The study of hydrocarbon chain mobility in membrane systems using spin‐label probes

The fluid nature of certain cell membranes and their relationship to growth temperature [ 1,2] is of particular interest. Highly fluid regions in cell membranes have now been demonstrated by both NMR [3] and ESR spectroscopy [4,5]. The latter technique makes use of spin labels which are incorporated in various ways into the membrane system. In a recent discussion, Crick [6] raised two points which could be important for the interpretation of spin label spectra of membrane systems. The first concerned the change in the magnitude of mobility of a spin label when a marked change occurs in the lipid system, e.g., the solidification of the hydrocarbon chains. The second concerned the hypothetical situation when two different phases exist in a membrane, would this heterogeneity be indicated by the ESR spectra? Our recent deuteron resonance experiments with Acholeplusma laidlawii B membranes [7] have indicated the heterogeneous nature of this membrane, rigid lipid domains coexisting with more fluid crystalline regions, and this has led us to consider these same points.