Compressed microfibril models of the native collagen fibril.

A three-dimensional crystal model for packing of collagen molecules (type I) in the native fibril has recently been proposed by Hulmes and Miller. It provides a straightforward explanation of the major features of the X-ray diffraction pattern, and is consistent with measurements of fibril density. However, there is independent evidence for a well defined microfibrillar substructure, which is absent from their model. This evidence, which is derived from electron microscopy and studies of in vitro assembly, the pattern of covalent crosslinks and sequence analysis, is convincing. Therefore, we have searched for a means to reconcile this conflict. We now propose two models which contain five-stranded microfibrils compressed to place molecules (in cross-section) on a pseudohexagonal lattice. The unit cells are equivalent or related to the cell proposed by Hulmes and Miller. In the simplest case, molecules, and thus microfibrils, are straight-tilted. However, it is not ruled out that molecules are supercoiled and microfibrils are straight. Noncrystallographic considerations favour supercoiling.