Nanopore Single-Molecule Analysis of Metal Ion-Chelator Chemical Reaction

Metal ions play critical roles in wide range of biochemical and physiological processes, but they can cause toxicity if excessive ingestion or misregulation. Chelating agents offer an efficient mean for metal ions intoxication and therapeutics of diseases. Studies on metal ion-chelator interactions are important for understanding the reaction mechanism and developing new specific metal chelator drugs. However, it remains a significant challenge to detect the metal ion-chelator interactions at the molecular level. Here, we report a label-free nanopore sensing approach that enables single-molecule investigation of the complexation process. We demonstrate that the chemical reaction between Cu²⁺ and carboxymethyl-β-cyclodextrin (CMβCD) in a nanoreactor is completely different from in the bulk solution. The formation constant (K_f = 4.70 × 10⁴ M^-1) increases 14 417-fold in the nanopore than that in the bulk solution (K_f = 3.26 M^-1). The bioavailable CMβCD as a natural derivative with higher affinity for Cu²⁺ could be used in the safe medicinal removal of toxic metal. On the basis of the different ionic current signatures across an α-hemolysin (α-HL) mutant (M113N)₇ nanopore lodged with a CMβCD adaptor in the presence and absence of Cu²⁺, the reversible molecular binding events to CMβCD can be in situ recorded and the single-molecule thermodynamic and kinetic information can be obtained. Interestly, we found that the Cu²⁺ binding leads to the increase of the channel current, rather than the blocking as usual nanopore experiment. The uncommon (on/off) characteristic could be very useful for fabricating the nanodevice. Furthermore, the unique nanopore sensor can provide a highly sensitive approach for detecting metal ions.