Figure 7.

Reversible binding of Cd²⁺ to P_CC. (A) Stacked current recording traces of P_CC, (AG)₆(L135C/G137C-AG)₁, at various concentrations of Cd(NO₃)₂ (cis), as labeled on the left of the traces. The conditions were the same as in Fig. 5. Only two current levels were observed, namely, the open pore (P_CC) and the pore with Cd²⁺ bound, as labeled on the right of the traces. All-points amplitude histograms are shown to the far left. The histograms are fitted to the sum of two Gaussian functions. (B) Reciprocals of the mean dwell times for the unoccupied pore ($1/{\overline{\tau }}_{\text{PCC}}$, black open rhombus) and the pore with bound Cd²⁺ ($1/{\overline{\tau }}_{{\text{Cd}}^{2+}\text{-}\text{bound}}$, gray solid square) versus the concentration of Cd(NO₃)₂. Each data point is the mean ± SD from three repeats. The association and dissociation of Cd²⁺ follow simple bimolecular and unimolecular kinetics, respectively. The binding of a second Cd²⁺ is not observed.