Noise-current generated by carrier-mediated ion transport at non-equilibrium

The measured spectral intensity Sj(f) of noise-current generated by carrier-mediated ion transport on bilayer membranes agrees under equilibrium and nonequilibrium conditions with the theoretically predicted behavior. It is shown that the shot noise intensity due to this ion transport mechanism yields a frequency independent level of Sj(f) at higher frequencies. The intensity of the shot noise contribution decreases with increasing voltage. Both experimentally and theoretically it could be shown that at nonequilibrium the Nyquist-theorem can no longer be applied for a description of the spectral intensity. Especially, the low frequency tail of Sj(f) is not proportional to the mean macroscopic steady state conductance. The transition of Sj(f) between the low and high frequency limit occurs in a frequency range which is related to the relaxation time constants of the transport system. In contrary to voltage jump current relaxation experiments where two relaxation times are predicted the theory predicts for the noise analysis a third relaxation time constant of a non-zero contribution to Sj(f). Besides the measurement of Sj(f) the corresponding autocorrelation function was determined. Comparison of both methods of noise analysis shows that for the carrier-mediated ion transport the determination of the autocorrelation function is the less appropriate approach due to principal methodical difficulties.