Electrical conductivity, transfer of hydrogen ions in lipid bilayer membranes and uncoupling effect induced by pentachlorobenzenethiol (pentachlorothiophenol)

Passive Permeability of Planar Lipid Bilayers to Organic Anions

The membrane permeability P of organic ions was reported to be governed by the structure of the permeating molecule. Thus far, it is unclear whether the ion structure alters membrane partition or translocation proper across the membrane. Here, we obtained P values for 24 anionic compounds (18 concrete values, 6 upper limits) measuring the current that they carry through folded planar lipid bilayers. The P values range over more than 10 log units. Our measured permeability values correlate well (r = 0.95; logRMSE 0.74) with the hexadecane/water partition coefficients of the respective chemicals predicted by the COSMO-RS theory. Other attempts to predict P from the partition coefficient of the neutral molecule and from the solvation energy (Born energy) that opposes transfer into the membrane once the molecule is charged were unsuccessful. The uncertainties in assigning an effective radius to nonspherical molecules were much too large. The observation underlines that the actual structure of the molecules needs to be considered to predict partition and thus P by the solubility-diffusion model.

Proton conductance caused by long-chain fatty acids in phospholipid bilayer membranes

Mechanisms of proton conductance (GH) were investigated in phospholipid bilayer membranes containing long-chain fatty acids (lauric, myristic, palmitic, oleic or phytanic). Membranes were formed from diphytanoyl phosphatidylcholine in decane plus chlorodecane (usually 30% vol/vol). Fatty acids were added either to the aqueous phase or to the membrane-forming solution. Proton conductance was calculated from the steady-state total conductance and the H+ diffusion potential produced by a transmembrane pH gradient. Fatty acids caused GH to increase in proportion to the first power of the fatty acid concentration. The GH induced by fatty acids was inhibited by phloretin, low pH and serum albumin. GH was increased by chlorodecane, and the voltage dependence of GH was superlinear. The results suggest that fatty acids act as simple (A- type) proton carriers. The membrane: water partition coefficient (Kp) and adsorption coefficient (beta) were estimated by finding the membrane and aqueous fatty acid concentrations which gave identical values of GH. For palmitic and oleic acids Kp was about 10(5) and beta was about 10(-2) cm. The A- translocation or "flip-flop" rate (ka) was estimated from the value of GH and the fatty acid concentration in the membrane, assuming that A- translocation was the rate limiting step in H+ transport. The kA's were about 10(-4) sec-1, slower than classical weak-acid uncouplers by a factor of 10(5). Although long-chain fatty acids are relatively inefficient H+ carriers, they may cause significant biological H- conductance when present in the membrane at high concentrations, e.g., in ischemia, hypoxia, hormonally induced lipolysis, or certain hereditary disorders, e.g., Refsum's (phytanic acid storage) disease.

Adsorption of ionized and neutral pentachlorophenol to phosphatidylcholine membranes

We have studied adsorption of pentachlorophenol (PCP) to phosphatidylcholine (PC) membranes by measuring the electrophoretic mobility of multilayered lipid vesicles in PCP solutions. PC vesicles become negatively charged due to the adsorption of ionized PCP, and we have found that their zeta potential depends upon the ionic strength and pH of the aqueous suspension. We have shown that the experimental results can be adequately accounted for in terms of a two-component Langmuir-Stern-Grahame adsorption model assuming that the 'PCP adsorption sites' are occupied either by the neutral (HA) or the ionized (A-) species. The characteristics of adsorption isotherms of the PCP - PC membrane are as follows: the association constants are KA = 55,000 dm3/mol, KHA = 279,000 dm3/mol; 4.3 PC molecules make up each PCP adsorption site at saturation; the linear partition coefficients are beta HA = (15.5 +/- 0.7) x 10(-5) m and beta A = (3.0 +/- 0.3) x 10(-5) m. The properties of PCP adsorption isotherms for PC membranes predict an increased pKa value of membrane-bound PCP, which has been observed in related studies.