RADIAL PACKING, ORDER, AND DISORDER IN COLLAGEN FIBRILS

Collagen fibrils resemble smectic, liquid crystals in being highly ord ered axially but relatively disordered laterally. In some connective t issues, x-ray diffraction reveals three-dimensional crystallinity in t he molecular packing within fibrils, although the continued presence o f diffuse scatter indicates significant underlying disorder. In additi on, several observations from electron microscopy suggest that the mol ecular packing is organized concentrically about the fibril core. In t he present work, theoretical equatorial x-ray diffraction patterns for a number of models for collagen molecular packing are calculated and compared with the experimental data from tendon fibrils. None of the m odels suggested previously can account for both the crystalline Bragg peaks and the underlying diffuse scatter. In addition, models in which any of the nearest-neighbor, intermolecular vectors are perpendicular to the radial direction are inconsistent with the observed radial ori entation of the principal similar to 4 nm Bragg spacing. Both multiple -start spiral and concentric ring models are devised in which one of t he nearest-neighbor vectors is along the radial direction. These model s are consistent with the radial orientation of the similar to 4 nm sp acing, and energy minimization results in radially oriented crystallin e domains separated by disordered grain boundaries. Theoretical x-ray diffraction patterns show a combination of sharp Bragg peaks and under lying diffuse scatter. Close agreement with the observed equatorial di ffraction pattern is obtained. The concentric ring model is consistent with the observation that the diameters of collagen fibrils are restr icted to discrete values.