Quantitative measurements of the intensity of the meridional reflections in the X-ray-diffraction pattern of α-keratin are shown to be consistent with a microfibril structure in which a surface lattice with an axially projected period around 200 Å is subject to a periodic interruption with an axially projected period of 470 Å. Taken in conjunction with recent evidence on the chemical structure of α-keratin and other inter-mediate filaments this finding enables an elaboration to be made of a model proposed earlier by RDB Fraser, TP MacRae, & E Suzuki (3. Mol. Biol. 108, 435–452, 1976.) for the α-helical framework of the microfibrii. The disposition and connectivity of the helical segments suggested here provides a straightforward explanation of a number of recent physicochemical and electron-microscopical observations on intermediate filaments and provides a starting point for the development of models for the framework of other intermediate filaments.

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