips. Urea and Formic Acid The action of two other protein denaturing reagents on this system has also been examined. Both 6M urea and õ7% [ormic acid weakened stratum corneum considerably. Formic acid (õ7%) disrupted and weakened the skin in a very short time (2 rain). Formic acid is known to break both hydrogen and salt linkages. Due to the intense weakening o[ the sample it was not possible to undertake the reversi- bility study (Table VII). The effect o[ tim urea was not as dramatic and the action was partly reversible. Urea has been implicated by several investigators as capable o[ weakening hydrogen bonds, hydrophobic bonds, or more possibly some combination o[ the two (23). The urea results upon stratum cor- neum are presented in Table VII. Table VII Effect of Protein Denaturating Agents on Work Index of Guinea Pig Stratum Corneum Reagent Work Index No. of Work Index No. of Solvent• Experi_ (Final Water • Experi- Water / ments •,Initi•l Water/ ments 6M ureaa 0.73 q- O.16b 7 0.90 q- 0.16 6• 97cffo formic acida 0.25 2 a . . Comments Partly reversible . . . 6M urea experiments. Average value from the skin of two animals. Mean and 95% confidence level (17). Reduction in number of experiments resulted from mechanical breakage of strips. 97% formic acid, skin from a single animal reversibility study not possible.