Fig. 1: Binding of [AuCl4]− with methionine (D91M) within an engineered MspA nanopore (MspA-M).
a The structure of MspA (PDB ID: 1uun32) and its sensing mechanism with [AuCl4]− ions. Pore engineering (D93N/D91M/D90N/D118R/D134R/E139K) was performed according to published methods with the exception of D91M for [AuCl4]− sensing. The mutant MspA (MspA-M) possesses eight identical methionine residues at position 91, and capable of binding multiple [AuCl4]− ions simultaneously. b, c Representative traces with all-points histogram for [AuCl4]− sensing by MspA-M at +100 mV with 1 μM and 10 μM HAuCl4 in cis, respectively. With 1 μM HAuCl4 in cis (b), most binding events are from single [AuCl4]− ions. Though rarely observed, it is possible to observe events resulted from simultaneous binding of two [AuCl4]− in the same pore, which accounts for ~3% of all the events acquired. However, with 10 μM HAuCl4 in cis (c), sequential binding events from multiple [AuCl4]− dominate. In stands for n [AuCl4]− ions simultaneously in the pore. d Event histogram for 5 min recording for [AuCl4]− binding by MspA-M with 10 μM HAuCl4 in cis. Gaussian fitting is performed for binding events from different numbers of [AuCl4]−. ΔI0−1: 11.3 ± 0.2 pA, N = 280; ΔI0−2: 24.0 ± 0.3 pA, N = 94; ΔI0−3: 40.1 ± 0.2 pA, N = 13 (ΔI0−1 stands for the current difference between I0 and I1; ΔI0−2 stands for the current difference between I0 and I2; ΔI0−3 stands for the current difference between I0 and I3). e Plot of the reciprocals of the mean inter-event intervals (τon) and plot of the reciprocals of the mean residence time (τoff) for single [AuCl4]− binding events versus [AuCl4]− ions concentration in cis. An abrupt increase in the event detection rates is observed at 1 μM. Further increase of HAuCl4 concentration in cis leads to more sequential binding from multiple [AuCl4]− (Supplementary Fig. 9). Mean ± Standard Deviation of τon, τoff are from three independent experiments (N = 3) with 10 min recording for each condition.
