XAFS Study of Coordination Structure of Cu(L-His)2 in Solution

Multiple coordination modes are present in the CuII-histidine complex in solution and the copper coordination environment varies with pH. In this work, we have investigated the coordination geometry of Cu(His)2 complex using X-ray absorption fine structure (XAFS) analysis. Copper K-edge XAFS spectra were acquired on aqueous Cu2+ samples with histidine at different pH values. The coordination environments were further confirmed by chemically modified histidine. Results show that the caboxylate groups coordinate at acidic condition, while amino and imidazole nitrogens get coordinated at higher pH. For the coordination geometry of Cu(His)2 in solution at physiological pH, the sixfold coordination is preferentially formed, while the fivefold coordination can co-exist in equilibrium.