The Permeability of Thin Lipid Membranes to Bromide and Bromine

Abstract

Thin lipid (optically black) membranes were made from sheep red cell lipids dissolved in n-decane. The flux of Br across these membranes was measured by the use of tracer 82Br. The unidirectional flux of Br (in 50-100 m NaBr) was 1-3 X 10 -2 mole/cm 2 sec. This flux is more than 1000 times the flux predicted from the membrane electrical resistance (> 108 ohmcm 2 ) and the transference number for Br-(0.2-0.3), which was estimated from measurements of the zero current potential difference. The Br flux was not affected by changes in the potential difference imposed across the membrane (4-60 mv) or by the ionic strength of the bathing solutions. However, the addition of a reducing agent, sodium thiosulfate (10 -3 M), to the NaBr solution bathing the membrane caused a 90 % reduction in the Br flux. The inhibiting effect of S203 suggests that the Br flux is due chiefly to traces of Br 2 in NaBr solutions. As expected, the addition of Br 2 to the NaBr solutions greatly stimulated the Br flux. However, at constant Br 2 concentration, the Br flux was also stimulated by increasing the Br-concentration, in spite of the fact that the membrane was virtually impermeable to Br-. Finally, the Br flux appeared to saturate at high Br 2 concentrations, and the saturation value was roughly proportional to the Br-concentration. These results can be explained by a model which assumes that Br crosses the membrane only as Br 2 but that rapid equilibration of Br between Br 2 and Br-occurs in the unstirred layers of aqueous solution bathing the two sides of the membrane. A consequence of the model is that Br-"facilitates" the diffusion of Br across the unstirred layers.

Facilitated Diffusion and Membrane Permeation of Fatty Acid in Albumin Solutions

“…At t=0: (11) (12) (13) These are arbitrary, given that at x=0 Eqs. (4) to (6) are satisfied, and could just as well be replaced by using zero concentrations throughout the slab.…”

“…When the linear distribution of C F [Eq. (13)] was replaced by a quadrature (parabolic dependence of C F on x) the differences became minor and insignificant within less than 5 s. This reflects the tendency of the solution to reach an equilibrium within a short time. Perhaps a more "natural" choice of initial conditions is the following: assume that there is a region that contains only albumin (with C A T concentration) and at a certain moment (t=0) we attach this region to the source, i.e., at x=0 this region has C A , C F , C AF values that are identical to the source equilibrium values, but the rest of the region (x>0) is still unaffected, specifically, (16) (17) (18) where the source concentrations are uniquely determined by Eqs.…”

“…1-2, the ones computed with Eqs. (11)- (13). The only difference is that now it takes longer to reach the steady state.…”

“…The need to account for unstirred layers of solution adjacent to membranes is clear. 6,13,14,28 Clearly, models assuming instantaneous equilibrium dissociation between plasma fatty acid and albumin in the unstirred layer or at the membrane surface cannot be used to assess the effects of slow association and dissociation. The conclusion of Van der Vusse et al 35 that a membrane receptor should be considered was based on the experimental observation that the transport of free unbound fatty acid was too low to account for fluxes observed in the presence of albumin.…”

“…Most of all, the controlling mechanisms of adsorption at the membranes' surfaces and transport the membrane across vary. 9,31,32,38 Gutknecht et al 13 showed that transmembrane transport of bromine was via the unionized form but that, like albumin-bound F, the diffusional transport of the ionized form in the stagnant layer contributed greatly.…”