STUDIES ON THE HYDROGENATION STEPS OF THE NITROGEN MOLECULE AT THE AZOTOBACTER-VINELANDII NITROGENASE SITE

We follow the initial activation of the nitrogen molecule at the FeMo cofactor of nitrogenase and subsequently model the hydrogenation of N- 2 up to the fourth protonation step using the intermediate neglect of differential overlap quantum-chemical model. The results obtained favo r a reaction mechanism going through hydrazido intermediates on the 4- Fe surfaces, externally to the FeMo cofactor. Calculations using densi ty functional theory on smaller model systems also support the suggest ed mechanism over other possible schemes that involve early release of the first molecule of ammonia as a product of the enzymatic reaction. We also demonstrate that dielectric stabilization due to the protein around the cofactor could lower markedly the barrier for the product r elease as an ammonium ion. (C) 1998 John Wiley & Sons, Inc.