Lipid phase transitions in cytoplasmic and outer membranes of Escherichia coli

The cytoplasmic and outer membranes containing either trans-delta-9-octadecenoate, trans-delta-9-hexadecenoate or cis-delta-9-octadecenoate as predominant unsaturated fatty acid residues in the phospholipids were prepared from a fatty acid auxotroph, Escherichia coli strain K1062. Order-disorder transitions of the phospholipids were revealed in both fractions of the cell envelope by fluorescent probing or wide angle X-ray diffraction. The mid-transition temperatures, Tt, and the range of the transition, delta-T, are similar in the outer and cytoplasmic membrane. Relative to the corresponding extracted lipids, 60-80% of the hydrocarbon chains take part in the transition in the cytoplasmic membrane whereas in the outer membrane only 25-40% of the chains become ordered. The results suggest that in the outer membrane part of the lipids form fluid domains in the form of mono- and/or bilayers.