Showing papers by "Douglas B. Kell published in 1985"

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

Abstract: 1. A system consisting of an 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. 2. 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. 3. 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. 4. 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. 5. Similar analyses are given for rotational and translational motions of phospholipids. 6. 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. 7. 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.

Dielectric Spectroscopy and Membrane Organisation

Abstract: The possible bases for field-mediated effects on cellular processes are reflected in the passive electrical properties of biological systems. The historical, present and prospective utility of dielectric spectroscopy in assessing the static and dynamic organisation of biological membranes is reviewed within this context. The basis for the view that the static capacitance of bioraembranes is as great as 1 fiF/cm2 is doubted; contributions from the (partially restricted) motions of membrane components, and of double-layer ions, probably contribute to this apparent value in bioraembrane vesicle suspensions. The importance of improving our knowledge of the static electrical capacitance of energy coupling membranes is stressed. Theoretical and experimental procedures for assessing the contribution of rotational and translational motions of membrane components, and of double-layer/membrane interactions, to dielectric spectra in the approximate frequency range 10 to 107 Hz are described. Finally, three outstandi...

Tinopal AN as a selective agent for the differentiation of phytopathogenic and saprophytic Pseudomonas species

Abstract: The selective toxicity of the respiratory inhibitor Tinopal AN (1,1-bis (3,N-5–dimethylbenzoxazol-2-yl) methine p-toluene sulphonate) towards phytopathogenic bacteria was investigated further and in general was confirmed using more than 160 additional strains of Pseudomonas spp. The mechanism(s) of the resistance shown by saprophytic fluorescent pseudomonads were studied to elucidate the differences between resistant saprophytic and sensitive phytopathogenic Pseudomonas species. Damage to, or partial removal of the cell wall of Tinopal AN-resistant Pseudomonas aeruginosa, resulted in a marked Tinopal AN-sensitivity, as judged by the ability of Tinopal AN to inhibit oxygen uptake. Furthermore, removal of part of the lipo-polysaccharide (LPS) component of the outer membrane also resulted in sensitivity. Mutants of Ps. aeruginosa with modified outer cell walls were tested for their reactions towards Tinopal AN, and two cell wall lipopolysaccharide (LPS) mutants of Escherichia coli (env Al) and Salmonella typhimurium (rfa) were, in contrast to the wild-type strains, found to be sensitive towards Tinopal AN. The results therefore suggest that the resistance of saprophytic pseudomonads towards Tinopal AN is due (at least in part) to the selectively permeable properties of the outer membrane of the cell wall. The usefulness of Tinopal AN for screening potentially phytopathogenic strains of Pseudomonas was confirmed.