On the dielectrically observable consequences of the diffusional motions of lipids and proteins in membranes

Abstract

A system consisting of any array of cylindrical, polytopic membrane proteins (or protein complexes) possessed of a permanent dipole moment and immersed in a closed, spherical phospholipid bilayer sheet is considered. It is assumed that rotation of the protein (complex) in a plane normal to the membrane, if occurring, is restricted by viscous drag alone. Lateral diffusion is assumed either to be free and random or to be partially constrained by barriers of an unspecified nature. The dielectric relaxation times calculated for membrane protein rotation in a suspension of vesicles of the above type are much longer than those observed with globular proteins in aqueous solution, and fall in the mid-to-high audio frequency range. If the long range lateral diffusion of (charged) membrane protein complexes is essentially unrestricted, as in the "fluid mosaic" membrane model, dielectric relaxation times for lateral motions will lie, except in the case of the very smallest vesicles, in the sub-audio (ELF) range. If, in contrast, the lateral diffusion of membrane protein complexes is partially restricted by "barriers" or "long-range" interactions (of unspecified nature), significant dielectric dispersions may be expected in both audio- and radio-frequency ranges, the critical (characteristic) frequencies depending upon the average distance moved before a barrier is encountered. Similar analyses are given for rotational and translational motions of phospholipids. At very low frequencies, a dispersion due to vesicle orientation might in principle also be observed; the dielectrically observable extent of this rotation will depend, inter alia, upon the charge mobility and disposition of the membrane protein complexes, as well as, of course, on the viscosity of the aqueous phase. The role of electroosmotic interactions between double layer ions (and water dipoles) and proteins raised above the membrane surface is considered. In some cases, it seems likely that such interactions serve to raise the dielectric increment, relative to that which might otherwise have been expected, of dispersions due to protein motions in membranes. Depending upon the tortuosity of the ion-relaxation pathways, such a relaxation mechanism might lead to almost any characteristic frequency, and, even in the absence of protein/lipid motions, would cause dielectric spectra to be much broader than one might expect from a simple, macroscopic treatment.

Cole–Cole, linear and multivariate modeling of capacitance data for on-line monitoring of biomass

“…It seems likely that the value of a is often due to a variety of effects, some of which are more or less important in different systems. The following are some of the suggested origins of this parameter's value: distribution of cell shapes and sizes [28], morphology of extra-cellular spaces [29], mobility of membranous proteins [30,31] and the fractal nature of dielectric relaxation [32]. Typical values of a for biological cells are in the order of 0.1 to 0.2 [13].…”

Dielectric spectroscopy of energy coupling membranes: Chloroplast thylakoids

“…(1) there is a strong coupling between double-layer and membrane component motions [9][10][11]66,73,74], and (2) that neither of these types of motion are restricted by hydrodynamic forces alone.…”

“…The observed relaxation time(s) of the latter type of mechanism [7,[9][10][11]311 are not obviously consistent with the view that the long-range mobility of membrane components is restricted by hydrodynamic forces alone, a point that is of special significance in view of present uncertainty concerning the randomness of the organisation of the protein complexes in the bacterial cytoplasmic membrane [32,33].…”

Forces, Fluxes and the Control of Microbial Growth and Metabolism: The Twelfth Fleming Lecture

“…Kell, 1980;Clarke et af., 1982Clarke et af., , 1985Carleysmith & Fox, 1984). To do so we must measure the properties of our fermenter broths, and one of the more obviously important variables is the microbial biomass concentration, something which has been notoriously difficult to measure (Harris & Kell, 19856;Meyer et al, 1985; Clarke et af., 1986). Now we saw from equation (1) that if we knew the volume fraction of the cells we could estimate the membrane capacitance per unit area by measuring the dielectric permittivity of the suspension.…”