Electrical fields, membrane interactions

Electric field sensitive dyes respond to changes in electrical membrane potential by a variety of different mechanisms with widely varying response times depending on their chemical structure and their interaction with the membrane. An understanding of the mechanisms of dye response and their response mechanisms is important for an appropriate choice of a probe for a particular application. The purpose of this chapter is, therefore, to provide an overview of the dyes presently available, how they respond to voltage changes, and give some examples of how they have been applied. Finally, because there is still scope for the development of new dyes with improved properties, some directions for future research will be discussed. 

Free diffusion of molecules in solution is characteristically a haphazard process with net directionality determined only by solute gradients and diffusion coefficients. Within cellular and extracellular spaces, however, diffusion can be strongly influenced by noncovalent interactions of solvent and solute molecules with membranes as well as the cellular and extracellular matrix. Channels and orifices can also alter the movement of solute and solvent molecules. These interactions can greatly alter the magnitude of the diffusion coefficient for a molecule from its isotropic value D in water to apparent diffusion coefficient D (which often can be directionally resolved into D, Dy, and D ). The parameter A, known as the tortuosity, equals DID y. In principle, A has X, y, and z components that need not be equal if there is any anisotropy in the local electrical fields or porosity of the matrix. 

Fig.3 A migrating zone of solute molecules (spots) interacting with lipid bilayers (rings) in a chromatographic or electrophoretic separation system. The free solute molecules move (arrows) relative to the liposomes or vesicles in a flow of eluent or in an electric field. The solute molecules may either partition into the membranes and diffuse between the external and internal aqueous compartments of the structures as depicted, or interact with the external surface of the membranes and stay outside.

It is important to note that the concept of osmotic pressure is more general than suggested by the above experiment. In particular, one does not have to invoke the presence of a membrane (or even a concentration difference) to define osmotic pressure. The osmotic pressure, being a property of a solution, always exists and serves to counteract the tendency of the chemical potentials to equalize. It is not important how the differences in the chemical potential come about. The differences may arise due to other factors such as an electric field or gravity. For example, we see in Chapter 11 (Section 11.7a) how osmotic pressure plays a major role in giving rise to repulsion between electrical double layers here, the variation of the concentration in the electrical double layers arises from the electrostatic interaction between a charged surface and the ions in the solution. In Chapter 13 (Section 13.6b.3), we provide another example of the role of differences in osmotic pressures of a polymer solution in giving rise to an effective attractive force between colloidal particles suspended in the solution. 

Essentially the only source of flow in a solid ion-exchange membrane (ion-exchanger) is electro-osmosis. This is a flow induced by the interaction of the electric field with the space charge distributed in the fluid present in the solid. In this respect, electro-osmosis may be regarded as a relative of electro-convection in a hydrodynamically free solution. 

Problems 2 and 3 are of direct relevance for an adequate understanding of concentration polarization at, respectively, composite heterogeneous and homogeneous permselective membranes. The main difference between these formulations is that in Problem 2, relevant for a composite heterogeneous membrane, the motion in a pore of the support is induced by the electro-osmotic slip due to the interaction of the applied electric field with the space charge of the electric double layer which is present already at equilibrium. 

In contrast to this, with a homogeneous membrane corresponding to Problem 3, the motion in a symmetry cell of the liquid boundary layer, adjacent to an electrically inhomogeneous membrane, is induced by the electric field interaction with an essentially nonequilibrium space charge, formed only in the course of the ionic transport itself. 

The possible existence of highly polarized metastable states in biological model membranes has led us to a preliminary model for specific interactions in membranes (14). We have assumed that biomolecules are capable of nonlinear polarization oscillations, when they are sufficiently excited by metabolic energy or by external means, e.g. electric fields. Restricting to the case of two interacting molecules for simplicity, the dynamics of the system is given by a set of nonlinear differential equations for the polarization P. of the molecule i ... 

Several functional sodium channel assays that use membrane potential sensing dyes and pharmacological activators have been described [99,106, 108,112]. In addition, there are recent reports of fluorescent sodium channel assays using changes in an electric field applied across cells to activate sodium channels [110, 111], thus avoiding potential interactions of the test compound with the channel activator. 

Because of the question as to the way in which the response of biological systems to outside stimulation is brought about, it will also be of particular interest to examine how the structure and function of tnopolymers can be affected by the variation of external parameters. In this regard the effects of the interaction of an electric field with biomolecular systems are of considerable significance, since electric fidds are known to occur in biological cells (especially at membranes) and to take part in regulation processes as well as in information transfer (as reflected, e.g., by nerve pulses). 

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