Interaction of the surface of biomembrane with solvents: structure of multilamellar vesicles of dipalmitoylphosphatidylcholine in acetone-water mixtures

Abstract

To investigate the interaction of the surface of biomembranes with solvents systematically, we have studied the structure and phase behavior of multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) in acetone-water mixture by X-ray diffraction and differential scanning calorimetry. The solubility of phosphorylcholine and l- α-glycerophosphorylchorine, which are the same molecular structure as the head group of phosphatidylcholine (PC), decreased with an increase in acetone concentration. This result indicates that acetone is a poor solvent for the hydrophilic segments of the surface of the PC membrane, and χ parameter (interaction energy parameter) of the hydrophilic segments of the membrane surface with solvent increases with an increase in acetone concentration. Main transition temperature of DPPC-MLV decreased with an increase in acetone concentration from 0% to 10% (v/v) acetone and increased from 10 to 20% (v/v) acetone concentration. X-Ray diffraction data indicated that DPPC-MLV in Region A (0% to 10% v/v acetone concentrations) was in L β′ phase (gel phase with tilted hydrocarbon chains) and that in Region B (10% to 92% (v/v) acetone concentrations) was in L β I phase (gel phase with interdigitated hydrocarbon chains). The biphasic effects of acetone concentration on the main transition temperature can be explained well by the formation of the L β I phase. Lamellar repeat periods of the DPPC-MLV decreased with an increase in acetone concentration in the Region B. At 92% (v/v) acetone, the L β I phase was transformed to L β′ phase. From 94% to 96% (v/v), a new X-ray reflection appeared in the intermediate-angle region having a spacing of 0.62 nm and WAXS reflections shifted to higher angle. These results suggests that a hexagonal packing of the head groups on the surface of DPPC bilayer is formed and the entire DPPC molecules are crystallized in the plane of the bilayer. As acetone concentration increased from 97% (v/v), the structure of DPPC-MLV changed gradually into the crystal structure of anhydrous DPPC. Above 99% (v/v) acetone, an X-ray diffraction pattern became almost the same as that of the anhydrous DPPC. These structural changes of DPPC-MLV are discussed by the effect of the χ parameter between the surface of DPPC-MLV and solvents. As acetone concentration increased, χ parameter of the hydrophilic segments of the surface of DPPC membrane with solvents increased, resulting in the decrease of the solvent content inside the DPPC-MLV.