| Summary: | 博士 === 國立清華大學 === 化學系 === 100 === S-nitrosation, coupling of NO and a cysteine-sulfur, has been identified as a post-translational modification of proteins to convey part of the ubiquitous influence of nitric oxide on thiol-dependent cellular-signaling transduction. Protein-bound dinitrosyl iron complexes (DNICs) were demonstrated to be a dominant species for producing cellular protein RSNOs. In order to uncover the DNIC-to-RSNO pathway, this biomimetic investigation on S-nitrosation of the coordinated thiolate in DNICs generating RSNOs was demonstrated. Consistent with the transformation of [(NO)2Fe(μ-StBu)]2 (1-tBuS) into the {Fe(NO)2}9 DNIC [(NO)2Fe(StBu)(MeIm)] (2-MeIm) upon addition of 1-methylimidazole (MeIm) into the THF solution of 1-tBuS, the dynamic interconversion between the {Fe(NO)2}9 [(NO)2Fe(S-NAP)(DMSO)] (2-DMSO) (NAP = N-acetyl-D-penicillamine) and [(NO)2Fe(μ-S-NAP)]2 (1-NAP) occurred in the DMSO solution of complex 1-NAP. In contrast to the reaction of 2-MeIm and bis(dimethylthiocarbamoyl) disulfide ((DTC)2) yielding the {Fe(NO)}7 [(NO)Fe(DTC)2] (3) (DTC = S2CNMe2), (tBuS)2 and NO(g), transformation of the {Fe(NO)2}9 2-MeIm (2-DMSO) into RSNOs (RS = tBuS, NAP-S) along with MNIC 3 facilitated by (DTC)2 and Brønsted acid demonstrates that Brønsted acid may play a critical role in triggering S-nitrosation from DNICs 2-MeIm and 2-DMSO to produce RSNOs. Transformation of DNICs into RSNOs may only occur on the one-thiolate-containing {Fe(NO)2}9 DNICs, in contrast to the protonation of the two-thiolate-containing DNICs [(NO)2Fe(SR)2]- by Brønsted acid yielding [(NO)2Fe(μ-SR)]2. Kinetic strudy on the transformation of [(NO)2Fe(SPh)(Me4Im)] (2-PhS) into MNIC 3 in the presence of Brønsted acid suggests a two-step intramolecular mechanism for the DNIC-mediated S-nitrosation. Conclusively, DNIC-to-RSNO requires a Brønsted acid and Lewis base pair to trigger the formation of RSNO. These results may rationalize that the known protein-Cys-SNO sites derived from DNIC were located adjacent to acid and base motifs, and no protein-bound-SNO characterized nowadays was directly derived from [protein-(cysteine)2Fe(NO)2] in biology.
DNICs can be classified into the EPR-active {Fe(NO)2}9 DNICs with a high-spin FeIII (S = 5/2) antiferromagnetically coupled to two triplet NO– (S = 2) as well as the EPR-silent {Fe(NO)2}10 DNICs with a high-spin FeII (S = 2) antiferromagnetically coupled to two triplet NO– (S = 2). Complexes {Fe(NO)2}9 [(NO)2Fe(N(mesityl)(TMS))2]– (9) and [(NO)2Fe(N(mesityl)(TMS))2] (10) are redox reversible interconversion. The electronic structure of the one-electron oxidized form, DNIC 10, was characterized by a detailed analysis of IR, 15N NMR, SQUID, XAS and X-ray structure, and is best described as [{FeIII(NO–)2}9- (L•)(L–)], instead of [{Fe(NO)2}8-(L–)2] (L = [N(mesityl)(TMS)]). That is, an aminyl radical can be stabilized by an electron-deficient {Fe(NO)2}9 fragment to yield the isolated complex 10. Our results bridging XAS study of the electronic richness of the {Fe(NO)2}9/10 core and the study of the electronic structures of the redox forms DNIC 9 DNIC 10 may point the way to understanding that all of tetrahedral DNICs isolated and characterized nowadays are confined in the {Fe(NO)2}9 and {Fe(NO)2}10 DNICs in chemistry and in biological system.
Dinuclear dinitrosyl iron complexes [(NO)2Fe(μ-NR2)]2 (NR2 = NPh2 (12), N(TMS)2 (13), N=CtBu2 (14)) display the shortest Fe-Fe bond distances of 2.43~2.58 Å, compared to the known terminal-nitrosyl diiron complexes containing thiolate-bridged ligands showing Fe…Fe distance of 2.62~2.79 Å. The electronic structure of complexes 12, 13 and 14, characterized by IR, UV-vis, Fe K-edge XAS, single-crystal X-ray structure and DFT calculation, is best described as S = 1/2 {FeIII(NO–)2}9 antiferromagnetically coupled with another S =1/2 {FeIII(NO–)2}9 via direct interaction of Fe-Fe bond in combination with indirect coupling through bridged ligands. Reduction on complexes 12, 13 and 14 may result in adding an electron on the dx2-y2-dx2-y2 antibonding, breaking metal-metal bond, and finally leading to degradation, compared to the reduction of [(NO)2Fe(μ-SR)]2 yielding [(NO)2Fe(μ-SR)]2–. Not only does development of the chemistry of NR2-RREs give a way for the new type of NO-donor drugs, but it may also provide another possible form for NO storage and transport in biological system.
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