COMPARISON OF X-RAY POWDER-DIFFRACTION DATA OF VARIOUS BACTERIAL LIPOPOLYSACCHARIDE STRUCTURES WITH THEORETICAL-MODEL CONFORMATIONS

X-ray powder-diffraction experiments have been performed on dry sample s of lipid A and various rough-mutant lipopolysaccharides (LPS) of Sal monella minnesota, Salmonella typhimurium and Escherichia coli. The di ffraction patterns obtained indicated exclusively lamellar, bilayered arrangements in all samples. The periodicities were found to be in the range 4.5 nm for lipid A to 8.8 nm for Ra-LPS. Upon treatment with wa ter-saturated air, swelling of the lamellar structures was achieved, a s indicated by shifts of reflections. The increase in bilayer dimensio ns normally was about 0.3 nm. X-ray intensities were used for the dete rmination of the inner bilayer structure, i.e. for calculation of the one-dimensional electron-density distribution across the bilayer. For lipid A and several Re-LPS, Rd2-LPS, Rd1-LPS and Rc-LPS samples, a str iking coincidence of the electron-density distributions in the lipid-A domain was found, suggesting that in all these structures the lipid-A portion is similarly arranged. For Rb, and Ra-LPS, the lipid-A domain could not be resolved due to the limited number of observed reflectio ns. For other Re-mutant lipopolysaccharide samples, quite different X- ray patterns were obtained. Some samples yielded diffraction patterns indicating a very high state of order in the lipid-A domain, whereas, in others, a significantly reduced order in the lipid-A domain was inf ered. Comparison of the X-ray data with features of a calculated three -dimensional molecular model of lipopolysaccharide revealed reasonable agreement in molecular dimensions and bilayer structure.