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The size and complexity of Mo-dependent nitrogenase, a multicomponent enzyme capable of reducing dinitrogen to ammonia, have made a detailed understanding of the FeMo cofactor (FeMoco) active site electronic structure an ongoing challenge. Selective substitution of sulfur by selenium in FeMoco affords a unique probe wherein local Fe-Se interactions can be directly interrogated via high-energy resolution fluorescence detected X-ray absorption spectroscopic (HERFD XAS) and extended X-ray absorption fine structure (EXAFS) studies. These studies reveal a significant asymmetry in the electronic distribution of the FeMoco, suggesting a more localized electronic structure picture than is typically assumed for iron-sulfur clusters. Supported by experimental small molecule model data in combination with time dependent density functional theory (TDDFT) calculations, the HERFD XAS data is consistent with an assignment of Fe2/Fe6 as an antiferromagnetically coupled diferric pair. HERFD XAS and EXAFS have also been applied to Se-substituted CO-inhibited MoFe protein, demonstrating the ability of these methods to reveal electronic and structural changes that occur upon substrate binding. These results emphasize the utility of Se HERFD XAS and EXAFS for selectively probing the local electronic and geometric structure of FeMoco.

A simple "diffusion-to-capture" model is used to estimate the upper limit to the growth rate of macromolecular crystals under conditions when the rate limiting process is the mass transfer of sample from solution to the crystal. Under diffusion-limited crystal growth conditions, this model predicts that the cross-sectional area of a crystal will increase linearly with time; this prediction is validated by monitoring the growth rate of lysozyme crystals. A consequence of this analysis is that when crystal growth is diffusion-limited, micron-sized crystals can be produced in ~1 s, which would be compatible with the turnover time of many enzymes. Consequently, the ability to record diffraction patterns from sub-micron sized crystals by X-ray Free Electron Lasers and micro-electron diffraction technologies opens the possibility of trapping intermediate enzyme states by crystallization.