PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 399 It has been shown from mechanical measurements on chemically modified fibers that the Post-Yield region slope is dependent on the cystine content of the fiber (72). This, however, is not the case for fibers in which the cystine content has been modified by a specialized feeding technique applied to the animal producing the fiber (73). An increase of 35% in the cystine content of the same wool fiber obtained by this latter method has negligible effect on the Post-Yield region of that fiber. It was shown for these fibers that the increase of cystine was produced in the matrix protein only, again emphasising the relationship between the Post-Yield region mechanics and the microfibrils of the cr-keratin structure. X-RAY DIFFRACTION In the Post-Yield region extension of the cr-keratin fibers results in an increasing loss of or-helical structure (66). For fibers extended at room temperature at strains of 50-60% all crystalline or-helical material has disappeared as far as X-ray diffraction measurements indicate. This disappearance of the ordered or-helical material with fiber extension is doubtlessly assisted by the association of the cr-helices with the covalent bonded network present in the microfibrils and responsible for the stiffening of the fiber structure in the Post-Yield region. The presence of such a network would create steric hindrance to the extension of each or-helical unit to its full /•-extended state. This would result in the partial extension of a large proportion of the or-helical units at a strain level lower than that expected if full unhindered extension occurred as in the Yield region. SUMMARY OF THE STATE OF KERATIN FIBER ON EXTENSION The evidence reviewed above consolidates the long accepted view of the two-phase nature of cr-keratin fibers, when considering their mechanical properties. Mechanically only two distinct phases can be separated out, and when considering the term "matrix" essentially we are taking into account not only the material seen under the electronmi- croscope as existing between the microfibrils, but all the material present, whose mechanical properties are indistinguishable from that of the matrix. What is the mechanical "microfibril" also does not necessarily correspond completely with the microfibril visible under the electronmicroscope. Side chains from or-helical compo- nents of the microfibrils certainly must interact with the matrix and the distinction of these into one or other phase is difficult. Table I sets out in summary the broad Table I A Description of the Major Molecular Events Occurring Within the Microfibrils and Matrix of a Keratin Fiber as Reflected by the Mechanical Properties of the Fiber. Region of Extension Microfibrils Matrix "Hookean" c• -- helices strained H-bonded water network + globular proteins in "gel" state Structure in "sol" state Structure in "sol" state Yield Post-yield + Covalent network under strain with possible bond breakdown

400 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS distinguishing mechanical events which occur in each of the two phases during the extension of the fiber. These are the events which are compatible with the physical evidence available and lead to an understanding of the mechanical properties of ce-keratin fibers. SET IN c•-KERATIN To obtain set in a fiber it is necessary not only to reduce the forces tending to return the fiber to its native configuration, but also to create new bonds in equilibrium with the setting strain. These latter bonds will oppose on release of the fiber any tendency for the fiber to return to its original state. In the case of "wet setting" of hair and wool fibers, the setting strain is applied to the fiber in a wet state, which assists in the rapid breakdown of weak interactions (hydrogen bonds, Coulombic interactions, etc.). If the fiber is dried while held strained, new interactions are formed between components of the keratin structure by the removal of water and the reduction of swelling. On release dry, the greater part of the fiber strain is retained, the fiber is set. However, if placed in water this set is released, and the fiber returns to its native configuration. This set is referred to as the Cohesive set, and is dependent on the breakdown and reformation of the Cohesive forces within the fiber. In commercial practice, where set in hair or wool textile material must be stable in the presence of moisture, it is necessary for the breakdown and reformation of bonds at the setting strain to involve covalent bonds, which will not be affected by the presence of moisture in the fiber. The disulphide bond formed by the diamino acid cystine present in c•-keratins is important in stabilizing the native configuration of the fiber. The breakdown and reformation of this bond via the mechanism of sulfhydryl-disulphide interchange (74) forms the basis of most techniques of "permanent" setting of c•-keratin fibers. Disulphide bonds in neutral or alkaline aqueous solutions can undergo a breakdown and reformation mechanism referred to as sulfhydryl-disulphide interchange in the presence of thiol groups by the following scheme: --S•- + --SbS•--•--Sb- + S•Sc-- This reaction results in the removal of disulphide bonds opposing the setting strain, the interchange favoring the breakdown of strained bonds, and their reformation in equilibrium, with the set configuration. In normal wool and hair about 0.5% of the amino acid residues are thioIs and the above mechanism proceeds in water at temperatures above 50-60øC. If more rapid interchange at lower temperatures is required to obtain this bond breakdown and reformation, it is necessary to form more thioIs by reduction of disulphide groups. The review of the chemistry involved, and the various techniques of setting of hair and wool, is beyond the scope of this consideration of the physical properties of ce-keratin fibers. However, it is necessary to note the effect of setting under certain conditions on the mechanical properties of the ce-keratin fibers and what this reveals with regard to.fine structure of the fibers. If a fiber is set by heating in distilled water at a strain corresponding to the Yield Region, the mechanical properties of the fiber are modified to a varying degree (see Figure 4). The stress-strain curve of the fiber is modified up to a definite strain level dependent on the setting strain, and beyond that level the stress-strain curve reverts to that of the unset fiber (75). The whole behavior of the fiber suggests that only zones