ABSTRACT
Electronic structure models of metalloenzyme active sites offer invaluable insight into determining atom-level details of the corresponding enzyme mechanism. Results from density functional theory (DFT) and other levels of computational chemistry can afford such models. However, these computational results must rst be spectroscopically validated-meaning they must be shown to accurately predict electronic structure properties. In the case of the tetranuclear manganese oxygen-evolving complex (OEC), which is the active site for water oxidation in photosystem II (PSII), DFT predictions have afforded several geometric models that were consistent with various x-ray diffraction and scattering results, although evaluation of the corresponding electronic structures would be more discriminating. To this end, many groups have used results from electron paramagnetic resonance (EPR) spectroscopy-in particular, advanced pulse EPR techniques-to probe the distribution of the unpaired electron spin about the OEC and the strength of the hyperne interaction between magnetic nuclei and this electron spin. This work has yielded a sea of magnetic parameters that collectively serve as a perspicacious judge of the computer-generated structural models of the OEC. In this chapter, we summarize these EPR-derived magnetic parameters for various states and inhibited forms of the OEC. These ndings have ruled out several proposed structures for the OEC and, with them, certain reaction pathways. Recently, however, one model (informed by the high-resolution crystal structure coordinates) has emerged that performs remarkably well, especially in predicting the magnetic parameters of the constituent 55Mn ions. Further renement of this model requires continued spectroscopic characterization of the OEC, and EPR spectroscopy will be a key tool to do so.
