Phase transition between hexagonal II (H-II) and liquid-crystalline phase induced by interaction between solvents and segments of the membrane surface of dioleoylphosphatidylethanolamine

We have investigated effects of several water-soluble organic solvents such as acetone, acetonitrile, and ethanol, which also have high solubility in alkane, on the structure and phase behavior of dioleoylphosphatidylethanolamine (DOPE) dispersion. X-ray diffraction data indicated that a phase transition from hexagonal II (H-II) to liquid-crystalline (L-alpha) phase in DOPE dispersion, occurred at 13% (v/v) acetone in H2O at 20 degrees C. The temperature of the L-alpha-H-II phase transition of DOPE dispersion increased with an increase in acetone concentration, and it was 37 degrees C at 20% (v/v) acetone. These results indicated that acetone stabilized L-alpha phase relative to H-II phase. Similar results were obtained in interactions of DOPE dispersions in H2O with acetonitrile or ethanol. X-ray diffraction data indicated that the H-II-L-alpha phase transition occurred at 9.0% (v/v) acetonitrile or at 9.8% (v/v) ethanol in water at 20 degrees C. The L-alpha-H-II phase transition temperature of DOPE dispersion increased with an increase in acetonitrile or ethanol concentration, and it was 66 degrees C at 20% (v/v) acetonitrile. Substitution of H2O by D2O (deuterium oxide) increased their threshold concentrations of the H-II-L-alpha phase transition induced by these organic solvents. A mechanism of these phase transitions and the effect of the substitution of H2O by D2O is proposed and discussed; an interaction free energy between solvents and the hydrophobic segments of the alkyl chains in the membrane surface, and also a packing parameter of the phospholipid may be main factors to explain these phenomena reasonably. (C) 1997 Elsevier Science B.V.