The dielectric properties of normal erythrocytes were compared to those of cells infected with the malarial parasitePlasmodium falciparum . Normal cells provided stable electrorotation spectra which, when analyzed by a single-shelled oblate spehroid dielectric model, gave a specific capacitance value of 12 ± 1.2 mF/m 2 for the plasma membrane, a cytoplasmic permittivity of 57 ± 5.4 and a cytoplasmic conductivity of 0.52 ± 0.05 S/m. By contrast, parasitized cells exhibited electrorotation spectra with a time-dependency that suggested significant net ion outflux via the plasma membrane and it was not possible to derive reliable cell parameter values in this case. To overcome this problem, cell membrane dielectric properties were instead determined from dielectrophoretic crossover frequency measurements made as a function of the cell suspending medium conductivity. The crossover frequency for normal cells depended linearly on the suspension conductivity above 20 mS/m and analysis according to the single-shelled oblate spheroid dielectric model yielded values of 11.8 mF/m 2 and 271 S/m 2 , respectively, for the specific capacitance and conductance of the plasma membrane. Unexpectedly, the crossover frequency characteristics of parasitized cells at high suspending medium conductivities were non-linear. This effect was analyzed in terms of possible dependencies of the cell membrane capacitance, conductance or shape on the suspension medium conductivity, and we concluded that variations in the membrane conductance were most likely responsible for the observed non-linearity. According to this model, parasitized cells had a specific membrane capacitance of 9 ± 2 mF/m 2 and a specific membrane conductance of 1130 S/m 2 that increased with increasing cell suspending medium conductivity. Such conductivity changes in parasitized cells are discussed in terms of previously observed parasite-associated membrane pores. Finally, we conclude that the large differences between the dielectrophoretic crossover characteristics of normal and parasitized cells should allow straightforward sorting of these cell types by dielectrophoretic methods.
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