Quantitative study of the fluidity of Escherichia coli membranes using deuterium magnetic resonance

Abstract

Specifically deuterated palmitic acid was incorporated into the membrane phospholipids of the L51 strain of E. coli. The cytoplasmic and outer membranes were separated using standard techniques and studied by deuterium NMR between 0.degree.-40.degree. C. Distinctive liquid-crystalline and gel spectra coexisted over a wide temperature range. The relative intensities of these spectra provided a direct measure of the fraction of the deuterium-labeled phospholipids in the fluid state as a function of temperature. Above 37.degree. C, the amount of immobilized or gel-phase phospholipid was less than 3% of the total phospholipid. The gel to liquid-crystalline transition region for the outer membrane was shifted upwards by .apprx. 7.degree. C relative to that of the cytoplasmic membrane, in agreement with previous studies. The orientational order in the fluid phase of both membranes decreased gradually with increasing temperature and was greater in the outer membrane than in the cytoplasmic membrane. The orientational order of the gel-phase component was the same for both membranes, within an experimental uncertainty of 10%, and was independent of temperature from 0.degree.-30.degree. C for the outer membrane and from 10.degree.-30.degree. C for the cytoplasmic membrane.