Selectivity for monovalent metal ions is an important facet of the function of the metalloregulatory protein CueR. 111Ag perturbed angular correlation of gamma-rays (PAC) spectroscopy probes the metal site structure and the relaxation accompanying the instantaneous change from AgI to CdII upon 111Ag radioactive decay. That is, a change from AgI, which activates transcription, to CdII, which does not. In the frozen state (-196 ºC) two nuclear quadrupole interactions (NQIs) are observed; one (NQI1) agrees well with two coordinating thiolates and an additional longer contact to the S77 backbone carbonyl, and the other (NQI2) reflects that CdII has attracted additional ligand(s). At 1 ºC only NQI2 is observed, demonstrating that relaxation to this structure occurs within approximate to 10 ns of the decay of 111Ag. Thus, transformation from AgI to CdII rapidly disrupts the functional linear bis(thiolato)AgI 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, 111Ag PAC spectroscopy may be applied to probe the flexibility of protein metal sites.