Comparative dynamics and location of chain spin-labelled sphingomyelin and phosphatidylcholine in dimyristoyl phosphatidylcholine membranes studied by EPR spectroscopy

The dynamics and environment of sphingomyelin spin-labelled at different positions in the N-acyl chain have been studied in dimyristoyl phosphatidylcholine bilayer membranes by using electron spin resonance spectroscopy. Comparison was made with phosphatidylcholine spin-labelled on the sn-2 acyl chain in the same host membrane. Spin-labelled sphingomyelin was found to mix well with the host phosphatidylcholine lipids in both gel and fluid phase membranes. At 1 mol%, mutual spin-spin interactions are no greater than for spin-labelled phosphatidylcholine. In the fluid membrane phase, the effective chain order parameters and polarity-sensitive isotropic hyperfine coupling constants of spin-labelled sphingomyelin display a similar dependence on the position of labelling to those of spin-labelled phosphatidylcholine. The values of both parameters are, however, generally larger for sphingomyelin than for phosphatidylcholine at equivalent positions of acyl chain labelling. This difference is attributed to the different chain linkage of sphingo- and glycero-lipids, combined with an offset of approximately one C-atom in transbilayer register between the respective N-acyl and O-acyl chains. In the gel phase, differences in chain configuration between sphingomyelin and phosphatidylcholine are indicated by differences in spin label spectral anisotropy between the two lipids, which appears to reverse towards the terminal methyl chain end.