Chemical Modeling of Biological Nitrogen FixationX IV FHMO study of probable Modes of μ_3(η~2) Coordination of Some Nitrogenase Substrates (Cyclopropene etc.)and Reductive-Hydrogenation Intermediates of N_2
- 化学化工－已发表论文 
[中文摘要]迄今所知的固氮酶十来种底物中,除极少数例外,大多数都是末端具有三重键(或环丙烯型的准三重键)的小分子。从这些底物的已知酶促反应和在固氨酶活性中心的相对配位亲合力,我们曾推断这些底物分子很可能是以相同的μ_3(η~2)型方式络合在固氮酶活性中心上的,并在此基础上提出了并联双座双立方烷型原子簇结构的活性中心模型。同时指出,对于那些末端有三重键但无“尾巴”的底物分子,如N≡N,C≡N~-,和HC≡CH,在三核活性中心(1Mo—2Fe)进行μ_3(η~2)型络合,有两种可能的方式即双端基加单侧基型络合和单端基加双侧基络合。本文报导了EHMO近似计算和定性的分子轨道讨论结果。结果表明:在环丙烯情况下,双(准)端基加单侧基型络合比单(准)端基加双侧基型络合在能量上要有利得多,而且它的络合键能相当大;在N≡N和HN≡C(假想的异氢腈酸)情况下,双端基(或准端基)加单侧基型络合比单端基加双侧基型络合在能量上亦较为有利;乙炔的情况也是这样,但两种络合方式的络合键能之差比较小。这些结果与下述的实验事实是一致的:环丙烯是固氮酶的底物,而且是N≡N酶促还原加氢的强竞争性抑制剂;两种按μ_3(η~2)型方式络合的炔烃过渡金属原子簇络合物都已发现。环丙烯和N≡N在三核活性中心的双端基(或准端墓)加单侧基型络合也能较好地说明环丙烯酶促还原加氮产物的结构选择性,和N2与固氮酶共同催化的专一性反应(D2+2H+2e-→2HD)的过渡态构型。[英文摘要]With only 2 exceptions, most of the 10-11 different types of nitroge-nase substrates known so far are small molecules with terminal triple-bonds, or cyclopropenc-type quasi triple bond. From the known nitrogenase-cata!yzed reactions of these substrates and their relative coordination affinities at nitrogenase active-center, it has been inferred by us that these substrate moleciles arc most probably coordinated jn similar μ3(η2) modes at nitrogenase active-center, and a twin-sited dicubane-cluster structural model of active-center has been proposed.It has also been pointed out that,for those substrate molecules with triple-bonds but without "tail"namely,cyclopropene,N≡N,C≡N-,and HC≡CH,both the donble-end-on-plus-single-side-on mode and the single-end-on-plus-double-side-on-mode of μ3(η2) coordination are compatible with the same trinuclear active-center(1Mo-2Fe).In the present work,results of approximate quantum-chemical EHMO calculations and qualitative molecular orbital discussions are presented,which show that in the case of cyclopropene,the double-end-on(pseudo)-plus-single-side-on mode of μ3(η2) coordination is energetically far more favorable than the alternative mode of μ3(η2) coordination;in the care of N≡N and HN≡C(hypothetical iso-hydrocyanic acid),the double-end-on-plus-single-side-on mode of μ3(η2) coordination are also energetically more favorable than the alternative mode of μ3(η2)coordination;this is still true in the case of HC≡CH,but here the difference in coordination bond energies for the 2 modes of μ3(η2) coordination is rather small.These results are in agreement with the known experimental fact that cyclopropene is a nitrogenase substrate and a strong competitive inhibitor of nitrogenase-catalyzed reduction of N≡N,and that both modes of μ3(η2) coordination are known for acetylenes in their transition-metal cluster complexes.The double-end-on(pseudo)-plus-single-side-on coordination of cyclopropene and of N≡N at the trinuclear active-center also provides satisfactory interpretation of the structural selectivity of the nitrogenase-catalyzed reduction products of cyclopropene,and the transition state configuration of the D2+2H++2e- → 2HD reaction specifically catalyzed by nitrogenase in the presense of N2.