Asymmetry potential in inhomogeneous membranes

Abstract

2721polar species, either in bulk water or at an aqueous interface, and (2) those which are dependent on the electrostatic self-energy of a polar or charged group. As the simplest example of the first class, consider the interaction of two spherical charges well separated in bulk liquid. From continuum electrostatics, this interaction is given by Coulomb's law as W = (q1q2)/trlz where ql and q2 are the charge magnitudes, t is the intervening dielectric, and rI2 is the separation of the charge centers. Assuming that charges q1 and q2 were not derived to explicitly take into account the dielectric constant of the water used in the simulation, the error introduced by using a TIP4P with a dielectric constant of 60 in the simulation will be 33%. Examples of problems of biological interest which are governed predominantly by electrostatic interaction energies include shifts in pK's of surface residues on mutation and association rates of substrates with enzymes. The simplest example of the second class of properties is the free energy of transfer of an ion from water to a nonaqueous environment. From continuum electrostatics the electrostatic component of this free energy is given by the Born expression W = q2/(2a)[ l / t il/t,] where a is the radius of the cavity formed by the ion in the two media and where ti and e, refer to the dielectric constants of the nonaqueous environment and of water, respectively. For the transfer of such an ion from water to a uniform dielectric with ti = 4, the error in free energy of transfer by using a water with a dielectric constant of 60 is only 1.7%. Examples of properties of biological or chemical interest which can be largely governed by electrostatic self-energies of charged or polar groups include transfer free energies, desolvation of charged or polar substrates on binding to enzymes, and protein folding. The example given above used ti = 4 to suggest an ideal protein interior. Thus interaction-energy properties but not solvation free energy properties will be sensitive to the use of a water model with a dielectric constant which is 60. It is worth making a final point concerning biological or chemical simulations which employ explicit water models. To obtain the results presented here and in other dielectric simulations, a reaction field has been introduced to account for the continuum beyond the cutoff used in the simulation. The dielectric properties of the model under study change considerably if such a convention is not introduced. In simulations of proteins in water, reaction fields have not to date been introduced. As such, the dielectric constant of the waters in such simulations is undetermined.In conclusion, our results indicated that the free energy method can give results comparable to the induced polarization method, though it produces 2-3 times the standard error when both are analyzed using block averages. Values for the dielectric constant of TIP4P determined by the free energy method and the polarization method are slightly higher than those reported for compara...

On the Methanol Permeability through Pristine Nafion® and Nafion/PVA Membranes Measured by Different Techniques. A Comparison of Methodologies

“…After each measurement the membrane was cleaned with distilled water and then left for 24 h immersed in distilled water to remove any residual concentration into the pores of the membrane. The asymmetry potential of the membrane was measured before performing another experiment following the procedure described in the literature [30].…”

“…It is known that platinum-ruthenium (Pt-Ru) is the best catalyst for methanol oxidation [30]. The mechanism of electro-oxidation of methanol is complex because the reaction involves a transfer of six electrons and the formation of several by-products and finally carbon dioxide.…”

Nernst−Planck Model Simulating the Electromotive Force Measured over Asymmetric Membranes with Special Reference to the Initial Time Method for Investigation of Surface Layers

“…[9][10][11] We would like to give an illustration of this method from a generalized Nernst-Planck-Donnan point of view. Furthermore, we have some experience with electrochemical characterization of membranes by irreversible thermodynamics, emf measurements and impedance spectroscopy techniques and with fitting the results to simple Nernst-Planck-Donnan expressions, especially cellulose acetate membranes.…”

Estimation of Diffusion Coefficients in Biomembranes

“…The initial time emf method was proposed to characterize the membrane phase of asymmetric membranes individually by means of emf measurements (17)(18)(19)(20). If the diffusion zone is narrow enough, the membrane properties may be assumed constant and the transport number may be calculated from the emf data.…”