BIOLOGICAL MATTER

Introduction

By dielectric spectroscopy, we investigate the dynamic processes in biological materials like proteins that often have considerable impact on biological functions in organisms. In addition, we have characterized the dielectric properties of human blood which are highly relevant to understand its interactions with electromagnetic waves. Another (quite funny) example is the measurement of a banana, demonstrating the occurrence of non-intrinsic dielectric effects.

1. Proteins

Proteins are crucial to almost every biological process. With dielectric spectroscopy, proteins can be examined not only in the solid state (i.e. as powder), but also under nearly in vivo conditions (dissolved in water). This is important, because water is essential for the biological functions of proteins and, hence, the investigation of the interaction of water with proteins is a very active field of research. As we are capable of determining the dielectric properties in a very broad frequency range, we are able to investigate many different aspects of protein dynamic, e.g., relaxation processes (see the schematic figure) and the "glass transition" of proteins. We also shed light on the water-protein interaction to evaluate the role of water for the protein dynamics and functionality.

Protein dynamics Schematic view of the typical loss spectrum of a protein solution. The β and γ relaxations arise from the tumbling motions of the protein and the water molecules, respectively. The origin of the δ relaxation, here indicated to arise from bound water molecules within the protein hydration shell, is still controversially discussed.

[from: M. Wolf, R. Gulich, P. Lunkenheimer, and A. Loidl, Relaxation dynamics of a protein solution investigated by dielectric spectroscopy, Biochim. Biophys. Acta. 1824, 723 (2012).]

Relevant publications from our group in this field are:

2. Blood

Blood is a highly functional body fluid. Its dielectric parameters are of relevance for various medical applications like cell separation, checking the deterioration of preserved blood, and dielectric coagulometry. In addition, the precise knowledge of the dielectric properties of blood is prerequisite for fixing limiting values for electromagnetic pollution, e.g., by mobile phones. For example, the ac conductivity directly affects the specific absorption rate (SAR), for which upper limits are set by law.

In our group, we have investigated the dielectric constant, loss and conductivity of blood as function of frequency, temperature and hematocrit value (Hct: volume fraction of red blood cells). Our measurements cover an exceptionally broad frequency range from 1 Hz to 40 GHz. Our measurements provide dielectric data on human blood of so far unsurpassed precision for a broad parameter range. All data are available in electronic form (see links below) to serve as basis for the calculation of the absorption rate of electromagnetic radiation and other medical purposes.

dielectric measurement of blood Dielectric measurement of blood with the open-end coaxial reflection method at 50 MHz - 40 GHz.

The obtained broadband dielectric spectra of blood show three major dispersions, termed β, γ and δ (see figure below). The β process, located around 10 MHz, is due to a Maxwell-Wagner relaxation caused by the inhomogeneities introduced by the presence of the red blood cells. Around 18 GHz, the γ relaxation arises from the tumbling of the dipolar water molecules in the blood sample. Between the β and γ relaxations, significant "δ" dispersion is observed, which, however, can be explained by a superposition of β and γ and is not due to an additional microscopic process often found in biological matter. The huge dispersion effect observed at the lowest frequencies (< 10kHz) is due to non-intrinsic electrode polarization. We find no evidence for a low-frequency α relaxation as reported for some types of biological matter.

dielectric spectra of blood Dielectric constant and real part of the conductivity of blood samples with different hematocrit values as function of frequency, measured at body temperature (310 K). The lines are fits assuming different models to account for the electrode-polarization effect and the β and γ relaxations.

[from: M. Wolf, R. Gulich, P. Lunkenheimer, and A. Loidl, Broadband dielectric spectroscopy on human blood, Biochim. Biophys. Acta. 1810, 727 (2011).]

Download of data:
(If you use them, please cite the paper listed below.)

For further details, see:

3. Banana

We used dielectric measurements of a banana to illustrate the occurrence of colossal dielectric constants and of P(E) hysteresis loops (like in a ferroelectric) due to purely non-intrinsic effects. It is shown, how the non-intrinsic nature of such effects can be easily identified by performing a variety of simple experiments.

banana Broadband dielectric spectrum of the dielectric constant of banana skin at room temperature. Colossal values of the dielectric constant (up to 109!) and several relaxation modes show up. Essentially, they arise from the ionic conductivity of the banana and are due to nonintrinsic effects like Maxwell-Wagner relaxations.

[from: A. Loidl, S. Krohns, J. Hemberger, and P. Lunkenheimer, Bananas go paraelectric, J. Phys.: Condens. Matter 20, 191001 (2008).]

For further details, see:


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