Fig. 4. Conformational changes induced in the lipid alkyl chains, by the presence of HFE and halothane. The results are represented as R , the ratio of the NMR deuterium order parameters for methylene groups as a function of their carbon number, along the lipid alkyl chains of the bilayer containing the agents to that of the lipids in the pure bilayer. The ratios (approximately 1) obtained both at low (solid squares) and high (solid triangles) HFE concentrations, indicate that the nonimmobilizer has a relatively small effect on the membrane structure. In contrast, results (open circles) from a MD simulation by Koubi et al.  22for a DPPC bilayer containing fifty mole percent halothane molecules, show that the anesthetic has a large influence on the acyl chains order parameters. The ratios R  for a low concentration of halothane, (approximately 1) and error bars, which are less than twice the size of the symbols, are omitted for clarity.