ISSN:
1573-904X
Keywords:
lipid bilayer membranes
;
bilayer permeability
;
unstirred boundary layers
;
pH effects
;
buffer effects
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract The diffusion of weak acids or bases across planar lipid bilayer membranes results in aqueous boundary layer pH gradients. If not properly taken into account, such pH gradients will lead to errors in estimated membrane permeability coefficients, P m. The role of the permeant concentration, the buffer capacity, and the physicochemical properties of both permeant and buffer on the magnitude and impact of such pH gradients have been explored. A theoretical model has been developed to describe the diffusion of both permeant and buffer species. Significant pH gradients develop depending on solution pH and the pK a's, concentrations, and P m values of both permeant and buffer. The relative error in experimentally determined P m values was calculated as the ratio, r, between apparent P m values (obtained from flux measurements using an equation which neglected boundary layer pH gradients) and its true value. Simulated r values ranged from 1 (0% error) to 〈0.01 (〉100% error) for weak acids, decreasing with decreasing buffer capacity and increasing solute flux. The buffer capacity required for an r 〉 0.95 was calculated versus pH for permeants varying in pK a and P m. Membrane-permeable buffers significantly reduce boundary layer pH gradients through a feedback effect due to buffer cotransport. Apparent P m values of p-hydroxymethyl benzoic acid across lecithin bilayer membranes at 25°C were obtained as a function of permeant concentration in various buffers [glycolic, 2-(N-morpholino)ethane-sulfonic, and formic acids]. Predictions agreed closely with experimental fluxes.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1018989107129
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