| Résumé | Dihydrosterculic acid (cis-9, 10-methylene-octadecanoic acid, 19:cp,cΔ⁹), specifically deuterated at several positions along the chain, has been incorporated biosynthetically into the membrane lipids of Acholeplasma laidlawii B. A detailed study of acyl chain order was carried out with deuterium nuclear magnetic resonance. The transition from the gel to the liquid-crystalline phase was determined to occur from -15 to 0°C, a range somewhat narrower than, but with a midpoint similar to , that found for membranes enriched in oleic acid. The acyl chains of 19:cp,cΔ⁹-containing membranes are less mobile in the gel state than those of oleic acid containing membranes. Above 0°C, the lipids are in the liquid-crystalline phase and give rise to powder spectra characteristic of axially symmetric motion. The C²H₂ segments near the cyclopropane ring gave rise to a quadrupolar powder pattern indicative of inequivalence of the two deuterons. The orientational fluctuations of the fatty acid chain segments in the membrane lipids are described in terms of deuterium order parameters. The overall ordering is greater everywhere than that in the case of oleoyl chains and features a maximum at the cyclopropyl moiety, in sharp contrast to the plateau found with saturated chains. Detailed analysis of the data for the cyclopropane ring indicates that the C-9-C-10 bond is inclined at 89° relative to the director of motional averaging, in sharp contrast to the 3° estimated for oleic acid in the same membranes. The effect of incorporation of cholesterol at 35 mol % lipid was examined. This had little effect on the breadth of the gel to liquid-crystal transtition but did result in a gel state with lipid that is rotationally more rigid. |
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