Multiple thermotropic phase transitions in Escherichia coli membranes and membrane lipids. A comparison of results obtained by nitroxyl stearate paramagnetic resonance, pyrene excimer fluorescence, and enzyme activity measurements.

At characteristic temperatures, membranes from Escherichia coli cells enriched in exogenous elaidic acid exhibit two abrupt changes in the slope of Arrhenius plots of two enzyme activities. For NADH oxidase, these changes occur at 27 degrees and 32 degrees, whereas for D-lactate oxidase, these changes occur at 31 degrees and 36 degrees. Pyrene excimer fluorescence and spin-labeled fatty acid paramagnetic resonance results indicate that the beginning, midpoint, and end of a single structural change(order leads to disorder transition) occurs at 25.5-29.0 degrees, 30.0-31.0 degrees, and 33.0-35.5 degrees, respectively. These data suggest that for NADH oxidase, the observed activity changes correspond to the beginning and midpoint of a single membrane lipid structural change, whereas for D-lactate, the activity changes correspond to the midpoint and end of that structural change. In addition to the membrane structural change spanning the range of 25.5-35.5 degrees, a second change (9.5-21.0 degrees) was also observed. This transition was detected by 5- and 16-2,2-dimethyloxazolidinyl-1-oxyl (doxyl) stearates, but not by 12-doxyl stearate or pyrene. Structural changes in the extracted lipids were observed in the temperature ranges 4.0-9.0 degrees, 14.0-20.0 degrees, and 25.0-35.5 degrees. The two higher ranges correlate well with the ranges for structural changes observed in the intact membrane. Observations of these multiple transitions in both intact membranes and extracted lipids strongly suggest that these lipids segregate into domains of different fluidity and composition.

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