SHINING LIGHT ON BIOLOGICAL NITROGEN FIXATION

Nitrogenases catalyze biological nitrogen fixation by reducing N₂ to NH₃ under ambient conditions. Beyond the Mo-dependent enzyme, V- and Fe-only nitrogenases exhibit reduced N₂ activity but enhanced reactivity toward alternative substrates such as CO, which can be converted to hydrocarbons. The electronic and structural origins of this substrate differentiation remain unresolved and likely reflect interplay between cofactor composition and the protein environment. X-ray spectroscopy provides direct access to these effects. In Mo nitrogenase, site-selective selenium substitution of belt sulfides, combined with Se Kα HERFD XAS and QM/MM calculations, has revealed how the protein matrix modulates metal-ligand covalency and active-site basicity. EPR and complementary X-ray methods further identify catalytically relevant intermediates. Extending these approaches to V- and Fe-only nitrogenases reveals systematic differences in cofactor electronic structure and protonation patterns that may explain divergent substrate selectivity, establishing a framework linking cofactor identity, protein environment, and catalytic function.