Fig. 3: Stochastic sensing of L-cysteine by Au(III) embedded MspA-M.
a A general schematic diagram of biothiols sensing. HAuCl4 were added in cis while the biothiols (R-SH) were added in trans. R represents the chemical structure of a specific type of biothiol other than the thiol group. b A molecular model for biothiols sensing. State 0 or 1 represents a methioine residue, without or with a [AuCl4]− embedment. State 1SH represents a biothiol bound with the pore restriction by taking the embedded Au(III) as an atomic bridge. c The chemical structure of L-cysteine (Cys). L-cysteine is a specific type of biothiol. d Binding of Cys with [AuCl4]− embedded MspA-M. The trace was recorded with a +100 mV applied voltage when 4 μM of HAuCl4 in cis and 40 μM of Cys in trans were present simultaneously. Resistive pulse signals in the trace result from binding of a [AuCl4]− or binding of a Cys by taking the bound Au(III) as an atomic bridge. Red triangles mark the events from Cys. The grey star marks the event of [AuCl4]−. Other unlabeled events are background signals as described in Supplementary Fig. 15. e A representative Cys blockage event. A representative Cys blockage event is composed of three states as described in b. State 1SH appears as a characteristic jitter signal. f The scatter plot of the relative blockade amplitude ΔI/I0 vs the dwell time from events of [AuCl4]− and Cys binding. The event amplitude (ΔI) for [AuCl4]− or Cys is defined as the current difference between state 1 or state 1SH in reference to state 0 (Supplementary Fig. 16). I0 is the open pore current, which was derived from the mean value of state 0. g The histogram of ΔI/I0 for single [AuCl4]− bindings and Cys bindings. h The histogram of the dwell time of state 1SH (toff,Cys) from a series of Cys binding events. The mean dwell time τoff,Cys was derived from the single exponential fitting result. The statistical data in f, g, and h were from a continuous 10 min recording (Methods). (Supplementary Tables 5 and 6).
