Cross-linking structure of keratin. IV. The number of cross-linkages in low-sulfur components and the volume fraction of high-sulfur domains in various α-keratin fibers

Various α‐keratin fibers that had been treated with an 11M LiBr solution containing N‐ethylmaleimide showed typical rubberlike elasticity in a solution composed of equal volumes of 8M LiBr and diethylene glycol mono‐n‐butyl ether. Stress—strain relations of the swollen fibers were treated with a two‐phase model: a mechanically stable phase of higher cross‐linked domains and a rubber phase with lower cross‐link density. Stress—strain curves for a variety of keratins (three different human hairs, six different wools, mohair, cashmere, llama, alpaca, angora, and opossum) were analyzed by applying non‐Gaussian chain statistics to the swollen keratin network, including microdomains, which act as reinforcing filler particles in rubber. The phase structures of unswollen domains and swollen rubber were considered to originate from different structural components characteristic of α‐keratin, namely, the high‐sulfur matrix and the low‐sulfur microfibrils being randomized by swelling. It has been suggested that (1) the modulus of swollen fibers increases with increase of the content of disulfide (SS) in keratins, (2) the volume fraction of high‐sulfur domains increases with increase of SS content, and (3) the number of intermolecular cross‐links in the rubber region of low‐sulfur proteins is virtually the same among keratins and reaches about 65–75% of the SS linkages in the corresponding proteins. Some discussion has been made on the SS bonding in situ, namely, SS linkages between the low‐sulfur proteins, between the low‐sulfur and the high‐sulfur proteins, and between the high‐sulfur proteins in keratins. © 1993 John Wiley & Sons, Inc.