Fluorine-19 nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. Effects of methyl-branch substitution and of trans unsaturation upon membrane acyl-chain orientational order.

The hydrocarbon-chain orientational order parameters of membranes of Acholeplasma laidlawii B enriched with straight-chain saturated, methyl iso-branched, methyl anteiso-branched, or trans-unsaturated fatty acids have been determined via fluorine-19 nuclear magnetic resonance spectroscopy (19F NMR). A theoretical description of the 19F NMR spectral line shape is presented that permits the determination of the orientational order parameters associated with specifically monofluorinated palmitic acid probes biosynthetically incorporated into membrane glycerolipids. Membrane orientational order profiles determined by 19F NMR in the case of straight-chain saturated fatty acid enrichment were qualitatively similar to profiles obtained by 2H NMR. The methyl iso-branch and methyl anteiso-branch structural substituents induced a local ordering while the trans double bond substituent induced a local disordering evident from alterations to the character of the orientational order profile. These various effects could be understood in terms of an altered probability of the occurrence of rotational isomerization in the presence of particular substituents. At 37 degrees C the overall orientational order decreased in the progression eta-acyl greater than iso branched greater than anteiso branched greater than or equal to trans double bonded. The relative overall order was then a direct function of the relative proximity of the membrane lipids to their respective gel to liquid-crystalline phase transitions. When observed at Tm + 15 degrees C, where the different species of fatty acids could be considered to be in a comparable thermodynamic state, the overall order decreased in the progression anteiso branch greater than trans double bond greater than iso branch greater than eta-acyl.(ABSTRACT TRUNCATED AT 250 WORDS)