BOND VALENCE SUM ANALYSIS OF METAL-LIGAND BOND LENGTHS IN METALLOENZYMES AND MODEL COMPLEXES

Bond valence sums (BVS's) have been calculated for a series of model complexes for important metalloenzymes. Values calculated from crystallographic distances predict the correct oxidation state of the metal center in a wide range of structural types. Many of the selected complexes have previously been studied by X-ray absorption spectroscopy, and first-shell metal-ligand bond distances have been determined by analysis of extended X-ray absorption fine structure (EXAFS). BVS's determined from the EXAFS distances also predict the correct metal oxidation states. Extension of the method to actual metalloproteins shows that the correct oxidation state can also be calculated from EXAFS distances in natural systems. The method can be applied to structures as widely varied as blue copper centers, oxo-bridged diiron centers, and mixed-valence forms of Fe-S clusters. The approach is then used to test possible oxidation-state assignments for the manganese clusters in the S1 and S2 states of the oxygen-evolving complex in photosystem II.