Effect of double bonds on the dynamic properties of the hydrocarbon region of lecithin bilayers

The motional properties of the hydrophobic fluorescent probe 1,6-diphenyl-1,3,5-hexatriene have been assessed in liposomes of various lecithins: dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine, 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), 1-palmitoyl-2-linoleoylphosphatidylcholine (PLPC), and 1-palmitoyl-2-arachidonoylphosphatidylcholine (PAPC). The temperature dependence of the steady-state fluorescence anisotropy was determined within the range of 10-50°C. Nanosecond measurements were also made and the results analyzed by a model of wobbling diffusion confined within a cone. In this treatment the cone angle (θ_c) obtained relates to the degree of order of the fatty acyl chains, and the wobbling diffusion constant (D_w) relates to the rate of motion of the hydrocarbon chains. θ_c was increased (order decreased) and D_w increased (rate increased) for the first double bond (POPC) as compared to the fully saturated DPPC. In all four unsaturated lecithins the temperature dependences of θ_c and D_w were similar: θ_c increased by about 4-5°, and D_w increased by a factor of 1.5 per 10°C. DPPC in the liquid-crystalline phase also showed a similar trend. The absolute values, on the other hand, showed some differences: for θ_c, DOPC (78° at 37°C) > PAPC (75°) > POPC (70°) ≃ PLPC (69°) > DPPC (20°), and for D_w, PAPC (0.28 ns^-1) > DOPC (0.22 ns^-1) ≃ PLPC (0.24 ns^-1) > POPC (0.21 ns^-1) > DPPC (0.05 n^-1). The differences among the unsaturated lecithins are relatively small suggesting that the first double bond introduced into a lecithin plays the most important role at physiological temperatures. In 1:1 mixtures of the disaturated lecithin, DPPC, with the unsaturated lecithins, the temperature of the phase transition of the former was lowered and broadened, by approximately the same amount, irrespective of the differences in unsaturation.