Modification of membrane lipid: physical properties in relation to fatty acid structure

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Abstract
Differential scanning calorimetry (DSC) and ESR measurements were made to characterize how modifications in the fatty acid composition of Escherichia coli affected the thermotropic phase transition(s) of the membrane lipid. When the fatty acid composition contained 20-60% saturated fatty acids, the DSC curves for isolated phospholipids and cytoplasmic membranes showed a broad (15-25.degree. C) gel to liquid-crystalline phase transition, the position of which depended on the particular fatty acid composition. Utilizing multiple lipid mutants, enrichment of the membrane phospholipids with a single long-chain cis-monoenoic fatty acid in excess of that possible in a fatty acid prototroph led to altered physical properties. A sharp thermotropic phase transition (1.degree. C) appeared at saturated fatty acid levels < 20% and gradually replaced the broad peak as the cis-monoenoic fatty acid content increased. These results were obtained with phospholipids, cytoplasmic membranes and whole cells. With these same phospholipids, plots of 2,2,6,6-tetramethylpiperidinyl-1-oxy partitioning and ESR order parameters vs. 1/T[absolute temperature] revealed discontinuities at temperatures 40-60.degree. C above the calorimetrically measured gel to liquid-crystalline phase transitions. Moreover, when the membrane phospholipids were enriched with certain combinations of cis-monoenoic fatty acids (e.g., cis-.DELTA.9-16:1 plus cis-.DELTA.11-18:1), the DSC curves showed a broad gel to liquid-crystalline phase change below 0.degree. C, but the ESR studies revealed no discontinuities at temperatures above those of the gel to liquid-crystalline transition. Enrichment of the membrane lipids with molecules in which both fatty acyl chains are identical cis-monoenoic residues apparently led to a distinct type of liquid-crystalline phase. Apparently, E. coli normally maintains a heterogeneous mixture of lipid molecules and by doing so prevents strong lipid-lipid associations that lead to the formation of lipid domains in the membrane.

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