Figure 3.

Experimental measurement of ionic current. (a) The electrolytic current through the 1.2-nm × 1.4-nm pore shown in Fig. 2c as a function of time with the membrane voltage at ϕ = 0.5 V. The open pore current, I₀, at this voltage is ∼0.9 nA (dotted line). Also shown (shaded) is the current through the pore with no DNA at ϕ = 0.25 and 0.5 V. (b) The electrolytic current through the same pore as a function of time with the membrane voltage at ϕ = 1.5 V (below the threshold) illustrating current enhancement above the open pore current and blockades. The open pore current, I₀, at this voltage is ∼2.6 nA (dotted line). (c) The same trace as panel b expanded ∼5.76 s to reveal current enhancement and current blockades occurring on a millisecond timescale. (d) The electrolytic current measured at ϕ = 1.5 V through the same pore over a longer timescale illustrating the long duration of the current transients and the persistence of the levels. (e) A histogram showing the frequency of values of the current observed over the 12-s interval shown in panel c. The shaded background is amplified ×25 to illustrate the frequency of current enhancements at twice the open pore current. For reference, arrows are shown at 4.33, 1.93, and 0.87 nA top to bottom.