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 m icellar to the lamellar phase transition of phosphatidylcholine/1-palm itoylphospharidylcholine (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 (Delta H) values of DPPC was observed. I ncreasing 16:0 LPC concentration, the phase transition was broadened a nd the transition enthalpy was decreased and finally totally disappear ed. The fluorescence probes of 3AS, 9AS, 12AS, and 16AP were employed, respectively, to detect the mobility of various sites of carbon chain s 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 fluidify gradient in b oth gel and liquid-crystalline phase, while in the presence of 14.1 mo l% and 27.0 mol% 16:0 LPC in the mixtures, the fluidity gradient tende d to disappear below 40 degrees C. In the case of 27.0 mol% 16:0 LPC i n the 16:0 LPC/DPPC mixtures the polarization of the sixteenth carbon of acyl chains was similar to that of the sixth at 10 degrees C. Small -angle X-ray diffraction showed that when increasing 16:0 LPC concentr ation there was a significant decrease in the 16:0 LPC/DPPC liposome t hickness. 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 red uced to 6.79 and 5.52 nm at 25 degrees C, respectively. Wide-angle X-r ay diffraction showed that a reflection appeared at 0.42 nm with a bro ad shoulder around 0.41 nm in pure DPPC at a lipid concentration of 30 0 mg/mL at 25 degrees C. In the 16:0 LPC/DPPC system, a single sharp r eflection appeared at 0.41 nm. It can be concluded that DPPC forms an interdigitated gel phase in the presence of 16:0 LPC concentration bel ow 30 mol%, above this concentration micellization of the bilayers occ urs. The interdigitated structures were destabilized slowly with 16:0 LPC concentration increasing, and finally the 16:0 LPC/DPPC bilayers f it into the micelles with its concentration up to 60 mol%.