Molecular tilting in dried elastoidin and its implications for the structures of other collagen fibrils

Wet elastoidin spicules (fish fin rays) yield low-angle meridional X-ray diffraction patterns which resemble those from tendons. However, when the spicule dries the meridian splits into the arms of a diagonal cross (sometimes only one arm appears). Of the possible explanations we reject shearing of the axial arrangement of molecules but confirm tilting. We suggest that, in three dimensions, the molecules are tilted at angles which vary from 0 degrees at the centre to some maximum value at the surface of the spicule, resembling torsion of the array of molecules. Molecular tilting probably occurs in other collagen fibrils.