Thermal Transitions in Keratin

Abstract

Wool fibers in water at room temperature were extended by constant loads chosen to produce extensions in the range 0% to 25%. After relaxation for 1 hr, the temperature was increased to 100° C at a constant rate of approximately 1° C/min. The length of the specimens increased progressively with the temperature; the increase was taken as a measure of a transformation in the fibers, which embodies both the normal α ⇄ β phase transition of α-helical proteins as well as other structural changes which may contribute to the free energy of the transition in keratin. On reduction of the temperature from a higher to lower value, there was generally no recovery of length, although in comparable experiments at constant extension there was a considerable recovery of the force.Under the present conditions, it was not possible to observe the length changes corresponding to the complete transformation. However, families of curves for arbitrary small extensions were obtained for different stresses and used to calculate "effective" thermodynamic parameters appropriate to the time scale of the experiments. The entropy of transition was found to be 6 cal/deg mole of residues, and the heat of transition was 3 kcal/mole of residues. Treatment by formaldehyde or by an anionic detergent appeared to elevate the temperature range of the transition region, whereas permanently setting the fiber or testing at pH 1 depressed it. Measurements on unmodified wool fibers in water at different temperatures indicated the existence of a viscous flow mechanism with an apparent activation energy of 28 kcal/mole.