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).

PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 403 proposed microfibrils consist of zones of purely c•-helices for a longitudinal distance of 1.44 nm in series with zones of 5.27 nm containing 2/3 c•-helical material and 1/3 non-helical high cystine material (see Figure 5). The purely c•-helical zones may be identified with the X zones, and the others with the Y zones. Calculations can be made on the extensibility of these X zones based on the extension of an a-helix being 125% when transformed to the/• state. The amount of strain corresponding to the unfolding of the c•-helices of the X zones in the Yield region = 125 x [1.44/(1.44 + 5.27)] = 26.8%, a very reasonable estimate compared with the value of 28% obtained from stress-strain data (75). The proportion for the microfibrils of c•-helical material in X zones on Fraser's data = [1.44/(1.44 + • X 5.27)] = 0.29 is in agreement with the value of 0.30 obtained by Bendit (66) for the number of c•-helices unfolded at the end of the Yield Region. The length of !.44 nm for the X zones means that the zones consist of short lengths of c•-helices, a little less than 3 turns. This would explain why there is no problem in the extension of these units into the /• state. Any wrapping around each other of the c•-helices due to Pauling's coiled-coiled proposals for the association of c•-helices (79) would be negligible over such a short distance. As already quoted, evidence from torsional data for fibers extended into the Post-Yield region indicates no increase in the stiffness of the matrix with the extension (71). In agreement with this and the "crystalline" sharpness of the change over events such as formation of free radicals (59) and the indication of mechanical irrecoverability (58), the series-zone model has associated the increased stiffness of the extending fibre in the Post-Yield region with the highly ordered micro fibrils. Other mechanical models have been proposed to explain the behaivor of c•-keratin fibers extended into the Post-Yield region (80). These, however, do involve change in the mechanical behavior of the matrix with extension of the fiber beyond the Yield region. The evidence from X-ray diffraction for the zonal nature of the microfibrils, the quantitative agreement of this evidence with the proposed series-zone model and the model's ability to explain the mechanical behavior of fibers modified by setting treatments, the two-stage super- contraction of fibers heated in concentrated aqueous lithium bromide solutions, all emphasize the usefulness of this model in our understanding of the behavior of c•-keratin fibers. ACKNOWLEDGEMENTS The author wishes to acknowledge the valuable support of Reckitts Toiletties International and the CSIRO Division of Textile Physics, Ryde, in the preparation of tfiis review paper. REFERENCES (1) W. T. Astbury and A. Street, X-ray studies of the structure of hair, wool and related fibres. I. General Phil Trans. Roy. Soc., 23OA, 75 (1931). (2) G. Blankenburg, Stress-strain properties and supercontraction of wool keratin in aqueous alcohol media. III Congress International de la Recherche Textile Lainiere (Paris) Section 2, 31 (1965). (3) J. B. Speakman, The intracellular structure of the wool fibre, J. Textile Inst., 18, T431 (1927). (4) H. Zahn, Wool is not keratin only. Proc. 6th Quinquennial International IVool Textile Research Conf, Vol. I, Pretoria (1980). (5) R. W. Moncrieff, IVool Shrinkage and Its Prevention, (National Trades Press, London, 1953).