| Abstract: | The 19F nuclear magnetic resonance (NMR) spectra of membranes of Acholeplasma laidlawii B enriched with one of a series of positional isomers of cis-octadecenoic acid plus small amounts of one 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 fluorine label in the liquid-crystalline state yielded an "order profile" with characteristics similar to those obtained via 2H NMR and which was 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 at 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 orientational 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.(ABSTRACT TRUNCATED AT 250 WORDS) |
