Relation between adsorption at an oil/water interface and membrane permeability

Kinetics and mechanisms of monolayer interactions V: surface activities of alkylamides, alkylmonocarboxylic acids, and alkylketones and their interaction energies with phospholipid monolayers

The adsorption free energies of the C1-C6 alkylamides, C3-C8 alkylketones, and C1-C10 alkylmonocarboxylic acids at the air-water interface, estimated from plots of the surface pressure (less than or equal to 5 dynes/cm) versus the bulk concentration, were linear functions of the total surface area per molecule (square angstroms per molecule), with a slope 46% higher for the alkylamides and 25% lower for the alkylketones than that for the monocarboxylic acids. The interaction energies of alkylamides with dipalmitoyl lecithin and dipalmitoyl phosphatidylethanolamine spread at the air-water interface, estimated from the surface pressure increase with increasing concentrations of the injected C1-C5 compounds, were linear functions of the total surface area per molecule. The diffusion free energies, delta Gdif, of the alkylamides within a phospholipid bilayer, predicted from the permeability equation and their interaction energies with dipalmitoyl lecithin monolayers by assuming the additivity of their free energies of adsorption and dehydration at the solution-bilayer interface, agreed with the literature data.

Thermodynamic analysis of nonelectrolyte permeation across the toad urinary bladder

Permeability coefficients (P's) and apparent activation energies (Ealpha's) for nonelectrolyte permeation across the toad urinary bladder have been analyzed in terms of the thermodynamics of partition between membrane lipids and water. Particular attention has been paid to the contributions made by -CH2- and -OH groups: on the average, the addition of one -CH2- group to a molecule increases P fourfold, while the addition of one -OH group reduces P 500-fold. Using these changes in P, we have calculated the incremental free energies (delta delta F), enthalpies (delta delta H), and entropies (delta delta S) for partition, hydration, and solution in membrane lipids. The results for toad bladder have been compared and contrasted with those extracted from the literature for red blood cells, lecithin liposomes, and bulk phase lipid solvents. The partition of -CH2- groups into toad bladder and red cell membranes is dominated by entropy effects, i.e., a decrease in entropy of the aqueous phase that "pushes" the group out of water, and an increase in entropy of the membrane lipid that "pulls" the group into the membrane. This process resembles that in "frozen" liposome membranes. In "melted" liposomes and bulk lipid solvents the free energy of solution in the lipid is controlled by enthalpy of solution. PArtition of -OH groups in all systems is governed by hydrogen bonding between the -OH group and water. However, the solution of the -OH group in the toad bladder membranes is complex, and processes such as dimer and tetramer formation in the lipid phase may be involved. The results presented in this and the previous paper are discussed in terms of the structure of phospholipid bilayer membranes. Attention is drawn to the possible role of structural defects in the quasi-crystalline structure of the lipid (so-called 2 gl klinks) in the permeation of small molecules such as water, urea, methanol and acetamide.

Temperature dependence of nonelectrolyte permeation across red cell membranes

The temperature dependence of permeation across human red cell membranes has been determined for a series of hydrophilic and lipophilic solutes, including urea and two methyl substituted derivatives, all the straight-chain amides from formamide through valeramide and the two isomers, isobutyramide and isovaleramide. The temperature coefficient for permeation by all the hydrophilic solutes is 12 kcal mol(-1) or less, whereas that for all the lipophilic solutes is 19 kcal mol(-1) or greater. This difference is consonant with the view that hydrophilic molecules cross the membrane by a path different from that taken by the lipophilic ones. The thermodynamic parameters associated with lipophile permeation have been studied in detail. DeltaG is negative for adsorption of lipophilic amides onto an oil-water interface, whereas it is positive for transfer of the polar head from the aqueous medium to bulk lipid solvent. Application of absolute reaction rate theory makes it possible to make a clear distinction between diffusion across the water-red cell membrane interface and diffusion within the membrane. Diffusion coefficients and apparent activation enthalpies and entropies have been computed for each process. Transfer of the polar head from the solvent into the interface is characterized by DeltaG(double dagger) = 0 kcal mol(-1) and DeltaS(double dagger) negative, whereas both of these parameters have large positive values for diffusion within the membrane. Diffusion within the membrane is similar to what is expected for diffusion through a highly associated viscous fluid.