Fluorine-19 nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. Orientational order in the presence of a series of positional isomers of cis-octadecenoic acid

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Abstract
The 19F NMR spectra of membranes of Acholeplasma laidlawii B enriched with 1 of a series of positional isomers of cis-octadecenoic acid plus small amounts of 1 of a number of isomers of monofluoropalmitic acid were interpreted in terms of an orientational order parameter (Smol). The variation of Smol with the position of the F label in the liquid crystalline state yielded an order profile with characteristics similar to those obtained via 2H NMR and which were relatively invariant regardless of the site of cis unsaturation. In the gel state, values of Smol approached the theoretical maximum, and the order profiles in the presence of different isomeric cis-octadecenoic acids displayed distinct dissimilarities. When the cis double bond was located proximal to the methyl terminus of the fatty acyl chain, a gradient of order across the bilayer was still evident in the gel state. When the cis double bond was located near the carbonyl head group, values of Smol were approximately equal to all chain positions. These observations were interpreted as indicating that in the gel state the stringency of packing restrictions is still subject to variation across the width of the bilayer. Relative overall orientation order among all isomers examined (specifically, 18:1c.DELTA.4, .DELTA.5, .DELTA.6, .DELTA.7, .DELTA.8, .DELTA.9, .DELTA.10, .DELTA.11, .DELTA.12, .DELTA.13, .DELTA.14 and .DELTA.15) varied directly as a function of proximity to the lipid gel to liquid-crystalline phase transition (Tm) (determined via differential scanning calorimetry) when compared at a constant temperature. Furthermore, when normalized with respect to the Tm, values of Smol all fell within a relatively narrow band which increased exponentially toward the theoretical maximum at temperatures below the Tm, indicating that the proportion of gel to liquid-crystalline lipid is the primary determinant of the overall orientational order. Nevertheless, isomers with the site of unsaturation near the center of the chain were revealed to be somewhat more ordering in the liquid-crystalline state than others following the normalization procedure, suggesting that structural considerations, although of secondary importance , were still significant.

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