CHEMISTRY OF KERATIN 229 the disulfide linkage has been re- placed by a bis-thioether linkage of the type--S(CH2),S--. While the position of the stress-strain curve for the fiber after such treatment has been slightly shifted from that of the original fiber, the shape of the curve and the recovery are very similar to those of the original fiber. A slight links is to strengthen the fibers and to suppress plastic flow. INFLUENCE OF CROSS LINKS ON OTHER PROPERTIES OF KERATIN FIBERS From the foregoing discussion it is apparent that the disulfide cross links have a marked influence on the mechanical properties of keratin o .......... fibers. It can be also demonstrated ß ................... ._,o.,,,-. that these cross links have a pro- found effect on certain chemical and biological properties of these ma- terials. For example, keratin fibers are distinguished from most protein systems by a resistance to attack by biological agents such as bacteria and enzymes. The resistance to digestion by enzymes is probably cross links to form a three-dimen- due to the unique structure of pep- tide chains joined by. disulfide •'•-i--•e•'• 'i sional network of extremely high LOAD IN GRAMS Figure 4.--Effect on stress-strain cycle of rebuilding cross links by reduction and alkylation with an alkyl dihalide. shift in the position of the curve is to be expected, since obviously the lengths of the cross linkages have been increased and one would ex- pect a slightly greater ease of exten- sion of the fiber. These experiments lead to a better understanding of the molecular basis of the elastic properties of keratin fibers. The long range elastic char- acteristics probably result from the ability of the flexible chain molecules to contract from the less probably stretched state to a more random form. The function of the cross molecular weight.• Rupture of the cross links results in a 'material which is readily digested in solu- tions of the proteolytic enzyme pepsin (8, 9, 10). It is interesting to note that al- though keratin as a protein contains most of the essential amino acids this resistance to attack by enzymes makes it worthless as a food source for humans or most animals. Rup- ture of the cross links, however, does yield a digestible material. A not- able exception, however, is found in the case of the moth larvae which encounters little difficulty with ker- atin materials. Apparently moth larvae can digest keratin but only after breaking its disulfide. cross

230 JOURNAL OF THE SOCIETY OF linkages by the action of an alkaline reducing agent present in their in- testinal tracts (11). For this reason it would appear probable that modified wools containing stable cross linkages su(!h as shown in equation (3) would be far more stable to biological agents and such stability was actually observed in experiment (5). REFERENCES (1) Mark, H.,"Advancing Fronts in Chemis- try," Vol. I, "High Polymers," New York, Reinhold Publishing Corp. (1945), p. 7. COSMETIC CHEMISTS (2) Harris, Mizell, and Fourt, Ind. 2•ng. Chem., 34, 833 (1942). (3) Harris and Brown, Textile Research •., 17, 323 (1947). (4) Patterson, Geiger, Mizell, and Harris, 5 •. Research Natl. Bur. StaNards, 27, 89 (1941). (5) Geiger, Kobayashi, and Harris, In. 2•n. Chem., 34, 1398 (1942). (6) Sookne and Harris, 7. Research Natl. Bur. Stanards, 19, 535 (1937). (7) Speakman, •'roc. Roy. Soc., BI03, 377 (1938). (8) Hack, Ramsay, and Harris, 5 •. Research Natl. Bur. Standards, 27, 181 (1941). (9) Geiger, Patterson, Mizel[, and Harris, Ibid., 27, 459 (1941). (10) Geiger and Harris, Ibid., 29, 271 (1942). (11) Lunderstrom, Lang, and Duspiva, Z. •'hisioL Chem., 237, 131 (1935). v,