Structural analysis of weakly ordered membrane stacks

The applicability of full- q- range models to fit low- resolution X- ray diffraction data from multibilayers exhibiting only weak quasi- Bragg peak scattering has been analysed. The models consider different structure factors, accounting for different types of lattice disorder caused by stacking faults or bending fluctuations. Numerical tests of the models, considering instrumental in influence of a line- focus collimation system, demonstrated that Bragg peak line shapes given by different lattice disorders cannot be discerned. However, line- shape parameters can be determined for a particular sample, if the type of disorder is known a priori. This has been verified by comparing the experimental results for the fluctuation parameter of palmitoyl- oleoyl phosphatidylcholine ( POPC) as a function of temperature with high- resolution data on the same lipid. Tests further show that the calculation of structural parameters, such as the membrane thickness or the extent of the interbilayer water region, is not obscured by the smearing imposed by the instrument. The model was further applied successfully to experimental data of lipid mixtures composed of sphingomyelin ( SM)/ POPC/ cholesterol and dipalmitoyl phosphatidylethanolamine ( DPPE)/ dipalmitoyl phosphatidylglycerol ( DPPG). The structural parameters determined give valuable insight into the physical state of the membrane system, which is not accessible when quasi- Bragg reflections only are considered.