Copper Dynamics in Doped Metal–Bis(histidine) Complexes

Electron paramagnetic resonance (EPR) temperature-dependent measurements were undertaken on three Cu(II)-doped metal–histidine complexes to assess copper site dynamic behavior. Previous single-crystal EPR analysis on two of these, zinc d,l-histidine pentahydrate (ZnDLH) and bis(l-histidinato)cadmium dihydrate (CdLH), found that doped Cu2+ can be modeled as hopping between two neighboring conformational states, with a temperature-dependent rate becoming large enough at room temperature to produce an “averaged” spectrum. By comparing spectra from their powdered form, we show that Cu2+ doped into a third system, Cd2+-d,l-histidine (CdDLH), also exhibits temperature-dependent EPR with features indicating a similar motional-averaging process. In addition, the change of g and copper hyperfine parameters from low to high temperature for CdDLH resembles that in ZnDLH, whereas the change in these parameters for CdLH is like that found in a fourth copper-doped system, zinc l-histidine dihydrate (ZnLH). Taken together, these results suggest that averaging motion between neighboring copper sites is common in metal–bis(histidine) compounds. More detailed studies on biological models are thus warranted, especially because they reveal unique relationships between structure, dynamic processes, and stability and can lead to a better understanding of the role played by site flexibility in copper proteins