Dipolar membrane potential

Considerations of the physical features that could control the transport and permeability of ions in membranes need to take into account the transmembrane potential. This potential, as determined experimentally, corresponds to the difference in the electrical potentials between the two aqueous phases away from the inner and outer membrane surfaces, and its steady-state value is related to such factors as the difference in ionic concentrations of the two aqueous phases, ionic permeability of the membrane, and the presence of any fixed charges or dipolar species in the membrane structure. Excitable cells such as those of nerve or muscle tissue exhibit resting membrane potentials of up to 90 mV and greater, whereas many of the small and nonexcitable cells (e.g., erythrocytes) exhibit smaller values of around 20 mV or so. 

Fig. 6 The electrical potential, ij/, profile across a lipid bilayer. The transmembrane potential, Aij/, is due to the difference in anion and cation concentrations between the two bulk aqueous phases. The surface potential, ij/s, arises from charged residues at the membrane-solution interface. The dipole potential, J/d, results from the alignment of dipolar residues of the lipids and associated water molecules within the membrane...

Dipolar Potential Anisotropy of lipid headgroup dipoles in organized membrane Electrostatic... 

The polar lipid headgroup zone contains the phospholipid and steroid phosphorus-nitrogen, carbonyl, hydroxyl and hydration water moieties which combine to establish a substantial dipolar potential. This positive potential is of a magnitude of several hundred millivolts across the membrane headgroup zone. In the membrane hydrocarbon interior, the electrostatic field must be at least 450 to 750 mV (14) and controls ion current across the interior as we 1 1 as possibly influencing selective ion adsorption to the membrane surface. 

The Gruen—Marcelja model could relate the hydration force to the physical properties of the surfaces by assuming that the polarization of water near the interface is proportional to the surface dipole density.9 This assumption led to the conclusion that the hydration force is proportional to the square of the surface dipolar potential of membranes (in agreement with the Schiby—Ruckenstein model),6 a result that was confirmed by experiment.10 However, subsequent molecular dynamics simulations revealed that the polarization of water oscillated in the vicinity of an interface, instead of being monotonic.11 Because the Gruen—Marcelja model was particularly built to explain the exponential decay of the polarization, it was clearly invalidated by the latter simulations. Other conceptual difficulties of this model have been also reported.12 13... 

Many membrane components such as phospholipid include moieties such as the C + = and 0 -P + exhibit polarisation. The membrane dipole potential ( )d has its origins in the dipole moments of polar groups from the lipidic components of the bilayer, it seems likely that the water molecules at the molecular surface of membrane also make a contribution (1). The organisation of the membrane components that contribute to this potential have been verified from neutron diffraction studies and NMR spectroscopy (23) and quite recently using cryo-EM techniques has also added quantitative estimations of the potential (24). These dipolar groups seem to be oriented in a way such that the potential located towards the hydrophobic interior of the membrane is positive with respect to the pole located towards... 

This expression, however, only deals with a single molecular dipole and as many such dipoles would be required to describe the overall membrane dipole potential but as a mean-field expression, this term is practical and culmulatively offers the approximation of the estimated dipolar organisation shown in Fig. 4. A further complication, however, involves the solvent environment and this too is also often dealt with as a mean field or in a continuum manner. But the relative permittivity (or dielectric constant) (sr) cannot be considered to possess the same value throughout the multiphase system represented by a membrane in an aqueous medium. The permittivity profile has been measured to vary from about 78.5 in the bulk aqueous... 

Figure 9.6 The inherent dipolar potential of the membrane as a trapezoidal potential field extending across the plane of the BLM. An external voltage can skew the electric field to act as a driving force for ion translocation.

At frequencies below 63 Hz, the double-layer capacitance began to dominate the overall impedance of the membrane electrode. The electric potential profile of a bilayer membrane consists of a hydrocarbon core layer and an electrical double layer (49). The dipolar potential, which originates from the lipid bilayer head-group zone and the incorporated protein, partially controls transmembrane ion transport. The model equivalent circuit presented here accounts for the response as a function of frequency of both the hydrocarbon core layer and the double layer at the membrane-water interface. The value of Cdl from the best curve fit for the membrane-coated electrode is lower than that for the bare PtO interface. For the membrane-coated electrode, the model gives a polarization resistance, of 80 kfl compared with 5 kfl for the bare PtO electrode. Formation of the lipid membrane creates a dipolar potential at the interface that results in higher Rdl. The incorporated rhodopsin may also extend the double layer, which makes the layer more diffuse and, therefore, decreases C. ... 

The foregoing points would not really be important issues however, if the dipole potential simply was a phenomenon that exists without any clear role in cell biology. This is not the case however, for as commented above, we have pioneered the concept that local dipolar fields appear to have profound effects on membrane function not least in modulating how molecules interact both with and within membranes. Furthermore, and perhaps even more importantly because it appears to modulate membrane protein structure and thus function. 

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