The mechanism of the stabilization of the hexagonal II (HII) phase in phosphatidylethanolamine membranes in the presence of low concentrations of dimethyl sulfoxide

Dimethyl sulfoxide (DMSO), a water-miscible organic solvent, has been used as a cryoprotectant for cells. It is known that DMSO stabilizes the H-II phase of phosphatidylethanolamine (PE) membranes rather than the L-alpha phase, while most other water-miscible organic solvents such as acetone and ethanol destabilize the H-II phase. To elucidate the mechanism for this stabilizing effect of DMSO on the H-II phase, we have investigated its effects on the structures and physical properties of PE membranes. X-ray diffraction data indicated that di-palmitoleoylphosphatidylethanolamine (DPOPE) membranes in H2O at 20 degreesC were in the L-alpha phase and that an L-alpha to H-II phase transition occurred at X=0.060 (mole fraction of DMSO) in water/DMSO mixtures. As the DMSO concentration increased, the basis vector length of the dioleoylphosphatidylethanolamine (DOPE)/ 16 wt% tetradecane membrane and also of the DPOPE/ 16 wt% tetradecane membrane in the H-II phase decreased, suggesting that the spontaneous curvature of these membranes increased. We have also investigated the effects of DMSO on the physical properties of the PE membranes, and compared them with those of acetone, As the DMSO concentration increased, the excimer to monomer fluorescence intensities of pyrene-phosphatidylcholine in the PE membranes decreased, indicating that the membrane fluidity decreased, and also the generalized polarization value of the Laurdan fluorescent probe in the DPOPE membrane increased, indicating that the polarity of the membrane interface decreased. On the other hand, acetone had the opposite effects to DMSO. The interaction free energy between the membrane surface segments and solvent increased with an increase in DMSO concentration. It decreased the amount of solvent in the membrane interface, inducing an increase in the spontaneous curvature. This can reasonably explain the effects of DMSO on the phase stability and the physical properties of the the membranes.