Relative Binding Affinity of Thiolate, Imidazolate, Phenoxide, and Nitrite Toward the {Fe(NO)2} Motif of Dinitrosyl Iron Complexes (DNICs): The Characteristic Pre-Edge Energy of {Fe(NO)2}9 DNICs

The synthesis, characterization, and transformation of the anionic {Fe(NO)2}9 dinitrosyl iron complexes (DNICs) [(NO)2Fe(ONO)2]− (1), [(NO)2Fe(OPh)2]− (2), [(NO)2Fe(OPh)(C3H3N2)]− (3) (C3H3N2 = imidazolate), [(NO)2Fe(OPh)(-SC4H3S)]− (4), [(NO)2Fe(p-OPhF)2]− (5), and [(NO)2Fe(SPh)(ONO)]− (6) were investigated. The binding affinity of ligands ([SPh]−, [−SC4H3S]−, [C3H3N2]−, [OPh]−, and [NO2]−) toward the {Fe(NO)2}9 motif follows the ligand-displacement series [SPh]− ∼ [−SC4H3S]− > [C3H3N2]− > [OPh]− > [NO2]−. The findings, the pre-edge energy derived from the 1s → 3d transition in a distorted Td environment of the Fe center falling within the range of 7113.4−7113.8 eV for the anionic {Fe(NO)2}9 DNICs, implicate that the iron metal center of DNICs is tailored to minimize the electronic changes accompanying changes in coordinated ligands. Our results bridging the ligand-substitution reaction study and X-ray absorption spectroscopy study of the electronic richness of the {Fe(NO)2}9 core may point the way to understanding the reasons for nature’s choice of combinations of cysteine, histidine, and tyrosine in protein-bound DNICs and rationalize that most DNICs characterized/proposed nowadays are bound to the proteins almost through the thiolate groups of cysteinate/glutathione side chains in biological systems.

本研究对阴离子{Fe(NO)2}9二硝基铁络合物（DNICs）的合成、表征及其转化进行了探究，包括[(NO)2Fe(ONO)2]− (1)、[(NO)2Fe(OPh)2]− (2)、[(NO)2Fe(OPh)(C3H3N2)]− (3)（C3H3N2 = 吡唑烷基）、[(NO)2Fe(OPh)(-SC4H3S)]− (4)、[(NO)2Fe(p-OPhF)2]− (5)和[(NO)2Fe(SPh)(ONO)]− (6)等。配体（[SPh]−、[−SC4H3S]−、[C3H3N2]−、[OPh]−和[NO2]−）对{Fe(NO)2}9结构基元的结合亲和力遵循配体取代系列[SPh]− ∼ [−SC4H3S]− > [C3H3N2]− > [OPh]− > [NO2]−。研究发现，在扭曲Td环境的Fe中心，来自1s → 3d跃迁的前缘能量在7113.4−7113.8 eV范围内，表明DNICs中的铁金属中心经过设计以最小化伴随配体变化而产生的电子变化。本研究将配体取代反应研究与{Fe(NO)2}9核心的电子丰度X射线吸收光谱研究相结合，可能为理解自然界在蛋白质结合DNICs中为何选择半胱氨酸、组氨酸和酪氨酸的组合提供途径，并解释了为何目前所表征/提出的多数DNICs在生物系统中几乎通过半胱氨酸/谷胱甘肽侧链的巯基团与蛋白质结合。