Lees Nicholas S, McNaughton Rebecca L, Gregory Wilda Vargas, Holland Patrick L, Hoffman Brian M
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
J Am Chem Soc. 2008 Jan 16;130(2):546-55. doi: 10.1021/ja073934x. Epub 2007 Dec 20.
Molybdenum-dependent nitrogenase binds and reduces N2 at the [Fe7, Mo, S9, X, homocitrate] iron-molybdenum cofactor (FeMo-co). Kinetic and spectroscopic studies of nitrogenase variants indicate that a single Fe-S face is the most likely binding site. Recently, substantial progress has been made in determining the structures of nitrogenase intermediates formed during alkyne and N2 reduction through use of ENDOR spectroscopy. However, constraints derived from ENDOR studies of biomimetic complexes with known structure would powerfully contribute in turning experimentally derived ENDOR parameters into structures for species bound to FeMo-co during N2 reduction. The first report of a paramagnetic Fe-S compound that binds reduced forms of N2 involved Fe complexes stabilized by a bulky beta-diketiminate ligand (Vela, J.; Stoian, S.; Flaschenriem, C. J.; Münck, E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522-4523). Treatment of a sulfidodiiron(II) complex with phenylhydrazine gave an isolable mixed-valence FeII-Fe(III) complex with a bridging phenylhydrazido (PhNNH2) ligand, and this species now has been characterized by ENDOR spectroscopy. Using both 15N, 2H labeled and unlabeled forms of the hydrazido ligand, the hyperfine and quadrupole parameters of the -N-NH2 moiety have been derived by a procedure that incorporates the (near-) mirror symmetry of the complex and involves a strategy which combines experiment with semiempirical and DFT computations. The results support the use of DFT computations in identifying nitrogenous species bound to FeMo-co of nitrogenase turnover intermediates and indicate that 14N quadrupole parameters from nitrogenase intermediates will provide a strong indication of the nature of the bound nitrogenous species. Comparison of the large 14N hyperfine couplings measured here with that of a hydrazine-derived species bound to FeMo-co of a trapped nitrogenase intermediate suggests that the ion(s) are not high spin and/or that the spin coupling coefficients of the coordinating cofactor iron ion(s) in the intermediate are exceptionally small.
钼依赖型固氮酶在[Fe7, Mo, S9, X, 高柠檬酸]铁钼辅因子（FeMo-co）处结合并还原N2。固氮酶变体的动力学和光谱研究表明，单个铁硫面是最可能的结合位点。最近，通过使用电子核双共振光谱（ENDOR），在确定炔烃和N2还原过程中形成的固氮酶中间体结构方面取得了重大进展。然而，从对具有已知结构的仿生配合物的ENDOR研究中得出的限制，将有力地有助于把实验得出的ENDOR参数转化为N2还原过程中与FeMo-co结合的物种的结构。第一个关于结合还原态N2的顺磁性铁硫化合物的报告涉及由庞大的β-二酮亚胺配体稳定的铁配合物（Vela, J.; Stoian, S.; Flaschenriem, C. J.; Münck, E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522 - 4523）。用苯肼处理硫化二铁（II）配合物得到了一种可分离的混合价态FeII-Fe(III)配合物，其带有一个桥连的苯肼基（PhNNH2）配体，并且该物种现在已通过ENDOR光谱进行了表征。使用15N、2H标记和未标记形式的肼基配体，通过一种结合了配合物（近乎）镜像对称性并涉及将实验与半经验和密度泛函理论（DFT）计算相结合的策略的程序，得出了-N-NH2部分的超精细和四极参数。结果支持在识别固氮酶周转中间体的FeMo-co结合的含氮物种中使用DFT计算，并表明来自固氮酶中间体的14N四极参数将有力地表明结合的含氮物种的性质。此处测量的大的14N超精细耦合与结合到捕获的固氮酶中间体的FeMo-co上的肼衍生物种的超精细耦合的比较表明，离子不是高自旋的和/或中间体中配位辅因子铁离子的自旋耦合系数异常小。
