Permeation of water through bilayer lipid membranes

Abstract

This paper, the fourth of a series, is concerned with the movement of water across the bilayer lipid membrane (BLM). The water permeability coefficients, Po, were determined for BLM formed from oxidized cholesterol, cholesterol-DAP (dodecyl acid phosphate), and cholesterol-HDTAB (hexadecyltrimethylammonium bromide). When NaCl was used as the solute, the value of Po for oxidized cholesterol membranes was found to be 8.4 ± 0.5 × 10−4 cm/sec at 22.5°C. In either NaI or CsCl solution, however, the value of Po was found to be about three times larger. The temperature dependence of Po for BLM generated from oxidized cholesterol in n-octane was also measured in the range of 22.5–44.0°C. Experimental activation energy was calculated to be 6.8 ± 0.5 kcal/mole. From the temperature-permeability coefficient data, various derived quantities from absolute reaction rate theory have been evaluated. The nature of pathway for water permeation and the factors governing the rate of water transport across the BLM are discussed in terms of the transition state theory. The results of this study suggest that the bifacial region of the membrane plays an important role in water permeability. During the last few years, a new experimental structure of less than 90 A thick has been extensively investigated. This membranous structure in aqueous solution known variously as bimolecular, bilayer, or black lipid membrane (abbreviated as BLM), possesses some of the properties of the biological membrane. These properties include thickness, certain electrical characteristics, bifacial tension, ionic selectivity, and water permeability. A detailed summary of the studies on BLM together with the relevant literature references may be found in a recent publication (1). In this paper we report the results of measurements of the osmotic permeability coefficient of water across BLM formed from various lipid solutions. Previously, the permeability coefficients of BLM have been measured by several groups of investigators (2–7). The values reported vary from 4.9–104 μ/sec. It is not certain from these studies whether this large discrepancy is due to the difference in the experimental techniques or the composition of the lipid solutions used for BLM formation or both. Our measurements with the BLM generated from various lipid solutions should therefore be of some interest. Regarding the mechanism of water permeation through the BLM, it is generally recognized that only a very limited amount of information can be derived from measurements of water permeability coefficient at a single temperature. Hence there is an obvious need to carry out studies at various temperatures. Further, one would like to raise the question of the nature of the interfacial region (i.e., the zone near the solution/membrane interface) upon the water permeability. Therefore, we have measured the permeability coefficients of water through the BLM as a function of (a) the temperature and (b) the ionic composition of the bathing medium. The permeability-temperature data permit an evaluation of the various derived quantities from the absolute reaction rate theory, from which certain speculations concerning the mechanism of water permeation through the BLM are now possible. The permeability coefficient data determined as a function of solute used provide an opportunity to examine the effect of ions on water permeation and possibly on the structure of the BLM/solution interface.