Effect of lysophosphatidylcholine on behavior and structure of phosphatidylcholine liposomes

Differential scanning calorimetry (DSC), fluorescence polarization and X-ray diffraction were performed to investigate the kinetics of the micellar to the lamellar phase transition of dipalmitoylphosphatidylcholine/1-palmitoylphosphatidylcholine (16:0 LPC/DPPC) liposomes at gel phase. With a 16:0 LPC concentration up to 27 mol% only the sharp main transition with relatively high enthalpy (ΔH) values of DPPC was observed. Increasing 16 : 0 LPC concentration, the phase transition was broadened and the transition enthalpy was decreased and finally totally disappeared. The fluorescence probes of 3AS, 9AS, 12AS, and 16AP were employed, respectively, to detect the mobility of various sites of carbon chains of DPPC or 16:0 LPC/DPPC liposomes. It was shown that DPPC liposomes formed in the absence of 16:0 LPC always had a fluidity gradient in both gel and liquid-crystalline phase, while in the presence of 14.1 mol% and 27.0 mol% 16:0 LPC in the mixtures, the fluidity gradient tended to disappear below 40°C. In the case of 27.0 mol% 16:0 LPC in the 16:0 LPC/DPPC mixtures the polarization of the sixteenth carbon of acyl chains was similar to that of the sixth at 10°C. Small-angle X-ray diffraction showed that when increasing 16:0 LPC concentration there was a significant decrease in the 16:0 LPC/DPPC liposome thickness. Thickness of the lipid layer of DPPC was 7.30 nm, but those of the samples containing 14.1 mol% and 27.0 mol% of 16:0 LPC were reduced to 6.79 and 5.52 nm at 25°C, respectively. Wide-angle X-ray diffraction showed that a reflection appeared at 0.42 nm with a broad shoulder around 0.41 nm in pure DPPC at a lipid concentration of 300 mg/mL at 25°C. In the 16:0 LPC/DPPC system, a single sharp reflection appeared at 0.41 nm. It can be concluded that DPPC forms an interdigitated gel phase in the presence of 16:0 LPC concentration below 30 mol%, above this concentration micellization of the bilayers occurs. The interdigitated structures were destabilized slowly with 16:0 LPC concentration increasing, and finally the 16:0 LPC/DPPC bilayers fit into the micelles with its concentration up to 60 mol%. Project supported by the National Natural Science Foundation of China.