Das Sandip, Devi Tarali, Goswami Mrigaraj, Yenuganti Mahesh, Bhardwaj Prabhakar, Ghosh Somnath, Chandra Sahoo Subash, Kumar Pankaj
Department of Chemistry, Indian Institute of Science Education and Research (IISER) Tirupati 517507 India
Humboldt-Universität zu Berlin, Institut für Chemie Brook-Taylor-Straße 2 D-12489 Berlin Germany.
Chem Sci. 2021 Jul 2;12(31):10605-10612. doi: 10.1039/d1sc00803j. eCollection 2021 Aug 11.
Nitrate reductases (NRs) are molybdoenzymes that reduce nitrate (NO ) to nitrite (NO ) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO from NO , which further produces nitric oxide (NO) either in acid-induced NO reduction or in the presence of nitrite reductases (NiRs). Here, we report a new approach of VCl (V ion source) induced step-wise reduction of NO in a Co-nitrato complex, [(12-TMC)Co(NO )] (2,{Co-NO }), to a Co-nitrosyl complex, [(12-TMC)Co(NO)] (4,{CoNO}), bearing an -tetramethylated cyclam (TMC) ligand. The VCl inspired reduction of NO to NO is believed to occur in two consecutive oxygen atom transfer (OAT) reactions, , OAT-1 = NO → NO (r) and OAT-2 = NO → NO (r). In these OAT reactions, VCl functions as an O-atom abstracting species, and the reaction of 2 with VCl produces a Co-nitrosyl ({CoNO}) with V-Oxo ({V[double bond, length as m-dash]O}) species, a proposed Co-nitrito (3, {Co-NO }) intermediate species. Further, in a separate experiment, we explored the reaction of isolated complex 3 with VCl, which showed the generation of 4 with V-Oxo, validating our proposed reaction sequences of OAT reactions. We ensured and characterized 3 using VCl as a limiting reagent, as the second-order rate constant of OAT-2 ( ) is found to be ∼1420 times faster than that of the OAT-1 ( ) reaction. Binding constant ( ) calculations also support our proposition of NO to NO transformation in two successive OAT reactions, as is higher than , hence the reaction moves in the forward direction (OAT-1). However, is comparable to , and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using N-labeled-NO and NO revealed that the N-atom in the {CoNO} is derived from NO ligand. This work highlights the first-ever report of VCl induced step-wise NO reduction (NRs activity) followed by the OAT induced NO reduction and then the generation of Co-nitrosyl species {CoNO}.
硝酸还原酶（NRs）是一种钼酶，在哺乳动物和植物中均可将硝酸盐（NO₃⁻）还原为亚硝酸盐（NO₂⁻）。在哺乳动物中，唾液微生物参与了从NO₃⁻生成NO₂⁻的过程，在酸诱导的NO₂⁻还原或存在亚硝酸还原酶（NiRs）的情况下，NO₂⁻会进一步生成一氧化氮（NO）。在此，我们报道了一种新方法，即VCl（V离子源）诱导钴-硝酸根配合物[(12-TMC)Co(NO₃)]（2，{Co-NO₃}）中的NO₃⁻逐步还原为钴-亚硝酰配合物[(12-TMC)Co(NO)]（4，{CoNO}），该配合物带有一个1,4,8,11-四甲基环四胺（TMC）配体。据信，VCl引发的NO₃⁻到NO₂⁻的还原发生在两个连续的氧原子转移（OAT）反应中，即OAT-1 = NO₃⁻ → NO₂⁻（反应速率r₁）和OAT-2 = NO₂⁻ → NO（反应速率r₂）。在这些OAT反应中，VCl充当氧原子提取物种，2与VCl的反应生成带有V-氧代（{V═O}）物种的钴-亚硝酰（{CoNO}），即一种推测的钴-亚硝酸根（3，{Co-NO₂}）中间体物种。此外，在另一个单独的实验中，我们研究了分离出的配合物3与VCl的反应，结果表明生成了带有V-氧代的4，这验证了我们提出的OAT反应序列。我们以VCl作为限量试剂确保并表征了3，因为发现OAT-2（k₂）的二级反应速率常数比OAT-1（k₁）反应的速率常数快约1420倍。结合常数（K）计算也支持我们关于在两个连续的OAT反应中NO₃⁻到NO₂⁻转化的观点，因为K₂高于K₁，因此反应朝着正方向（OAT-1）进行。然而，K₃与K₂相当，因此确定了第二个OAT反应（OAT-2）的顺序。使用N标记的NO₃⁻和NO₂⁻对这些反应进行的机理研究表明，{CoNO}中的N原子源自NO₃⁻配体。这项工作首次报道了VCl诱导的逐步NO₃⁻还原（NRs活性），随后是OAT诱导的NO₂⁻还原，然后生成钴-亚硝酰物种{CoNO}。
