Intramembrane diffusion coefficient

Eqn. 5 provides a very clear theoretical basis for the data of Fig. 1 (and similar data on other systems, as we shall see). The measured permeability coefficients for a set of solutes should parallel the measured partition coefficients, if the model solvent corresponds exactly in its solvent properties to the permeability barrier of the cell membrane. In addition, the molecular size of the solute is very likely to be an important factor as it will affect the diffusion coefficients within the membrane barrier phase. Data such as those of Fig. 1 will convince us that we have in our chosen solvent a good model for the solvent properties of the membrane s permeability barrier. We can now calculate values of PLx/K for the various solutes, and obtain estimated values of the intramembrane diffusion coefficient, and are in a position to study what variables influence this parameter. Fig. 3 is such a study in which data from Fig. 1 are plotted as the calculated values of f>n,c,n/A.t (calculated as P/K) against the molecular weight of the permeating solute. The log/log plot of the data has a slope of — 1.22, which means that one can express the dependence of diffusion coefficient on molecular weight (A/) in the form where... 

This steep molecular weight dependence of the calculated intramembrane diffusion coefficient is somewhat unexpected. For diffusion in water, the experimental values of the diffusion coefficient bear an or dependence, a form... 

Fig. 3. Calculated relative intramembrane diffusion coefficients across the Nitella cell membrane as a function of molecular weight of the permeant. Ordinate logarithm of the ratio of the permeability coefficient (in 10 cm/s) to the olive oil/water partition coefficient for the permeants of Fig. 1. Abscissa logarithm of their molecular weights. The solid straight line is the Unear regression of log(P/K) on log M with slope of — 1.22. The dashed lines are at a distance of one standard deviation away from the regression line.

Fig. 5. Calculated relative intramembrane diffusion coefficients across the human red blood cell membrane as a function of molecular weight. Abscissa molecular weight on a hnear scale. Ordinate permeability coefficients from Fig. 4 divided by relevant octanol/water partition coefficients (listed in (41).

Mas.s selectivity of intramembrane diffusion coefficients and solvent validity indices... 

When Sy. is recorded, data were fitted to Eqn, 9, When just is recorded, this is the slope of the regression line of (log of) intramembrane diffusion coefficient on log molecular weight. 

Fig. 10. Calculated intramembrane diffusion coefficients across membranes of (A) Nil 8 hamster fibroblasts and (B) the HTC rat hepatoma cell line, as a function of molecular weight of the permeant. Ordinate logarithm of membrane thickness, taken as 50-10 cm, multiplied by the permeability coefficient (in cm/sec) and divided by the octanol/water partition coefficient. Abscissa logarithm of molecular weight. Data taken from Giorgi and Stein [19]. Permeants indicated as follows large circle, mean value for all the steroids of Fig. 8 Ur, urea Thw, thiourea Gly, glycerol Ap, antipyrine. Regression lines through the points have slopes of —3.9 for the Nil 8 cells and —3.7 for the HTC cells.

Available data on the permeability of cell membranes are consistent with the view that the vast majority of substances find the major barrier to their trans-membrane movement to be the tightly packed proximal chains of the phospholipid hydrocarbons, together with the cholesterol moieties and the glycerol backbone. This region has solvent properties well modelled by the less non-polar organic solvents (ether and even w-octanol). The data reveal a steep mass selectivity (three to six power inverse dependence of intramembrane diffusion coefficient on molecular... 

Experimental results from our previous studies [26] demonstrated that the diffusional resistance in the liquid phase is negligible under representative experimental conditions and intramembrane transport is the rate-limiting step for the alum recovery process. The following provides a stepwise protocol for the determination of self-diffusion coefficient values and the alum recovery rate for the DMP. 

Based on simulation results,i the authors concluded that it is possible to obtain conversions higher than the thermodynamic equilibrium value, for a given set of operating conditions related with the pressure difference between the retentate and permeate sides, Thiele modulus (ratio between a characteristic intramembrane diffusion time and a characteristic direct reaction time), and contact time (ratio between the maximum possible flux across the membrane for a reference component and the total molar feed flow rate) parameters. For a given stoichiometry, the authors found a possible conversion enhancement when, globally, the diffusion coefficients of the reaction products were higher than the ones of the reactants, or vice versa for the sorption coefficients (that is, the ones for the reactants higher than the ones for the reaction products). Also, it was found that an enhancement or a detriment of the conversion could be obtained based solely on the pressure difference between the two sides of the membrane. 

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