EPR Studies of Copper Dynamics in Doped Biological Crystals

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Electron Paramagnetic Resonance (EPR) spectroscopy will be utilized to study dynamic behavior exhibited by copper in crystals of biological models. Knowledge of dynamic processes in these systems gives important insight into the stability of unusual copper coordination like those frequently found in biological macromolecules. Features of the dynamic processes are normally described by a critical temperature, above which motion becomes rapid enough to average out characteristics of individual species, rate(s) of transition, activation energy (ies) and energy differences of the possible states. This project will determine these parameters and in doing so will provide both a qualitative and quantitative understanding of the potential energy surfaces of the metal complex. This leads to a better understanding of the relationship between stability and structure of copper in metalloproteins. The previous lack of suitable copper model systems showing dynamic behavior has been circumvented by the realization in a survey of copper-doped cadmium-crystals that when Cu2+ replaces Cd2+, the copper complex, in all cases, exhibits dynamic behavior. This is most likely the result of the cadmium ion posessing a larger cage of surrounding ligands than the replacing copper ion. Four such model crystal systems have been initially identified, Cu2+-doped into Cd2+-histidine, Cd2+-theophylline, Cd2+-glycylglycine HCl and Cd2+-glutamate, and are proposed here for detailed EPR investigations. The long term objective is to use this information in studies where copper can act as a surrogate in cadmium-binding proteins and in the newly discovered cadmium-dependent carbonic anhydrase.