A comparative study of diffusive and osmotic water permeation across bilayers composed of phospholipids with different head groups and fatty acyl chains.

Osmotic and diffusive water permeability coefficients Pf and Pd were measured for lipid vesicles of 100-250 nm diameter composed of a variety of phospholipids with different head groups and fatty acyl chains. Two different methods were applied: the H2O/D2O exchange technique for diffusive water flow, and the osmotic technique for water flux driven by an osmotic gradient. For phosphatidylcholines in the liquid-crystalline state at 70 degrees C, permeability constants Pd between 3.0 and 5.2.10(-4) cm/s and ratios Pf/Pd 7 and 23 were observed. The observation of a permeability maximum in the phase transition region and the fact that osmotically driven water flux is higher than diffusive water exchange suggest that water is diffusing through small transient pores arising from density fluctuations in the bilayers. The Pd values depend on the nature of the head group, on the chemical structure of the chains, and on the type of chain linkage. In the case of charged lipids, the ionic strength of the solution has a strong influence. For phosphatidylethanolamines, phosphatidic acids, and ether phosphatidylcholines, permeability constants Pd were considerably lower (2-4.10(-6) cm/s at 70 degrees C). For liquid-crystalline phosphatidylcholines, a strong reduction of Pd after addition of ethanol was observed (2-4.10(-6) cm/s at 70 degrees C). The experimental values are discussed in connection with different permeation models.