Low‐sulfur proteins from α‐keratins. Interrelationships between their amino acid compositions, α‐helix contents, and the supercontraction of the parent keratin

Abstract

Abstract S ‐Carboxymethyl (SCM) kerateine preparations from a range of keratins were fractionated by acid precipitation into low‐sulfur (SCMKA) and high‐sulfur (SCMKB) fractions. Amiono acid analyses and optical rotatory dispersion measurements on the SCMKA fractions from different keratins indicated that the proportion of α‐helical material (30–64%) increased with increasing leucine and glutamic acid contents and decreased with increasing SCM‐cysteine and proline contents. It is shown that these variations in α‐helix contents are due mainly to differences in the conformations of the principal components of the SCMKA fractions, although there is also a smaller contribution arising from contamination with varying amounts of non helical components. The thiol + disulfide contents for the parent keratins were measured and compared with the SCM ‐cysteine contents of the solubilized fractions. In general the SCM‐cysteine contents of the SCMKA fractions increased with increasing thiol + disulfide content of the parent keratin, but these values were not directly proportional. The extent of total supercontraction in 8M LiBr solution was measured for the parent keratins and shown to bear no significant relationship to their disulfide contents. From the extents of first‐stage and total supercontractions the disulfide contents of the contractile structures in the fibers were calculated according to a model described previously and compared with the analytical values for the SCM ‐cysteine contents of the SCMKA and SCMKB fraction. Data for both fractions were consistent with the model in that they were in each case related to the calculated values by a constant factor. However, because of simplifying assumptions in the model, it was not possible to identity positively one or other of these protein fractions with the contractile elements in the fiber.