FIGURE 6.

Power spectral density of noise in the current through a single fully open channel increases as pH of 1 M KCl solution is shifted to lower values. Background spectrum (curve 1) was measured for the membrane with a single OmpF channel at 0 mV. The low-frequency noise of the channel at pH 6.0 (curve 2) is orders-of-magnitude smaller than the noise of the same channel at pH 3.7 (curve 3). At f < 500 Hz the spectra of low-pH fluctuations can be approximated by single Lorentzians (shown for curve 3). Except for the background spectrum, the applied voltage was 100 mV.