Influence of the supramolecular structure of free lipid A on its biological activity.

The three-dimensional supramolecular structures and the states of order of the acyl chains of lipid A from different Gram-negative species were investigated at 40 degrees C, high water content (80-90%), and different [lipid A]/[Mg2+] molar ratios using synchrotron radiation X-ray diffraction and Fourier-transform infrared spectroscopy. Measurements were made on free lipid A from Salmonella minnesota R595, mono- and bi-phosphoryl, as well as those from the non-enterobacterial strains Rhodobacter capsulatus 37b4, Rhodopseudomonas viridis F, and Rhodocyclus gelatinosus 29/1. Parallel to differences in their chemical primary structure, the structural polymorphisms and states of order at 37 degrees C of the non-enterobacterial lipid A were found to be different from those of enterobacterial lipid A. A clear correlation between the supramolecular structure and previously determined biological activities was found. Lipid A with a strong preference for lamellar structures (Rb. capsulatus and Rp. viridis) are endotoxically inactive and lack cytokine-inducing capacity; the compounds assuming a mixed lamellar/nonlamellar structure (monophosphoryl lipid A from S. minnesota) are of lower toxicity in vivo, but may induce cytokines in vitro; those lipid A with a strong tendency to form non-lamellar inverted structures (lipid A from S. minnesota and Rc. gelatinosus) exhibit full endotoxicity in vitro and in vivo. In contrast, anti-complementary activity is most pronounced for compounds with lamellar and least expressed for those with inverted structures. The states of order at 37 degrees C vary non-systematically, exhibiting the highest values for lipid A of S. minnesota and the lowest for that of Rc. gelatinosus. We propose to extend the term 'endotoxic conformation', which is used to describe the conformation of a single lipid A molecule required for optimal triggering of biological effects, to 'endotoxic supramolecular conformation' which denotes the particular organization of lipid A aggregates in physiological fluids causing biological activity.