Electrondensity profile determination of bacterial photosynthetic membranes

Abstract

X-ray diffraction experiments with stacks of wet, oriented bacterial thylacoids were performed. The main result is the determination of the electrondensity profile, which is the projection of the electrondensity of the membrane stack onto the stacking axis. Our evaluation procedure takes into account statistical deviations of the quasi-crystalline arrangement of membranes in the stack, differences between the electrondensities in the interthylacoid space and the thylacoid lumen, the statistical distribution of the stack lengths and differences between the mean electrondensity of the stacks and the surrounding medium. Electronmicroscopical determination of parameters describing the statistical lattice deviations in freeze fractured specimens, prepared in the same way as for X-ray diffraction, showed that the influence of membrane undulations can be neglected. They also provided information concerning the validity of the model assumption of lattice deviations of the second kind [27]. The procedure for evaluation which we describe also yielded an absolute calibrated electrondensity scale for the final structure. This profile enabled to be determined the distribution of the membrane components with respect to their position along the stacking axis to be estimated.