Modern theories of liquids and the diffuse equatorial X-ray scattering from collagen

The idea that the lateral arrangement of a proportion of molecules in the collagen fibril displays a short-range two-dimensional liquid-like order is investigated and shown to be true. Theoretical calculations of a dense disordered assembly of hard discs yield X-ray scattering curves which possess the features of the near equatorial diffuse scattering from rat-tail tendon collagen. The theoretical model used is the integral equation formulation of Percus & Yevick [Phys. Rev. (1958). 110, 1] known to be almost exact for hard-disc potentials. The molecular diameter and number density of molecules obtained from the comparison with experiment are close to those suggested by Woodhead-Galloway, Hukins & Wray [Biochem. Biophys. Res. Commun. (1975). 64, 1237-1244] to explain the ordered part of the collagen and to those suggested by Katz & Li [J. Mol. Biol. (1973). 73, 351-369] on the basis of an investigation of the density of wet tendon. A brief discussion of elastoidin is included. Discrepancies between the observed and predicted scattering are discussed.