Correction of electrode polarization contributions to the dielectric properties of normal and cancerous breast tissues at audio/radiofrequencies.

Spurious contributions from electrode polarization (EP) are a major nuisance in dielectric measurements of biological tissues and hamper accurate determination of tissue properties in the audio/radiofrequencies. Various electrode geometries and/or treatments have been employed traditionally to reduce EP contributions, although none succeeded to completely remove EP from measurements on tissues for all practical frequency ranges. A method of correction for contributions of EP to the dielectric properties of tissues is proposed. The method is based on modeling the electrode impedance with suitable functions and on the observation that certain parameters are only dependent on electrodes properties and can thus be determined separately. The method is tested on various samples with known properties, and its usefulness is demonstrated with samples of normal and cancerous human female breast tissue. It is observed that the dielectric properties of the tissues over the frequency range 40 Hz-100 MHz are significantly different among different types of breast tissue. This observation is used further to demonstrate that, by scanning the tip of the measuring dielectric probe (with modest spatial resolution) across a sample of excised breast tissue, significant variations in the electrical properties are detected at a position where a tumor is located. This study shows that dielectric spectroscopy has the potential to offer a viable alternative to the current methods for detection of breast cancer in vivo.

Protein influence on the plasma membrane dielectric properties: in vivo study utilizing dielectric spectroscopy and fluorescence microscopy.

We have investigated the origin of the dielectric response of the plasma membrane of living yeast cells (Saccharomyces cerevisiae) by using radiofrequency dielectric spectroscopy. The cells were genetically engineered to overexpress in the membrane of yeast cells a G protein-coupled receptor--the Sterile2-alpha factor receptor protein (Ste2p)--fused to the green fluorescent protein (GFP). Presence of the Ste2-GFP proteins in the plasma membrane was confirmed by exciting the cells at 476 nm and observing with a confocal microscope the emission characteristic of the GFP from individual cells. The dielectric behavior of cells suspended in KCl solution was analyzed over the frequency range 40 Hz-110 MHz and compared to the behavior of control cells that lacked the ability to express Ste2p. A two-shell electrical cell model was used to fit the data starting from known structural parameters and adjustable electrical phase parameters. The best-fit value for the relative permittivity of the plasma membrane showed no significant difference between cells expressing Ste2p (1.63+/-0.11) and the control cells (1.75+/-0.16). This result confirmed earlier predictions that the dielectric properties of the plasma membrane in the radiofrequency range mostly reflect the properties of the hydrophobic layer of the membrane, which is populated by the hydrocarbon tails of the phospholipids and hydrophobic segments of integral membrane proteins. We discuss ways by which dielectric spectroscopy can be improved to be used for tag-free detection of proteins on the membrane.

Nonlinear optical studies of heme protein dynamics: implications for proteins as hybrid states of matter.

Protein structure is fundamentally related to function. However, static structures alone are insufficient to understand how a protein works. Dynamics play an equally important role. Given that proteins are highly associated aperiodic systems, it may be expected that protein dynamics would follow glass-like dynamics. However, protein functions occur on time scales orders of magnitude faster than the time scales typically associated with glassy systems. It is becoming clear that the reaction forces driving functions do not sample entirely the large number of configurations available to a protein but are highly directed along an optimized pathway. Could there be any correlation between specific topological features in protein structures and dynamics that leads to strongly correlated atomic displacements in the dynamical response to a perturbation? This review will try to provide an answer by focusing upon recent nonlinear optical studies with the aim of directly observing functionally important protein motions over the entire dynamic range of the protein response function. The specific system chosen is photoinduced dynamics of ligand dissociation at the active site in heme proteins, with myoglobin serving as the simplest model system. The energetics and nuclear motions from the very earliest events involved in bond breaking on the femtosecond time scale all the way out to ligand escape and bimolecular rebinding on the microsecond and millisecond time scale have been mapped out. The picture that is emerging is that the system consists of strongly coupled motions from the very instant the bond breaks at the active site that cascade into low frequency collective modes specific to the protein structure. It is this coupling that imparts the ability of a protein to function on time scales more commensurate with liquids while simultaneously conserving structural integrity akin to solids.

Non-Debye dielectric relaxation in biological structures arises from their fractal nature.

What differentiates biological tissues from one another, thereby allowing their accomplishment of a physiological function, is their organization at supracellular and cellular levels. We developed general dielectric models for Cantorian (or treelike) fractal networks of transmission lines that mimic supracellular organization in numerous biological tissues and tissue surfaces, and which are compatible with both in vitro and in vivo measuring techniques. By varying a set of adjustable physical and geometrical parameters pertaining to the structure, we could numerically reproduce a variety of dielectric dispersion curves-most of them of a composite type-that suitably described experimental data from relatively organized biological tissues. We therefore conclude that the well-documented non-Debye dielectric behavior of biological structures reflects their self-similar architecture.

