A Single-Molecule Observation of Dichloroaurate(I) Binding to an Engineered Mycobacterium smegmatis porin A (MspA) Nanopore.

Gold(I) compounds are known to bind sulfur-containing proteins, forming the basis in the design of gold(I)-based drugs. However, the intrinsic molecular mechanism of the chemical reaction is easily hidden when monitored in ensemble. We have previously demonstrated that Mycobacterium smegmatis porin A (MspA) can be engineered (MspA-M) to contain a specialized nanoreactor to probe chemical reactions involving tetrachloroaurate(III). Here, we provide further investigations of coordination interactions between dichloroaurate(I) and MspA-M. Gold compounds of different coordination geometry and valence states are as well probed and evaluated, demonstrating the generality of MspA-M. With single-molecule evidence, MspA-M demonstrates a preference for dichloroaurate(I) than tetrachloroaurate(III), an observation in a single molecule that has never been reported. By counting the maximum number of simultaneous ion bindings, the narrowly confined pore restriction also efficiently distinguishes dichloroaurate(I) and tetrachloroaurate(III) according to their differences in geometry or size. The above demonstration complemented a previous study by demonstrating other possible gold-based single-molecule chemical reactions observable by MspA. These observations bring insights in the understanding of gold-based coordination chemistry in a nanoscale.