INTERMEDIATE FILAMENT PACKING IN ALPHA-KERATIN OF ECHIDNA QUILL

The swelling and associated physical behavior of wool alpha-keratin fi bers when immersed in a solution of formic acid in water have given ri se to a detailed model for hexagonal packing of the intermediate filam ents in hard alpha-keratin. Low-angle synchrotron x-ray diffraction st udies of the alpha-keratin fibers of dry and fully hydrated echidna qu ills not only provide extremely accurate measurements of the radii of the intermediate filaments and their center-to-center spacing, but als o reveal the superposition of two separate infinite lattices in the me ridional direction, one the accepted infinite repeat length of alpha-k eratin, 47.0 +/- 0.2 nm, and the other 62.6 +/- 0.2 nm. Despite consid erable overlapping, there are sufficient reflections unique to each la ttice. These results and the equatorial data are consistent with the h exagonal packing suggested from the swelling experiments.