Order-disorder conformation transition of hydrocarbon chains in lipopolysaccharide from Escherichia coli.

Abstract

Thermal phase transitions in isolated lipopolysaccharide from Escherichia coli B/r have been studied using N-phenyl-1-naphthylamine as fluorescence indicator and by X-ray diffraction analyses. Measurements of fluorescence changes revealed a transition temperature tt of 22°C with a range of transition of 10.5°C. Wide-angle X-ray diffraction analyses showed that lipopolysaccharide in aqueous suspension forms a lipid bilayer in which, below tt, the ordered state of hydrocarbon chains is less well developed than in most biological membranes or lipids; instead of the 0.42-nm reflection attributable to the β-type ordered conformation of hydrocarbon chains, a 0.433-nm reflection was found in lipopolysaccharide. Thermal phase transitions probably depending on lipopolysaccharide and as measured with N-phenyl-1-naphthylamine were found in some cell envelope preparations (tt∼ 25–29°C) while others did not exhibit phase transitions; in the latter cases EDTA treatment evoked the appearance of such transitions. It was not possible to decide if these transitions were due to lipopolysaccharide-phospho-lipid interaction because studies with model systems were not successful: temperature-dependent fluorescence changes in mixed phospholipid-lipopolysaccharide preparations only revealed the two separated lipopolysaccharide and phospholipid transitions. Addition of dimyristoyl phosphatidylethanolamine (tt= 48°C) to cell envelopes (tt∼ 33°C) of a heptose-less mutant of E. coli K-12 led to the appearance of only one new tt at 42°C. It appears that external phospholipid can be incorporated into the altered outer membrane of the mutant and there interact with lipopolysaccharide.