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

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Abstract
The hydrocarbon-chain orientational order parameters of membranes of A. laidlawii B enriched with straight-chain saturated, methyl iso-branched, methyl anteiso-branched or trans-unsaturated fatty acids were determined via 19F NMR spectroscopy. A theoretical description of the 19F NMR spectral line shape is presented than 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; 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.degree. C the overall orientational order decreased in the progression .eta.-acyl > iso-branched > anteiso branched .gtoreq. 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 [melting point] + 15.degree. 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 > trans double bond > iso branch > .eta.-acyl. The overall ordering effect of these substituents, observed upon elimination of the effect of relative proximity to the Tm, could be interpreted in terms of their effects upon membrane acyl-chain packing.