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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

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posted on 05.10.2009, 00:00 by Ming-Che Tsai, Fu-Te Tsai, Tsai-Te Lu, Ming-Li Tsai, Yin-Ching Wei, I-Jui Hsu, Jyh-Fu Lee, Wen-Feng Liaw
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.