The limiting collagen microfibril The minimum structure demonstrating native axial periodicity

Collagen fibers were grown from solutions of acid-soluble or neutral salt-soluble collagen in 0.5M acetic acid by rapid dialysis. The collagen was obtained under conditions where protease inhibitors were present at every stage of extraction and purification. Under the conditions used, length-wise but not lateral filament growth proceeded rapidly and gel-like networks were formed. Water readily exuded from the networks. The networks were stretched to fibrous form during drying. Small-angle X-ray diffraction showed the stretched fibrils to be highly ordered, showing up to 20 orders of the 670 Å meridional periodicity. Intermediate- and wide-angle photographs show equatorial reflections at a spacing corresponding to ≈12.5 Å which is related to the intermolecular distance but none related to a microfibrillar packing at the 35-40 Å level. Electron microscopy of the gel networks before stretching shows the presence of thin filaments with diameters predominantly in the 35-40 Å range. No cross-striated fibrils are seen in electron micrographs of either stretched fibers or unstretched fibers. Thus, intermolecular packing in accord with the 670 Å axial periodicity can take place within ≈40 Å diameter thin filaments. These correspond to the structures previously postulated to be collagen 'microfibrils'.