Non-raft forming sphingomyelin-cholesterol mixtures.

Sphingomyelin from biological membranes forms segregated domains with cholesterol in fluid bilayers. However, a synthetic form of sphingomyelin with an oleoyl chain linked to sphingosine is not incorporated into cholesterol-rich domains. We have studied the properties of mixtures of oleoyl-sphingomyelin and cholesterol as well as mixtures of oleoyl-sphingomyelin with 1-stearoyl-2-oleoyl-phosphatidylcholine by DSC and NMR. Cholesterol has a high miscibility with oleoyl-sphingomyelin and it does not separate in crystalline form until the mol fraction of cholesterol reaches a value above 0.6. A large fraction of the cholesterol crystals that are formed are in the monohydrate form. Furthermore, these crystals rehydrate relatively rapidly compared with pure cholesterol crystals in the absence of phospholipid. The environment of the carbonyl group of the phospholipid indicates that it is similar to other forms of sphingomyelin with saturated acyl chains. Also similar to other forms of sphingomyelin, the quaternary ammonium group of oleoyl-sphingomyelin is more rigid than that of phosphatidylcholines, as indicated by the strong resonance observed with cross-polarization/magic angle spinning. Additionally, oleoyl-sphingomyelin produces a larger alteration than egg sphingomyelin of the phase transition of 1-stearoyl-2-oleoyl-phosphatidylcholine. These studies indicate that oleoyl-sphingomyelin, unlike saturated forms of sphingomyelin, does not form segregated domains with cholesterol because of its greater miscibility with phosphatidylcholine.