Dielectric spectroscopy of Tobacco Mosaic Virus

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Abstract

The dielectric properties of the Tobacco Mosaic Virus (TMV) have been measured using time domain dielectric spectroscopy (TDDS) in the temperature range from 1 to 40 °C. A single dielectric dispersion is observed in the MHz range. The activation energy of the process is found to be in the range 1–2 kcal/mol. The experimental data could not be completely accounted for by current theoretical models, but evidence indicates that the dielectric loss arises from polarisation of charge on and around the virus.

Introduction

Dielectric spectroscopy has been used as a tool to investigate the biological properties of cells and organelles for over 75 years. In 1925, Fricke [1], [2] derived the capacitance and thickness of the erythrocyte membrane from impedance measurements. Following on from the pioneering work of Debye [3] on polar molecules, Oncley [4], Onsager [5] and Kirkwood [6] investigated the dielectric properties of proteins. In the 1960s, Schwan amongst others [7], [8], laid the foundations for the understanding of the dispersion of a biological cell suspended in an electrolyte according to the theories of Maxwell [9] and Wagner [10], [11]. Since then, the dielectric properties of cells and bacteria have been extensively studied by a number of workers [12], [13], [14], [15], [16], [17], [18] and the observed dielectric data generally interpreted as arising from interfacial polarisation effects at high frequencies and from relaxation of the double layer at much lower frequencies.

The dielectric properties of protein molecules has also been the subject of investigation by a number of groups [3], [14], [19], [20] and the data interpreted in terms of the relaxation of a fixed dipole, in combination with surface conductance effects. However, investigations of the dielectric properties of viral particles has received scant attention and to the best of our knowledge there is only one publication reporting the dielectric properties of the un-enveloped Alfalfa Mosaic Virus [21]. The authors treated the virus as a rod-like molecule and compared their experimental data with theoretical predictions concerning the polarisation of a counterion atmosphere surrounding a rod-shaped polyelectrolyte. In this paper, we report measurements of the dielectric properties of Tobacco Mosaic Virus (TMV). Dielectric data was collected using time domain dielectric spectroscopy (TDDS) covering a wide frequency range from 100 kHz to 1 GHz. In an attempt to ascertain the nature of the process responsible for the observed dielectric dispersion, the activation energy of the relaxation was also measured.

Section snippets

Virus

TMV is a rod-shaped virus approximately 280 nm in length and 18 nm in diameter. The coat protein surrounds an RNA core with 6240 nucleotides and an average of 2.13 nucleotides per angstrom length. The molecular weight of the sodium salt of the TMV particle is estimated to be 4×107, and its density is 1.325 g cm−3. (For further details of the virus structure, see, for example, Ref. [22]).

The TMV used in our work was strain U1, raised in Nicotiana tabacum cv. Petite Havana SR1, and purified by

Results

The dielectric loss data for a suspension of TMV at different concentrations (measured at 25 °C) are presented in Fig. 1 for the frequency range 100 kHz–100 MHz. This figure does not include dielectric data for frequencies greater than 100 MHz because the signal level is low and noisy. The magnitude of the dielectric dispersions are plotted after subtracting the dc conductivity in the time domain. Analysis of the spectra has shown that the best fit can be obtained by using the Cole–Cole

Discussion

The mechanisms responsible for the dielectric loss of TMV are unknown. The TMV particle contains a double-stranded RNA molecule, which is enclosed by a cylinder of protein molecules. This system is symmetric, the permanent dipole moments of the proteins are expected to cancel and the RNA, being double stranded, is unlikely to possess a permanent dipole moment. As indicated above, the measured activation energy of the loss process is very small, so that we can exclude permanent dipole

Conclusions

The observed dispersion can be partly described using the simple counterion polarisation models of Mandel and Oosawa. However, the number of charges predicted by the counterion polarisation model is much lower than the charge density of TMV estimated from the known protein structure.

The activation energy data points to some mechanism involving charge polarisation. The measured value of ~1.5 kcal/mol is similar to that measured for systems where polarisation of an ion cloud is considered to be

Acknowledgements

We would like to thank Ms. Janet Laird for assistance in the sample preparation. This work was supported by Leverhulme Trust Foundation.

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