Molecular packing in type I collagen fibrils.

Previous studies of the X-ray diffraction pattern of the crystalline regions of type I collagen fibrils yielded information on the unit cell parameters and also the orientation of the pseudo-hexagonally packed molecular segments in the overlap region. The absence of Bragg reflections at high angles attributable to the molecular segments in the gap region led to the suggestion that these segments were more mobile than those in the overlap region. We report a study of the low-angle Bragg reflections in a search for information about the nature of the orientation and packing of the molecular segments in the gap region. We conclude that the (m = 0, n = 0) helix layer plane of the molecular segments in the overlap region makes little or no contribution to the Bragg reflections at low angles, and identify three possible origins for the observed low-angle reflections in the electron density contrast associated with: (1) the "hole" created by the missing molecular segment in the gap region; (2) the telopeptides; or (3) the axial regularities in amino acid residues of a particular type, with periodicities of D/5 or D/6. Sufficient information is available to investigate the first two of these possibilities, and the results obtained suggest specific arrangements for the molecular segments in the overlap and gap regions, and specific connectivities between the molecular segments in successive overlap regions. In addition, we have examined the amino acid sequence and identified features related to the mobility of the molecular segments in the gap region and to the regions where it is thought that molecules are kinked.