Flexibility of the CueR Metal Site Probed by Instantaneous Change of Element and Oxidation State from Ag(I) to Cd(II)

Selectivity for monovalent metal ions is an important facet of the function of the metalloregulatory protein CueR. (111)Ag perturbed angular correlation of γ‐rays (PAC) spectroscopy probes the metal site structure and the relaxation accompanying the instantaneous change from Ag(I) to Cd(II) upon (111)Ag radioactive decay. That is, a change from Ag(I), which activates transcription, to Cd(II), which does not. In the frozen state (−196 °C) two nuclear quadrupole interactions (NQIs) are observed; one (NQI(1)) agrees well with two coordinating thiolates and an additional longer contact to the S77 backbone carbonyl, and the other (NQI(2)) reflects that Cd(II) has attracted additional ligand(s). At 1 °C only NQI(2) is observed, demonstrating that relaxation to this structure occurs within ≈10 ns of the decay of (111)Ag. Thus, transformation from Ag(I) to Cd(II) rapidly disrupts the functional linear bis(thiolato)Ag(I) metal site structure. This inherent metal site flexibility may be central to CueR function, leading to remodelling into a non‐functional structure upon binding of non‐cognate metal ions. In a broader perspective, (111)Ag PAC spectroscopy may be applied to probe the flexibility of protein metal sites.