Fluorine-19 nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. A physical, biochemical, and biological evaluation of monofluoropalmitic acids as membrane probes

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Abstract
19F NMR spectroscopy offers unique advantages for studies of lipid fatty acyl chain order and dynamics in model and biological membranes. However, the geminal difluoromethylene fatty acids commonly employed as 19F membrane probes appreciably perturb the organization of model membranes and biomembranes. Thus, specifically labeled monofluoropalmitic acids were synthesized and evaluated for suitability as membrane probes. Differential scanning calorimetric studies of aqueous dispersions of several bis-(monofluoropalmitoyl)phosphatidylcholines reveal that a F substitution near the carbonyl group of palmitic acid has only a modest effect on the thermotropic phase behavior of these model membranes and that substitutions in the center or toward the methyl terminus are relatively nonperturbing. All bis(monofluoropalmitoyl)phosphatidylcholines tested exhibit nearly ideal mixing in all proportions with dipalmitoylphosphatidylcholine. The thermotropic phase behavior of membranes of the simple, cell-wall-less prokaryote A. laidlawii B is also not detectably altered by the presence of appreciable amounts of biosynthetically incorporated monofluoropalmitic acid. The biosynthetic incorporation of even large amounts of monofluoropalmitic acids into the membrane lipids of A. laidlawii B has no effect upon the growth and survival of this organism. The presence of exogenous monofluoropalmitic acids in the growth medium does not alter the polar head group composition or lipid/protein ratio of the A. laidlawaii B membrane. All monofluoropalmitic acids tested are biosynthetically incorporated as well as palmitic acid itself and distribute relatively evenly between the various membrane glyco- and phospholipids. Since in most respects these monofluorinated palmitic acids are considerably less perturbing than the geminal difluoro fatty acids thus far studied, they appear to be the 19F fatty acid probes of choice for NMR studies of membranes.