Intermembrane distance in multilamellar vesicles of phosphatidylcholine depends on the interaction free energy between solvents and the hydrophilic segments of the membrane surface.

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) and 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) in dimethylformamide (DMF)-water mixture by X-ray diffraction and differential scanning calorimetry. The solubility of phosphorylcholine, which is the same molecular structure as the head-group of phosphatidylcholine (PC), decreased with an increase in DMF concentration. This result indicates that DMF is a poor solvent for the hydrophilic segments of the surface of the PC membrane, and interaction free energy of the hydrophilic segments of the membrane surface with solvents increases with an increase in DMF concentration. X-ray diffraction data indicated that DPPC-MLVs were in the bilayer gel phase from 0 to 80% (v/v) DMF, and that the spacing (lamellar repeat period) and intermembrane distance of DPPC-MLV decreased with an increase in DMF concentration. Main transition temperature and pre-transition temperature of DPPC-MLV increased with an increase in DMF concentration, and above 50% (v/v) DMF there was no pre-transition. In the interaction of POPC-MLV with DMF, X-ray diffraction data indicated that POPC-MLVs were in L(alpha) phase (liquid-crystalline phase) from 0 to 80% (v/v) DMF, and that the spacing and intermembrane distance of POPC-MLV decreased with an increase in DMF concentration. These results are discussed by the change of the interaction free energy between the hydrophilic segments of the membrane surface and solvents. As DMF concentration increases, this interaction free energy may increase, resulting in the decrease of the intermembrane distance of PC-MLVs.