Calorimetric and spectroscopic studies of lipid hydrocarbon chain order in the Acholeplasma laidlawii membrane.

The thermotropic phase behavior of Acholeplasma laidlawii B membranes, whose lipids were biosynthetically highly enriched with one of a wide variety of different types of fatty acyl groups, was studied by differential scanning calorimetry. The membrane-lipid-hydrocarbon chain orientational order of these membranes, in both the gel and liquid-crystalline states, was also investigated by 19F-nuclear magnetic resonance spectroscopy, using biosynthetically incorporated monofluoropalmitic acid probes. The gel to liquid-crystalline phase transition midpoint temperature, Tm, for a series of membranes containing different types of fatty acyl groups, but with a similar number of carbon atoms, decreases in the order: linear saturated greater than methyl isobranched greater than methyl anteisobranched greater than or equal to trans-monounsaturated greater than cis-monounsaturated. At comparable reduced temperatures below their respective Tm values, the degree of hydrocarbon chain order correlates strongly and positively with Tm, indicating that Tm is determined primarily by the efficiency of chain packing in the gel state. At comparable "reduced" temperatures above Tm, the degree of hydrocarbon chain orientational order is moderately and inversely correlated with Tm. At any one absolute temperature in the fluid state, however, orientational order does correlate weakly and inversely with Tm, due to a small inherent decrease in order with temperature. Thus, to a first approximation, membrane lipid order in the liquid-crystalline state is directly proportional, and membrane lipid fluidity inversely proportional, to the gel to liquid-crystalline phase transition temperature. However, the magnitude of this effect lessens with decreasing proximity to the Tm. Finally, evidence is presented that the presence of membrane protein has little if any effect on the average orientational order of the membrane lipid hydrocarbon chains in either gel or liquid-crystalline states.