No abstract

Low-frequency dielectric properties of biological tissues, characterized by αand β-dispersions, are reviewed with emphasis on physical mechanisms. Ion activities, tissue microstructure and composition are discussed. A new mechanism associated with membrane permeability is included. The counterion layer (electrical double layer) phenomenon is discussed. Electrode polarization, which always causes problems with low-frequency dielectric measurements, is also discussed.

A hypothesis for the regulation of some sugar transport systems by the bacterial phosphoenolpyruvate:sugar transport system postulates an interaction between IIIGlc of this system and the carrier whose activity is regulated. We have studied this interaction in more detail, employing one of these transport systems, the lactose carrier of Escherichia coli. Purified IIIGlc of the phosphotransferase system interacted directly with the lactose carrier. The binding of IIIGlc to lactose carrier required the presence of the non‐phosphorylated form of IIIGlc and substrates of the carrier and exhibited a stoichiometry of 1.2± 0.2 mol IIIGlc/mol lactose carrier. The Kd of lactose carrier for IIIGlc was 10 ± 5 µM. IIIGlc is apparently unable to interact with a mutant lactose carrier which still binds but does not transport galactosides. The binding of IIIGlc to the lactose carrier results in a 3.5‐fold increase in the apparent affinity of galactosides for the carrier. Significantly, the binding of IIIGlc to the lactose carrier results in an inhibition of galactoside translocation both in membrane vesicles and liposomes reconstituted with the purified lactose carrier. This inhibition may thus be the basis for the well‐documented phenomenon of inducer exclusion.

Principles of dielectric property measurement by microwave free-space transmission measurements are presented, and the important sources of errors in such measurements are discussed. A system, including a vector network analyser, horn/lens antennas, holder for grain and oilseed samples and a radiation absorbing enclosure that was used for such measurements is described, and the techniques and procedures followed to obtain reliable permittivity data for wheat, shelled corn (maize) and soybeans are outlined. Data illustrating linear relationships between microwave attenuation and phase shift per unit sample thickness, each divided by the bulk density of the granular materials, and frequency and moisture content are presented graphically. The linear dependence of calculated permittivity components, dielectric constant and loss factor, on bulk density is also shown, and permittivity components for wheat, corn and soybeans are listed for reference at frequencies from 5 to 17 GHz at different densities and moisture levels at about 23 °C. Permittivity values are also listed for the same three commodities, adjusted to a medium density value through use of the Landau and Lifshitz, Looyenga dielectric mixture equation, for the total range of moisture contents at 10 GHz and at the same temperature.