CHEMISTRY OF KERATIN 227 reactions may be presented as follows: W--S--StoW + 2HS--CH2--COOH • 2W--SH + (S--CH•.--COOH)• (1) WSH + RX --• W---SR + HX (where X = . el, Br, etc.) (2) 2WSH + (CH•),•X2 -,- 2HX + W--S--(CH2).--S--W (3) Reaction of the type shown in equa.tions 1 and 2 result in perma- nent rupture of the cross links reac- tion with dihalides as shown in equation 3 result in th• formation of new cross links in which the sulfur atoms of the cystinc are connected by short hydrocarbon chains. The reaction in equation 2 and 3 offers a means of preparing a large number of wool derivatives. Thus it was not only possible to investigate the nature of the sulfur linkages and their relation to the structure of the fiber but also to study the effects on the properties of introducing into the fiber new groups having widely different properties. ELASTIC BEHAVIOR OF WOOL AND CHEMICALLY MODIFIED WOOLS Measurements of the elastic prop- erties were made on individual fibers by a modification of the method described earlier (6). In this method the 30 per cent index is the energy required to stretch a fiber to 30 per cent elongation after a treatment, divided by the similar energy requirement prior to the treatment. This index is based on Speakman's demonstration that wool fibers can be elongated 30 per cent without permanent defor- mation or weakening, if the duration of the strain is short (7). In the former investigations from this lab- oratory, however, the stress-strain characteristics of fibers were de- termined only during the extension process. In the present work, this procedure was modified so that measurements of the behavior of the fiber during its retraction could also be obtained. Figure 1 shows the behavior of a typical wool fiber during two suc- O--INITIAl ELONGATION O-$EC0ND ELONGATION t• {3_ uJ LOAD IN GRAMS Figure l.--Typica] stress-strain cycle ot' an untreated wool fiber. cessive stress-strain determinations. The fiber was allowed to relax for approximately 24 hours between the first and second extensions. It is noteworthy that the entire stress- strain cycle is reproducible. This fact makes it possible to compare the stress-strain characteristics of a particular fiber in retraction as

228 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS well as in extension, before and after a modification of its chemical structure. When the cross linkages have been permanently ruptured by the re- duction process, followed by alkyla- tion of the sulfhydryl groups with an alkyl monohalide of low molecular weight, such as methyl iodide or ethyl bromide, the resistance to ex- tension is greatly altered, as shown in Figure 2. Much less energy is re- g LOAD IN GRAMS Figure 2.--Effect on stress-strain cycle of rupturing cross links. quired to elongate the fiber after the cross links have been split, the 30 per cent index dropping to the low value of 0.26. In this material the S-shaped curve, characteristic of the original wool fiber and of other highly elastic materials, is not ob- served in extension to only 30 per cent, but can be demonstrated in experiments involving higher ex- tensions. The whole scale of the relations of stress to strain has been shifted toward greater extensions for smaller forces. It should be added that the recovery from 30 per cent extension to the original length is complete and rapid. If, after reduction, the cross links are largely rebuilt by reoxidation of the sulfhydryl groups to the disul~ fide form, the wool recovers to a large extent its original properties,' as shown in Figure 3. The small UNTREATEO REDUCED 6 REOXlOIZEO •c - .o •o I , 2 4 LOAD IN GRAMS Figure 3.--Effect on stress-strain cycle of reduction of cross links followed by reoxida~ tion with oxygen. discrepancy may arise either from the possibility that reoxidation has not been complete or that a few SH groups had reacted, during the re- oxidation, with sulfhydryl groups other than those with which they were combined in the original fiber. Alkylation of the reduced wool with an alkyl dihalide, such as methylene bromide or trimethylene bromide, results in the re-formation of cross linkages, but in this case,