Electron spin resonance probing of lipopolysaccharide domains in the outer membrane of Escherichia coli

Abstract

Listen

Translate article

(1) Scatchard analysis of the binding of the cationic electron spin resonance probe 4-(dodecyldimethylammonium)-1-oxyl-2,2,6,6-tetramethylpiperdine bromide to lipopolysaccharide and to phospholipid indicated that this probe has a 5 fold greater affinity for anionic lipopolysaccharide than for phospholipid. In the intact outer membrane this cationic probe likely associates with the lipopolysaccharide-containing outer monolayer. (2) The temperature dependence of the mobility of the cationic spin probe in the outer membrane indicates that a structural transition occurs at 9°C in the outer monolayer. (3) A similar 9°C transition was detected in the outer membrane using the spin probe 5-doxyl stearate. This anionic probe has been shown to preferentially partition into the phospholipid enriched domains of the outer membrane. (4) A porin-lipopolysaccharide-peptidoglycan complex probed with the cationic probe also was shown to undergo a thermally induced structural change at 9°C. (5) Purified lipopolysaccharide, however, was shown to have structural transitions at 20°C and 40°C. (6) It is proposed that a structural rearrangement of the intact outer membrane occurs at approx. 9°C in both the lipopolysaccharide and phospholipid domains of the outer membrane. Furthermore, this structural transition appears to be highly dependent on lipid-protein interactions. A second thermotropic transition that occurs in the outer membrane at approx. 40 to 42°C appears to result mainly from changes in the lipopolysaccharide domain structure.