398 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS •-keratin as judged by X-ray diffraction measurement had been unfolded. The process of unfolding the crystalline u-helical structure by extension and forming extended /3-keratin had been shown by Astbury (67) to be quite recoverable. When the fiber is returned on release to its original length, provided the temperature at which the extension occurred was not higher than about 50øC for a fiber in water, the/3-keratin disappeared and the •-keratin structure returned. The evidence appears clear, that in the Yield region the mechanical transformation of Burte-Halsey units from state A to state B, and units of crystalline u-helical material being unfolded forming extended /3-keratin are the same event. INFRA-RED ANALYSIS AND ELECTRICAL CONDUCTIVITY Infra-red absorption measurements on the Amide-N-H showed that if •x-keratin fibers were immersed in heavy water, exchange of -NH to --ND occurred primarily in the non-helical amide groups. However, when cr-keratin fibers in D20 are extended into the Yield region (and further) the deuteration occurs of the helical amide groups opened up by the extension (68). The unfolding of the •x-helices occurs with an apparent absoprtion of water by the opened up microfibrillar structure. Microscopic measurements by Haly (69) have shown that extension into the Yield region for a fiber in water is accompanied by an increase in moisture content for the fiber. Measurements of changes of electrical conductivity with extension into the Yield region also clearly indicate that the process of extension of a fiber is accompanied by moisture uptake into the micro fibrils. If in a high humidity environment (--- 90% Relative Humidity) a fiber is extended to a value of strain in the Yield region and held at this extension, the conductivity of the fiber initially drops markedly, and then recovers to an equilibrium value at a rate determined by the diffusion of water from the environment into the fiber (70). The initial extension opens up the micro fibrils so that water is transferred from the matrix. This results in a rapid loss of conduction by the depleted network of water molecules in the matrix responsible for the proton semi-conduction mechanism. Water to replace this depletion must come from the environment, and as shown by Algie this replacement may be hindered by the application of a barrier coating on the fiber surface. THE POST YIELD REGION MECHANICAL The stiffening of •x-keratin fibers on extension into the Post-Yield region was shown by Speakman (3) to be independent of the moisture content of the fiber. The increase of the incremental longitudinal modulus was shown to result from a covalently bonded network involving the cystine bond. Torsional data (71) obtained for fibers longitudi- nally extended to as high as 60% strain indicated no increase in the torsional rigidity of the fiber with strain, thus suggesting that the increased stiffness of the Post-Yield region is produced by material forming a part of the microfibrillar structure rather than the matrix. The sharpness of the onset of irrecoverability (58) of mechanical properties as a fiber is extended into the Post-Yield region also suggests that the change of events from the Yield to Post-Yield region involves highly ordered structures as may be associated with the micro fibrils.

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