Planar liquid-like arrangement of photopigment molecules in frog retinal receptor disk membranes

Abstract

Low-angle X-ray diffraction arising from 40 to 50 Å particles within wet frog retinal receptor disk membranes at 26 °C was not consistent with a planar crystalline lattice of the particles within the disk membranes. The nature of the diffraction suggested the possibility of a planar liquid-like arrangement of the particles. Such an arrangement is supported by the observation that the planar ordering of the particles is easily altered by their interaction with globular protein molecules non-specifically adsorbed to the disk membranes. In view of the above, we obtained diffraction patterns from our wet disk membrane preparations at several temperatures between 4.5 and 42.5 °C, and applied a Fourier analysis to the diffracted intensities appropriate for a planar liquid-like arrangement of the 40 to 50 Å particles. The analysis gave the planar radial distribution function description of the supposed planar liquid-like arrangement of the particles. These radial distribution functions, derived from the diffracted intensities, were examined in terms of their shape and variation with temperature, and compared with the known predictions from statistical mechanics for a liquidlike arrangement of particles whose pair potential contains both attractive and repulsive terms. This comparison for the derived radial distribution functions demonstrated that the observed diffraction data from the 40 to 50 Å particles were indeed consistent with a planar liquid-like arrangement of these particles within the disk membrane. Our radial distribution function analysis allowed model scattering factors for the particles to be tested. It was found that only hard sphere cross-sectional electron densities for the particle with diameters of 40 to 44 Å or reasonably hard, soft-sphere cross-sectional electron densities, with a core of uniform electron density 38 to 40 Å in diameter and a total diameter of 44 to 46 Å, gave good agreement. A similar analysis was applied to the diffracted intensities arising from the antirhodopsin molecules adsorbed to the wet disk membranes which had been treated with our antirhodopsin serum and is discussed relative to the preceding paper (Blasie, Worthington & Dewey, 1969). A comparison of the radial distribution functions for the antirhodopsin molecules adsorbed to the antirhodopsin serum treated disk membranes and the 40 to 50 Å particles of the untreated disk membranes at identical temperatures showed the particles to be the photopigment molecules. The mathematical derivation of the planar radial distribution function and a critical evaluation of the errors involved are presented in the Appendices.