Curvature properties of novel forms of phosphatidylcholine with branched acyl chains

We studied the properties of a series of phosphatidylcholine molecules with branched acyl chains. These lipids have previously been shown to have mark ed stimulatory effects on the side-chain cleavage activity of cytochrome P4 50SCC (CYP11A1), an enzyme of the inner mitochondrial membrane. The synthet ic lipids used were diacyl phosphatidylcholines with the decanoyl, dodecano yl or tetradecanoyl chain having a hexyl, octyl or decyl straight chain ali phatic branch at the 2-position. All three lipids lowered the bilayer to he xagonal phase transition temperature of dielaidoyl phosphatidylethanolamine , the lipids with longer acyl chains being more effective in this regard. A s pure lipids all of the forms were found by X-ray diffraction to be predom inantly in the hexagonal phase (H-II) over the entire temperature range of 7-75 degrees C. The properties of the H-II phase were unusual with regard t o the small size of the lattice spacings and the small temperature dependen ce of the spacings. We used tetradecane to relieve hydrocarbon packing cons traints to determine the intrinsic radius of curvature of the lipid monolay er. The elastic bending modulus was measured in the presence of tetradecane by introducing an osmotic gradient across the hexagonal phase cylinders wi th aqueous solutions of poly(ethylene glycol). The elastic bending modulus was found to be higher than that observed with other lipids and to increase with temperature. Both the small intrinsic radius of curvature and the hig h elastic bending modulus indicate that the presence of these lipids in bil ayer membranes will impose a high degree of negative curvature strain.