Organic solvents induce interdigitated gel structures in multilamellar vesicles of dipalmitoylphosphatidylcholine

We have investigated the effects of water-soluble organic solvents which also have a high solubility for alkanes, on the structure and phase behavior of multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC-MLV). Of these solvents, we selected five organic solvents; such as, acetonitrile, acetone, propionaldehyde, ethanol, and tetrahydrofuran. The main transition temperature of DPPC-MLV decreased with an increase in acetonitrile concentration from 0% to 6.0%(v/v) acetonitrile, and increased above 6.0%(v/v). X-ray diffraction data indicated that a phase transition form L(beta)' to L(beta)I phase (interdigitated gel phase) in DPPC-MLV, occurred at 5.0%(v/v) and DPPC-MLV were completely in the L(beta)I phase above 6.0%(v/v) acetonitrile at 20 degrees C. Results of the excimer method (Yamazaki, M. et al. (1992) Biochim. Biophys. Acta 1106, 94-98) supported the above results; the ratio of excimer to monomer fluorescence intensity (E/M) of pyrene-PC in DPPC-MLV rapidly decreased at 5.1%(v/v) and E/M became very low above 6.0%(v/v) acetonitrile. By the excimer method, we have found that other organic solvents; such as acetone, propionaldehyde, and tetrahydrofuran, induced a phase transition for L(beta)' to L(beta)I phase in DPPC-MLV. Threshold concentrations of acetone, ethanol, propionaldehyde, and tetrahydrofuran for this phase transition at 20 degrees C were 9.4%(v/v), 5.5%(w/v), 3.5%(w/v), and 3.7%(w/v), respectively. Substitution of H2O by D2O (deuterium oxide) increased the threshold concentrations of all the organic solvents. A mechanism of these phase transitions and the effect of the substitution of H2O by D2O is proposed and discussed; a concept of the chi parameter, which is an interaction energy parameter between the surface segments of DPPC-MLV and solvents, may explain these phenomena reasonably.