Effects of poly( l-lysine) on the structural and thermotropic properties of dipalmitoylphosphatidylglycerol bilayers

Abstract

The effects of poly( l-lysine) on the structural and thermotropic properties of dipalmitoylphosphatidylglycerol (DPPG) bilayers were studied with differential scanning calorimetry (DSC), X-ray diffraction and freeze-fracture electron microscopy. For thermal behavior, in the DPPG/poly( l-lysine) system the main transition temperature rises to 45.7°C and the pretransition disappears in opposition to pure DPPG vesicles. An additional transition appears approximately at 36°C for the DPPG/poly( l-lysine) system after incubation at 4°C for two months. The incubated sample gives a X-ray diffraction pattern having several additional reflections in the range of 0.2–0.9 nm at 15°C. These results suggest that even in the presence of poly( l-lysine) the DPPG bilayers form the subgel (L c) phase after the long incubation at a low temperature. The X-ray diffraction measurements indicate that the structure of the L c phase for DPPG/poly( l-lysine) system is different from that of pure DPPG bilayers. On the other hand, in the gel (L β′) phase, the wide-angle X-ray diffraction pattern suggests that the presence of poly( l-lysine) hardly affects the packing of hydrocarbon chains in the DPPG bilayers. The small-angle X-ray diffraction and freeze-fracture electron microscopy exhibit that the DPPG/poly( l-lysine) system forms a tightly packed multilamellar structure in which the poly( l-lysine) is intercalated between the subsequent DPPG bilayers.