A Model for the Mechanical Properties of the α-Keratin Cortex

Abstract

Chemical and structural analysis of low and high sulphur protein extracts from α- keratin fibers has indicated that the low sulphur protein represents the protein of the microfibrils with its organized α-helical structure. Further, the high sulphur protein, together in some α-keratins with high glycine tyrosine protein, is the matrix protein. This latter protein, because of its high proportion of hydrophobic residues and covalent crosslinks of the diamino acid cystine, exists as a globular protein formed in the moist environment of the growing cortical cell. The matrix in a wet α-keratin fiber consists of water together with the globular high sulphur protein. The water acts as a continuous three-dimensional polymer network outside the globular matrix protein, interacting with the hydrophilic residues on the surface of the protein. From these observations, a simple microfibril-matrix model of the cortex of α-keratin fibers is proposed. This model quantitatively predicts the mechanical stiffening of a fiber being extended in water from the yield to post-yield regions of the load-extension curve. The prediction is obtained by two independent approaches that give similar results. The success of this model suggests that in considering the structural state of the cortex of α-keratin fibers, the biological origin, the chemical data such as amino acid residue sequences, and the physical measurements as applied for standard polymers need to be more dosely considered.