Multifrequency method for dielectric monitoring of cold-preserved organs.

To answer a growing need for non-invasive monitoring of biological organs, we have developed an automated system capable of repeated dielectric measurements over the frequency range 10 kHz-100 MHz. Further, we propose a novel method of data analysis that may convert the acquired, individual dispersion curves into a diagram of the time course of specific phenomenological parameters, such as the characteristic frequency. By using this new procedure, unattended, long-term monitoring of temporal changes in the dielectric behaviour of excised liver lobes stored at 4 degrees C was successfully realized. The 'multifrequency' method presented here was definitely superior to the conventional 'fixed-frequency' method in providing reliable results.

A quantitative approach to the dielectric properties of the skin.

The results of measurements using an open-ended coaxial probe of the audio/radiofrequency dielectric properties of human skin in vivo, either dry or moistened with physiological saline, are reported. Permittivity and conductivity dispersion curves were parametrized by using a newly reported dispersion function (Raicu V 1999 Dielectric properties of biological matter: model combining Debye-type and 'universal' responses Phys. Rev. E 60 4677), and the results obtained are discussed in the light of the recent advances in this field. It is suggested that the coaxial probe reports on the properties of the superficial layer, the stratum corneum, when the skin surface is dry, whilst the signal from deeper skin layers becomes dominant after wetting the skin with conductive physiological saline.

Dielectric dispersion of biological matter: model combining Debye-type and "universal" responses.

The remarkably broad dielectric dispersions exhibited by solid dielectrics are well-known examples of the failure of Debye's relaxation theory; such dispersions are much better represented by a "fractional power law" described by Jonscher [A. K. Jonscher, Nature 267, 673 (1977)] as the "universal dielectric response." As it happens, however, recent experimental advances in this field suggest that neither of the two approaches is general enough to cope with the dielectric response of biological tissues, which combines striking features from both types of behavior. A phenomenological function is therefore proposed, which not only reproduces observations on biological tissues but also includes all of Jonscher's "universal response," the Debye, Cole-Cole, and Davidson-Cole functions, as its special cases.

Effects of cetyltrimethylammonium bromide (CTAB) surfactant upon the dielectric properties of yeast cells.

The dielectric properties of yeast cells in the absence and presence of cetyltrimethylammonium bromide (CTAB) were investigated. The surfactant concentration range was between 0.0 and 1.0 mM. The experimental permittivity and conductivity spectra of frequency were analyzed by means of the two-shell electrical cell model (Irimajiri et al., Bull. Inst. Chem. Res., Kyoto Univ. 69 (1991) 421-438), and the electrical phase parameters of cells were subsequently evaluated. The cytoplasm conductivity and the conductivity of the vacuole interior decreased drastically by treating the cells with surfactant. The apparent capacitance of the plasma membrane increased systematically from 0.65 microF/cm2, for untreated cells, up to about 0.75 microF/cm2, at 0.3 mM CTAB. This growth was ascribed to the increase in the folding of the membrane surface associated with the surfactant-induced cell shrinkage. A further addition of the surfactant entailed a gradual decrease of the capacitance that was assigned to the membrane solubilization by the surfactant molecules. Within the accuracy of the data, the specific capacitance of the vacuole membrane was nearly constant (0.544+/-0.021 microF/cm2) over the whole surfactant concentration range. Also, the cytoplasm permittivity remained constant at 64.3+/-4.5.

Dielectric properties of yeast cells as simulated by the two-shell model.

The paper reports a re-evaluation of the previous studies on yeast by considering the influence of vacuole upon the dielectric properties of the cell. In this respect, relative permittivity and conductivity of yeast cells dispersed in KCI solutions of various concentrations were measured in the frequency range from 0.1 to 100 MHz. The analysis of data revealed that the beta-dielectric dispersion of yeast cell suspensions is a composite of three (or probably four) distinct sub-dispersions. Since the dielectric response of the cell wall was experimentally avoided (according to Asami et al. (1976) J. Membr. Biol. 28, 169-180), the two-shell model, related to the plasma membrane and the vacuolar membrane, respectively, appeared to be the best approximation for yeast cells. The most relevant parameters obtained with the aid of the two-shell model were as follows. Specific capacitance of the plasma membrane and the vacuolar membrane were 0.703 +/- 0.011 microF/cm2 and 0.483 +/- 0.029 microF/cm2, respectively; electrical conductivity of the cytoplasm and the vacuole interior were 0.515 +/- 0.028 S/m and 3.22 +/- 0.48 S/m; finally, the permittivity of the cytoplasm was 50.6 +/- 2.