Single-channel recordings to determine the conductance state in which reaction occurred. Each panel shows current versus time, with the average current levels for the closed, open, and reacted states indicated by the dashed lines labeled C, O, and R, respectively. (A) About 50 min before the start of the record, 65 pg G309C was added to the cis compartment; 100 µg MTS-ET was added to the trans compartment ∼15 min later. The record begins with a 43-pS channel open at 100 mV. Several brief flickers toward the closed level can be seen. A drop to the reacted level of 31 pS, marked by a downward-pointing arrow, appears to have occurred from the open state. The reaction took place ∼9 s after the channel opened. (B) About 75 min before the start of the record, 180 µg MTS-EA was added to the trans compartment. Three additions of S305C totaling 18 ng were made to the cis compartment 37–73 min before the start of the record. The record begins with a 38-pS channel open at 60 mV. A brief flicker to the closed level can be seen. A drop to the reacted level of 11 pS, marked by an upward-pointing arrow, appears to have occurred via the closed state. The reaction took place ∼6 s after the channel opened. For both panels, the solutions were as in Fig. 2. The filtering frequency was 1,000 Hz, and the sample interval was 0.15 ms. The current traces are shown as small closed circles connected by lines, with each circle representing one sampled data point. The zero-current level is slightly below the average closed level.