PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 401 C ! 0 LOAD '--'=' Figure 4. The typical load-extension curve of a wool fiber in water at 20øC after having been held strained 10% for one minute in boiling, distilled water and then released for one hour in water at: (i) 20øC, (ii) 53øC, (iii) 100øC. Also shown is the curve in an environment of 63% relative humidity and 20øC for a fiber set and released as for case (iii). In all cases the modification due to setting mechanical properties of the fiber is limited to the range ce to 15, a range defined by the setting strain. within the microfibrils of the fiber have been modified by the setting treatment, and the zones affected relate to the structure unfolded by the setting strain. The presence of these affected zones has been confirmed by torsional experiments on hair fibers set at various strains within the Yield region (76). These torsional experiments showed a linearly proportional relationship between setting strain and zones present in the set keratin-structure. SERIES-ZONE MODEL As a further refinement in our understanding of the mechanical properties of ot-keratins, the series-zone model (77) was proposed based on results obtained for longitudinal stress-strain behavior in water and in concentrated lithium bromide of unmodified and of set fibers (75). The model proposes the existence of two varieties of alternating zones along the microfibrils called X and Y, differing in their stability. The ot-helices within the X zones are the 30% that unfold in the Yield region of the stress-strain curve of a keratin fiber, and the Y zone ot-helices are unfolded with ,extension into the Post-Yield region. The opening up of the X zones is quite recoverable with no covalent bond breakdown involved, whereas the opening up of the Y zones as the fiber is extended into the Post-Yield region involves the breakdown of covalent interactions (disulphide bonds) which stabilize these zones. This series-zone

402 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS model, which is superimposed on microfibrils of the two-phase model, has been successful in explaining many mechanical phenomena for keratin fibers, such as the two-stage contraction behavior in concentrated lithium bromide solution (77). Recent interpretation of low angle X-ray diffraction data (78) by Fraser et al. has led to a suggested model for the microfibrils as shown in Figure 5. In cross section the ! Figure 5. Structure of a microfibril in c•-keratin as proposed by Fraser et a/.(78). The microfibril consists of three units, each containing four protofibrils with three protofibrils on the outside of the microfibril and one protofibril in the core of the microfibril. Each protofibril consists of a coiled rope with segments containing two or three c•-helices followed by a nonhelical structure. Within each unit the helical segments come together to form a purely helical component, 148.6/•k long, alternated by nonhelical segments of 52.7 •. Between the three units the helical components are staggered by 67.1 • relative to each other. (a) Radial projection of the surface of the microfibril illustrating the nine protofibrils on the outside of the microfibril with the helical segments clear and the nonhelical parts covered with random lines. (b) Portion of a unit on the outside of a microfibril with two ce-helices in one of the protofibrils. (c) Cross section of the microfibril with three helices per protofibril, illustrating the relative position of the four protofibrils in the one unit between the dotted radii. (1 • = 0.1 nm).