Intermediate Filament Packing in α-Keratin of Echidna Quill

The swelling and associated physical behavior of wool α-keratin fibers when im mersed in a solution of formic acid in water have given rise to a detailed model for hexagonal packing of the intermediate filaments in hard α-keratin. Low-angle syn chrotron x-ray diffraction studies of the α-keratin fibers of dry and fully hydrated echidna quills not only provide extremely accurate measurements of the radii of the intermediate filaments and their center-to-center spacing, but also reveal the superpos ition of two separate infinite lattices in the meridional direction, one the accepted infinite repeat length of α-keratin, 47.0 ± 0.2 nm, and the other 62.6 ± 0.2 nm. Despite considerable overlapping, there are sufficient reflections unique to each lattice. These results and the equatorial data are consistent with the hexagonal packing suggested from the swelling experiments.