A cationic electron spin resonance probe used to analyze cation interactions with lipopolysaccharide

The partitioning of a cationic electron spin resonance probe, 4-(dodecyl dimethyl ammonium)-1-oxyl-2,2,6,6,-tetramethyl piperidine bromide, into lipopolysaccharide from $\text{Escherichia}\text{coli}$ W1485 was shown to increase markedly above approximately 15°C, presumably reflecting a thermal transition. Partitioning was also highly dependent on probe and lipopolysaccharide concentrations, and Scatchard analysis of electrodialyzed lipopolysaccharide revealed a single non-interactive binding site for the probe. Several cations were able to displace probe bound to this site. At concentrations above 30 μM, Ca²⁺ and Mg²⁺ displaced probe bound to electrodialyzed lipopolysaccharide while various polyamines and other cations were less effective. Since this probe is very sensitive to the environment of the lipopolysaccharide, it should prove to be a valuable tool in analyzing lipopolysaccharide structure and interactions with other molecules.