Chain-length dependence of n-alkane solubility in phosphatidylcholine bilayers: a 2H-NMR study

Phosphatidylcholine bilayer membranes containing 2H-labelled n-alkanes have been studied by 2H-NMR as a model system for the investigation of molecular theories of general anesthesia. The solubilities of n-alkanes in lipid bilayers have been determined by measurement of the relative intensities of a powder pattern signal arising from orientationally ordered, membrane-soluble alkane and a sharp signal in the 2H-NMR spectrum resulting from isotropically reorienting alkane. The ordering profiles for the ordered n-alkane as determined from the quadrupole splittings for different segments along the chain are similar to those described earlier for n-hexane, n-octane and n-dodecane, suggesting that the restricted motions undergone by the n-alkanes of chain length from 6 to 19 are basically similar. For this homologous series of n-alkanes, it was found that membrane solubility dropped sharply at an alkane chain length which depended on lipid chain length, degree of unsaturation, cholesterol concentration in the bilayer, and temperature. The results show that the incorporation of n-alkanes in lipid bilayers is a complex function of lipid composition. The implications of these results in relationship to the observed 'cut-off' in anesthetic potency in the n-alkane homologous series are discussed.