Spectral density of voltage

Initial measurements on valinomycin-doped membranes (13, 14) showed that lipid bilayers, which provide cells with an effective permeability barrier, are equilibrium objects by this criterion. Within an accuracy of several percent, the experimentally obtained values of the spectral density of voltage noise showed agreement with those calculated from relation 1. Figure 1 illustrates this agreement for three valinomycin concentrations. For the valinomycin-K+ system chosen for these experiments and for the frequency range used in measurements, the dispersion in membrane impedance was caused only by geometrical capacitance of the bilayer the characteristic times of the transport process itself were too small to influence impedance in this range. 

Figure 1. The agreement of the spectral density of voltage fluctuations from valinomycin-modified phospholipid bilayers at equilibrium conditions (13, 14) with the Nyquist relation 1. An aqueous 0.01-M KCl solution at 33 °C was used in the experiments. Bilayer direct current resistances and valinomycin solution concentrations were 0.52-Mfl and 1.5 X 10 8 M (l), 0.19 Mfl and 5 X 10 8 M (2), and 0.055 Mi2 and 1.5 X 10 7 M (3). Solid lines are drawn in accordance with relation 1 for the impedance of a parallel resistance-capacitance (RC) circuit using foregoing resistance values and a value of membrane geometrical capacitance.

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