Metal ion probes are used to assess the accessibility of cysteine side chains in polypeptides lining the conductive pathways of ion channels and thereby determine the conformations of channel states. Despite the widespread use of this approach, the chemistry of metal ion-thiol interactions has not been fully elucidated. Here, we investigate the modification of cysteine residues within a protein pore by the commonly used Ag(+) and Cd(2+) probes at the single-molecule level, and provide rates and stoichiometries that will be useful for the design and interpretation of accessibility experiments.
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