On the dielectrically observable consequences of the diffusional motions of lipids and proteins in membranes

Abstract

• 1. The dielectric properties of suspensions of intact cells of Methylophilus methylotrophus, Paracoccus denitrificans and Bacillus subtilis have been measured in the frequency range 1 kHz to 13 MHz. All possess a pronounced dispersion corresponding in magnitude and relaxation time to the “β-dispersion” in a terminology defined by Schwan [Adv. Biol. Med. Phys. 5: 147–209 (1957)]. The latter two strains, but not M. methylotrophus, also possess a substantial α-dispersion. The relaxation time of the β-dispersion of B. subtilis is significantly lower than that of the other two strains, due to the higher internal K⁺ content of this Gram-positive organism.

• 2. Treatment of P. denitrificans or B. subtilis with lysozyme greatly reduces the magnitude of the α-dispersion; in the latter case it is virtually abolished.

• 3. The magnitude of both the α- and β-dispersions of protoplasts of these organisms is significantly decreased by treatment with the cross-linking reagent glutaraldehyde, indicating that diffusional motions of the lipids and/or proteins in the protoplast membranes contribute to the dielectric relaxations observed in this frequency range. Such motions cannot be unrestricted, as in the “fluid mosaic” model, since the relaxation times of the lipids and proteins, if restricted by hydrodynamic forces alone, should then correspond, in protoplasts of this radius (0.4–0.5 μm), to approximately 10 Hz.

• 4. Even after treatment of the (spherical) protoplasts with glutaraldehyde, the breadth of the remaining β-dispersion is still significantly greater than (a) that of a pure Debye dispersion and (b) that to be expected solely from a classical Maxwell-Wagner-type mechanism.

• 5. It is recognised that the surfaces of the protein complexes in such membranes extend significantly beyond the membrane surface as delineated by the phospholipid head-groups; such molecular granularity can in principle account for the broadened dielectric relaxations in the frequency range above 1 kHz, in terms of the impediment to genuinely tangential counterion relaxation caused by the protruding proteins themselves.

• 6. The relaxation time of a previously observed, novel, low-frequency, glutaraldehyde-sensitive (μ-) dispersion in bacterial chromatophore suspensions, as well as that of their α-dispersion, is significantly increased by increasing the aqueous viscosity with glycerol. This finding is consistent with the view that, from a dielectric standpoint, the motions of charged proteins (and lipids) in biological membranes are rather tightly coupled to those of the adjacent ions and dipoles in the electric double layer.

• 7. Mebbrane vesicles of P. denitrificans do not possess a μ-dispersion as marked as that of chromatophores. As with chromatophores, their α-dispersion is somewhat decreased by glutaraldehyde treatment. The relative lack of a μ-dispersion in these vesicles may be related to their different polarity relative to that of bacterial chromatophores; alternatively, and perhaps additionally, the longrange lateral mobility of lipids and proteins in this system may be even more restricted than in chromatophores.

• 8. Overall, our results draw attention to the fact that the motions of proteins, lipids and double-layer species can contribute to the audio- and radiofrequency dielectric properties of microbial cell, protoplast and vesicle suspensions, and indicate that both the magnitude and the rate of such relaxations depend rather finely on the intimate molecular structure and organisation of the bacterial cytoplasmic membrane and its superincumbent double layers.