CHEMISTRY OF KERATIN 225 ing, twisting, or crushing. The im- portance of this property becomes most apparent when we consider the uses to which wool is put in industry, namely, in the fabrication of such materials as blankets, cloth- ing, and carpets. Thus, a wool blanket is warmer than a cotton blanket, but not because the wool fiber is intrinsically warmer than the cotton fiber. It is a well-known fact that if two blankets of similar construction and thickness, one of wool and one of cotton, are sub- jected to thermal transmission studies, they show closely similar in- sulation values. This is due to the fact that the insulation value of these systems is largely dependent on the large number of small air pockets trapped in the structures. However, empirical experience has indicated that the wool fabric is more satisfactory, and this appears to result from the fact that a cotton blanket during use, and especially when wet, tends to mat and, there- fore, loses a portion of its insulating qualities whereas the wool blanket, largely because of the elasticity of the wool fiber, tends to retain its essential thickness and thus main- tain itself as a better insulator over a longer period of time. In clothing these same considerations are im- portant, but in addition, the elastic- ity of the wool fiber results in a fabric which will recover its shape more satisfactorily after deforma- tion than cotton, linen, rayon, etc.-- fabrics which tend to crease readily on similar deformation. Finally, in the case of carpets, the important feature of the wool pile is that it re- covers well after it has been de- formed by applying a load. From the foregoing discussion, it is not surprising that nature should have furnished the human and animal body with protective and insulative tissues such as hair, skin, and fingernails which are highly elastic. In view of the importance of this property of elasticity in keratin fibers it is particularly de- sirable to examine in more detail, those aspects of their molecular structure which contribute to their unique behavior as fibers. CHEMICAL NATURE OF WOOL Keratin is composed principally of protein substances and as such is a polycondensation product of amino acids in which the different amino acids are linked together to form the polypeptide chain shown in the following scheme: R • I --HN--CH--CO--NH--CH--CO--NH-- I ' CH-- I R R 2 It will be immediately obvious that two molecular properties of these chains are of importance in terms of the principles proposed earlier. First, because of free rota- tion about the carbon-carbon and carbon-nitrogen bonds, such chains exhibit great flexibility which makes possible a large number of con- figurations. Thelong-r,•nge extensi- bility of keratin fibers can be as- cribed to the opening of these folds the elastic properties to the tend-

226 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ency of the extended chain to as- sume a more probable or stable form which is the folded form. The second factor influencing the prop- erties of such chains is the presence of relatively large size chains (R groups in the scheme of the poly- peptide chain) which prevent close packing of the protein molecules. It is estimated that close to 50% of the weight of the fiber is in the side chains (2). In other words the com- bination of the flexible chains and a structure that would fit poorly into a lattice structure would be expected to yield a system with rubber-like properties. Based on present knowledge of fiber structure, however, a system made up of flexible polar molecules which pack poorly would be ex- pected to exhibit relatively low strength, especially in the wet state (3). Since this is not in accord with the known properties of keratin fibers, it is necessary to search fur- ther for additional factors account- ing for the observed properties. A more detailed examination of the constitution of the keratin protein reveals the presence of a relatively high percentage of the double amino acid, cystine. NH• NH• CH---CH•--S--S--CHz--CH HOOC COOH Although cystine contains two amino and two carboxyl groups it appears that all these groups are in- volved in peptide linkages (4). On the basis of this, at least the major portions of the cystine must form parts of two separate polypeptide chains and accordingly the disulfide group becomes a connecting or cross link between polypeptide chains. Keratin fibers must therefore be considered a three-dimensional net- work of polypeptide chains held together by the disulfide groups of the amino acid cystine. Such a concept suggests that the role of cystine in keratin must be an im- portant one and indeed it can be shown that many of the •chemical, physical, and biological properties of keratin are dependent on the presence of these cross links (2, 4, 5). Such a structure suggests that the modification of the mode of linkage of the sulfur cross links should lead to profound alterations in the properties of the fiber. In subsequent investigations, it was found that the disulfide group was readily ruptured by a number of reducing agents, especially of the water-soluble type such as thio- glycolic acid (4, 5). If the solution of such reducing agents was highly alkaline, the cross links were rup- tured and in addition the freed poly- peptide chains went into solution. This is today the basis for a number of depilating systems. On the other hand if a portion of the disul- fide groups was ruptured and the pH was kept below about pH 10, the original fiber structure was retained and further the reduced fiber could be subsequently treated with alkyl dihalides to form a whole series of keratin derivatives (4, 5). The