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

Abstract

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 α) phase in DOPE dispersion, occurred at 13% (v/v) acetone in H 2O at 20°C. The temperature of the L α-H II phase transition of DOPE dispersion increased with an increase in acetone concentration, and it was 37°C at 20% (v/v) acetone. These results indicated that acetone stabilized L α phase relative to H II phase. Similar results were obtained in interactions of DOPE dispersions in H 2O with acetonitrile or ethanol. X-ray diffraction data indicated that the H II-L α phase transition occurred at 9.0% (v/v) acetonitrile or at 9.8% (v/v) ethanol in water at 20°C. The L α-H II phase transition temperature of DOPE dispersion increased with an increase in acetonitrile or ethanol concentration, and it was 66°C at 20% (v/v) acetonitrile. Substitution of H 2O by D 2O (deuterium oxide) increased their threshold concentrations of the H II-L α phase transition induced by these organic solvents. A mechanism of these phase transitions and the effect of the substitution of H 2O by D 2O 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.