The conformations of nitroxide-labelled fatty acid probes of membrane structure as studied by 2H-NMR.

The electron spin resonance (ESR) spectrum of a nitroxide spin probe intercalated in a membrane is influenced by the amplitude of anisotropic motion of the nitroxide group and by the geometry of the oxazolidine ring of the nitroxide. In the analysis of the ESR spectra of nitroxide-labelled fatty acid probes, it is generally assumed that the five-membered oxazolidine ring system is oriented rigidly perpendicular to the long molecular axis of the probe. This assumption is tested in the present study, using 2H-NMR of specifically deuterium-labelled nitroxide spin probes. Evidence is presented that the nitroxide does not display the assumed geometry in membranes. The departure from this geometry depends on the position of the nitroxide label on the acyl chain, with a more pronounced departure for position 5 relative to position 12. These and previous data provide an explanation for the discrepancies between spin-probe ESR and 2H-NMR order parameters in membranes.