Fluorine-19 nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. Gel-state disorder in the presence of methyl iso- and anteiso-branched-chain substituents

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Abstract
The hydrocarbon chain orientational order parameters of membranes of Archoleplasma laidlawii B enriched with large quantities of a linear saturated, a methyl iso-branched, or a methyl anteiso-branched fatty acid plus small quantities of various isomeric monofluoropalmitic acid probes were determined via 19F NMR over a range of temperatures spanning the gel to liquid-crystalline phase transitions (determined by differential scanning calorimetry). Membrane orientational order profiles in the liquid-crystalline state were generally similar regardless of the particular fatty acyl structure, showing a region of relatively constant order preceding a region of progressive decline in order toward the methyl terminus of the acyl chain. In the gel state, the order profile of the linear saturated fatty acid enriched membranes was characteristically flat, with little head to tail gradation of order. The methyl iso-branched and the methyl anteiso-branched enriched membranes exhibited a local disordering in the gel phase reflected in a very pronounced head to tail gradient of order, which remained at temperatures below the lipid phase transition. The methyl iso- and anteiso-branched fatty acid enriched membranes were overall more disordered than the membrane containing only linear saturated fatty acyl groups. At a constant value of reduced temperature below the lipid phase transition, overall order decreased in the progression 15:0 > 16:0i > 16:0ai, suggesting that these methyl-branched substituents lower the lipid phase transition by disrupting the gel phase lipid chain packing. At a constant value of reduced temperature above the lipid phase transition, overall order decreased in the progression16:0ai > 16:0i > 15:0. The apparently more highly ordered liquid-crystalline state of the branched-chain lipids may simply be the result of an absolute temperature effect rather than reflecting any real differences in chain packing above the lipid phase transition.

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