Relevance and Evaluation of Hydrogen and Disulfide Bond Contribution to the Mechanics of Hard α-Keratin Fibers.

We propose a simple mechano-chemical model for the dependence of Young's modulus of α-keratin fibers on the hydrogen and disulfide bonds existing in the matrix and evaluate the relative change in bonding following an oxidative chemical treatment. Atomic force microscopy nanoindentation of longitudinal and cross sections of the fiber showed that, although the oxidative treatment breaks a significant amount of disulfide bonds, it introduces compensatory hydrogen bonds that maintain fiber elasticity at values comparable with those of the untreated fiber under dry conditions. The striking influence of humidity on the hydrogen bonding in keratin fibers is also evaluated. The hydrogen bonds are labeled as "type 1" and "type 2" hydrogen bonds; newly formed hydrogen bonds, type 2, are more labile than those native (type 1) to the matrix. Examining their contribution to Young's modulus of the matrix allowed for quantifying the loss of disulfide bonds, and the result matched the decrease in cystine, measured by amino-acid analysis, caused by the oxidative treatment.