Density and Structure of Alpha-Keratin

Abstract

A MODEL of the two-phase structure of fibrous α-keratin has been evolved, starting with the α–β fold of Astbury et al.¹ and thence via the concept of α-helices to the coiled-coil models of Pauling and Corey² and of Crick³, and at present to the detailed schemes of packing recently proposed by Fraser, MacRae and Rogers⁴ and by Johnson and Sikorski⁵. The successive stages of this development have resulted primarily from attempts to give a more accurate explanation of the observed X-ray diffraction patterns. In addition, however, each advance has been judged by its success in explaining the ‘high’ densities found for keratin, and all have been found wanting because they led to lower crystalline densities than the values actually measured (definitive values given by Fraser and MacRae⁶ range from 1.283 g/c.c. for cow horn to 1.335 g/c.c. for echidna quill tip, at 25° C). This reasoning appears to rest on a false analogy with polymers, involving the assumption that the density of the crystalline phase is necessarily greater than that of the non-crystalline phase. Because this assumption is still influencing current thought on keratin ultra-structure it seems worth while to point out three deficiencies in the polymer analogy.