PHYSICAL PROPERTIES OF ALPHA-KERATIN FIBERS 391 109 pascals as the stationary value agrees well with the equilibrium Young's Modulus with which it should correspond. At the temperature of liquid nitrogen (-196øC) no segmental mobility exists in the keratin fiber structure (23), and any mechanical distortion of the structure results in bond deformation only. The Young's Modulus of c•-keratin fibers under these conditions is 9.6 x 109 pascals which is a value close to that of ice, indicating the correspondence to the expected stiffness of a hydrogen bonded network (24). Similar values of longitudinal mechanical stiffness were obtained in experiments in which the strain was of very short duration (-10 microseconds), under which circumstances segmental mobility was again eliminated (25). The values of Young's Modulus quoted for keratin fibers in water correspond to an aqueous medium at a pH around neutrality. If an c•-keratin fiber is tested in aqueous media of varying pH at 20øC it is found that the mechanical stiffness of the fiber (26) remains unchanged from neutrality down to pH 3. Below pH 3 the Young's Modulus decreases by about 40% until a plateau is reached at pH 1. This phenomenon is reversible and indicates the reversible breakdown of Coulombic interactions (salt links) from pH 3 to pH 1 as the increased hydrogen ion concentration neutralizes the side chain carboyxlic acid groups (-COO- ---• -- COOH), resulting in a loss of interaction with positively charged groups in the side-chains of the polypeptides in the keratin structure. About 6-7 x 108 pascals of the equilibrium Young's Modulus values of 1.4 X 109 pascals are due to Coulomibic interactions (27,22). Although Coulomibic interactions are present throughout the keratin structure, only those shielded from water molecules, that is not interacting to any degree with water molecules, are involved in opposition to mechanical distortion. These are the interactions producing the stiffness which makes such a major contribution to the Young's Modulus of the fiber (28). X-RAY DIFFRACTION AND THE o•-HELIX Astbury (29) demonstrated that with extension of an c•-keratin fibre in the "Hookean" region the folded molecular structure corresponding to the material responsible for the high angle X-ray diffraction pattern changes by the equivalent amount corresponding to the overall strain of the fiber. This folded structure in c•-keratin has been shown by Pauling (30) to correspond to the helical structure designated as the c•-helix present in a large proportion of proteins. The mechanical opposition to extension of c•-helices has been demonstrated to be due mainly to hydrogen bonds present between the turns of the helices (31). X-ray evidence suggests that water sorbtion in an c•-keratin fiber is mainly confined to the non-crystalline regions (32). While the crystalline o•-helical structures are intact the stiffness of the c•-keratin fibers in water corresponds to a Young's Modulus of the order of 109 pascals. However, if the c•-helices are randomized (33) (as discussed below) in aqueous solutions of lithium bromide at concentrations greater than 6.4M, the Young's Modulus drops to the value of a material in an elastomeric state (of the order of 10 y pascals). OPTICAL BIREFRINGENCE In keratin fibers the optical birefringence of the fiber is mainly a measure of crystallinity (34). Fibers placed in aqueous solutions of lithium bromide (33) of a progressively increased molarity from pure water to a concentration of 5M, experience a relatively

392 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS small change in both birefringence and Young's Modulus. However, from a 6M to 7M concentration of lithium bromide the optical birefringence drops drastically to a value less than 0.2 of the value for the fiber in water. Completely concomittant with this rapid drop of birefringence, the Young's modulus of the fiber as indicated previously drops by a factor in excess of 20 to a value corresponding to a material in the elastomeric state. The c•-helices have been transformed to random coils and no longer form crystalline structures. Crystallinity within the c•-keratin structure, which involves the ordering of the c•-helices within that structure, appears to be the main factor contributing to the stiffness of the fiber in water at extensions within the Hookean region. INFRA RED ANALYSIS The infra-red absorption of c•-keratin fibers (35) exhibits the characteristics found in protein spectra generally, such as the amide A, I, and II bands. By the application of polarized infra-red and the measurement of absorption with the planes of polarization parallel and perpendicular to the fiber direction (dichroism), it has been shown that the amide NH and CO groups in keratin are preferentially ordered in the fiber direction (36). Further, when the fibers were immersed in heavy water (D20) within 24 hours about 70% of the amide hydrogens were replaced by deuterium (37). The remaining amide -NH groups were highly dichroic, the dichroic ratio for -NH groups increasing by the deuteration from about 1.5 to 5.5. This means that the undeuterated NH groups remaining correspond to highly oriented structures. A theoretic estimate from the dichroic ratio (38) shows that the 30% undeuterated groups could be accounted for by 80% perfectly aligned c•-helical material with about 20% randomly oriented undeuter- ated -NH groups. Further, the infra-red data means that this highly organized 30% of the structure of c•-keratin fibers in the time range of 24 hours exhibits negligible association with water as far as the amide --NH groups are concerned. Again the evidence points strongly to the lack of interaction of water with the organized c•-helical structure within the keratin fibers. ELECTRICAL CONDUCTIVITY AND THE DIELECTRIC PROPERTIES The electrical conductivity of an c•-keratin fiber is very dependent on the water content of the fiber. The keratin-water system acts as a protonic semi-conductor in which the mechanism of transfer of the proton is similar to that proposed in ice, consisting of a rotation of water molecules and proton jumps between two equilibrium positions between the oxygen atoms of neighboring water molecules. 39 The conductivity requires a continuous hydrogen bonded network of water molecules, and it is the sensitivity to the number of protonic pathways throughout this network that results in an increase of electrical resistivity of wool fibers at 25øC from 6 x 106 ohm-cm at 25% water content to 3 x 1012 ohm-cm at 7% water content based on the mass of dry keratin. The conductivity also depends on the freedom of rotation of the water molecules. The temperature coefficient of the conductivity corresponds to an activation energy barrier of 30K.cal./g. mole for a nearly dry fiber reducing to 15K.cal./g. mole at 25% water content, indicating a freeing within the structure of the movement of protons as the water content of the fiber is increased (40). With the small extensions corresponding to strain in the "Hookean" region the electrical conductivity of the fibers exhibits an insignificant drop. The network of water