Staying in the tradition of Astbury's hypothesis about the role of the α ⇔ β trans formation for the stress/ strain-curve of wool fibers and of Feughelman's X/Y-zones model, the interrelation between the morphological structure of keratin fibers and the shape of their stress/strain-curve in water is reevaluated. The yield and post-yield regions can be attributed to the opening up of two distinctly different and well defined portions of the monomer of the intermediate filament; the increased slope in the post- yield region can be attributed to the influence of the sulfur bonds in one of the segments. The role of the sulfhydryl-disulfide interchange reaction for the appearance of the post-yield region is pointed out and the molecular mechanisms for achieving the max imum possible strain are discussed.
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