Investigation into the acyl chain packing of endotoxins and phospholipids under near physiological conditions by WAXS and FTIR spectroscopy

The acyl chain packing of various endotoxins and phospholipids was monitore d via the main wide-angle reflection between 0.410 and 0.460 nm by wide-ang le X-ray scattering (WAXS) and via the absorption band of the symmetric str etching vibration of the methylene groups v(s)(CH2) around 2849 to 2853 cm( -1) by Fourier-transform infrared spectroscopy. The lipids investigated inc luded various rough mutant (R) and smooth form (S) lipopolysaccharides (LPS ) differing in the length of the sugar portion, lipid A, the "endotoxic pri nciple" of LPS, and various saturated and unsaturated phospholipids with di fferent head groups under a near physiological (greater than or equal to 85 %) water content. The packing density of the saturated endotoxin acyl chain s is lower than those of saturated phospholipids but similar to those of mo nounsaturated phospholipids, each in the gel phase, The hydrophobic moiety of endotoxins thus exhibits significant conformational disorder already in the gel phase. The acyl chain packing of the endotoxins decreases with incr easing length of the sugar chain lengths, which seems to be relevant to the observed differences in biological activity. For Re-LPS with different cou nterions (salt forms), in the presence of externally added cations or at re duced water content (50%), no change of the acyl chain packing density is d educed in the X-ray data, although the FT-IR data indicate its increase. Th e position of the v(8)(CH2) vibration is, thus, only a relative measure of lipid order, in particular when lipids with different head groups and in th e presence of external agents are compared. (C) 1999 Academic Press.