Synthesis, X-ray Structures, Electronic Properties, and O₂/NO Reactivities of Thiol Dioxygenase Active-Site Models

Abstract

Mononuclear nonheme iron complexes that serve as structural and functional mimics of the thiol dioxygenases (TDOs), cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), have been prepared and characterized with crystallographic, spectroscopic, kinetic, and computational methods. The high-spin Fe(II) complexes feature the facially-coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (Ph2TIP) ligand that replicates the three histidine (3His) triad of the TDO active sites. Further coordination with bidentate L-cysteine ethyl ester (CysOEt) or cysteamine (CysAm) anions yielded five-coordinate (5C) complexes that resemble the substrate-bound forms of CDO and ADO, respectively. Detailed electronic-structure descriptions of the [Fe(Ph2TIP)(LS,N)]BPh4 complexes, where LS,N = CysOEt (1) or CysAm (2), were generated through a combination of spectroscopic techniques [electronic absorption, magnetic circular dichroism (MCD)] and density functional theory (DFT). Complexes 1 and 2 decompose in the presence of O2 to yield the corresponding sulfinic acid (RSO2H) products, thereby emulating the reactivity of the TDO enzymes and related complexes. Rate constants and activation parameters for the dioxygenation reactions were measured and interpreted with the aid of DFT calculations for O2-bound intermediates. Treatment of the TDO models with nitric oxide (NO) – a well-established surrogate of O2 – led to a mixture of high-spin and low-spin {FeNO}7 species at low temperature (−70 °C), as indicated by electron paramagnetic resonance (EPR) spectroscopy. At room temperature, these Fe/NO adducts convert to a common species with EPR and infrared (IR) features typical of cationic dinitrosyl iron complexes (DNICs). To complement these results, parallel spectroscopic, computational, and O2/NO reactivity studies were carried out using previously-reported TDO models that feature an anionic hydrotris(3-phenyl-5-methyl-pyrazolyl)borate (Ph,MeTp−) ligand. Though the O2 reactivities of the Ph2TIP- and Ph,MeTp-based complexes are quite similar, the supporting ligand perturbs the energies of Fe 3d-based molecular orbitals and modulates Fe-S bond covalency, suggesting possible rationales for the presence of neutral 3His coordination in CDO and ADO.

Spectroscopic and computational studies of reversible O2 binding by a cobalt complex of relevance to cysteine dioxygenase

“…46 More recently, our group employed the tris(4,5-diphenylimidazol-2-yl)phosphine chelate (2-TIP Ph2 ; Scheme 3) to generate the related CDO mimic [Fe(2-TIP Ph2 )(CysOEt)]BPh4 (2). 49 The 2-TIP Ph2 and Tp Ph,Me supporting ligands both replicate the facial N3 coordination of the 3His triad, but the former matches the neutral charge of the enzymatic environment. Complexes 1and 2 are pale yellow due to the presence of overlapping S → Fe(ii) charge transfer (CT) transitions in the near UV region, which we deconvoluted with the aid of MCD spectroscopy.…”

“…In our previous report, this limitation was overcome by treating the CDO models with the O2-surrogate NO, which generated a mixture of EPR-active Fe/NO adducts possessing S = 3/2 or 1/2 ground states. 49 Another well-established strategy for probing the geometric and electronic structures of nonheme iron enzymes (and related model complexes) involves replacement of the Fe(ii) center with a divalent metal ion that is more spectroscopically "accessible", such as Co(ii), Mn(ii), or Cu(ii). [50][51][52][53] In addition to enhancing the spectroscopic features of the enzymes, metal-substitution experiments often yield valuable insights into O2activation mechanisms, as recently demonstrated by Que and Lipscomb in their studies of Co(ii)-and Mn(ii)-substituted versions of homoprotocatechuate-2,3-dioxygenase, a MNID that catalyzes the oxidative ring-cleavage of its catechol substrate.…”

A Structural and Functional Model for the Tris‐Histidine Motif in Cysteine Dioxygenase

“…[4] The FeÀN py distances in 2 are shortert han the FeÀN His bonds in CDO, while those of the Fe II -Tp complexes and theird erivatives( 2.11-2.28 )a re longer, and this is due to electronic effects (e.g.,t he pK a values, see above)a nd the geometries (5-vs. 6-ring heterocycles). [7][8][9][10] The similarity of the structurally well-definedF e II model 2 with that of the enzyme is visualized by the overlay plot of 2 with the active site structure of CDO ( Figure 1): the main difference between the two geometries is the torsion aroundt he FeÀ N heterocycle axes, well visualized in Figure 1. We propose that this is due to the rigidity of our model ligandsL ,w here the only flexibility in the tripodals tructure is the torsion aroundt he C alkyl ÀC heterocycle bond, and the lowest energy conformation obviously is that with the three pyridine rings parallel to the molecular C 3 axis.H owever,w ea nticipate only am inor electronic perturbation due to this major structurald ifference, since pbondingi nvolving pyridinea nd imidazole groups coordinated to an iron center is known to be of only minor importancea t most.…”

“…Similar spectralf eatures wereo bservedf or the Cys-bound Fe II -CDO [4,6] as well as in other model complexes. [7][8][9][10] All possible electronics pin states of 3 and 4 were optimized using DFT methods. The optimized geometries of 3 and 4 along with computed bond parameters, spin density plots and relative energies between different spin states are given in the Supporting information, Ta ble S4.…”

“…[9] Brunold and Fiedler studied Fe II complexes of the tris(4,5-diphenyl-1-methyl-imidazol-2-yl)phosphine ligand Ph 2 TIP as am odel for the 3-His triad of the thiol dioxygenase (TDO). [10] We use at ris(pyridyl)-based ligand system (Scheme 2), which enforces an environment to the metal center (geometry and electronics) that is very similart ot he tris-His active site of CDO (vide infra for the observed structuralp roperties;p K a (protonated heterocycles): 7.0 vs. 5.2 vs. 2.5 for imidazole, pyridine and pyrazole, respectively). Here, we present results that show that the Fe II complex of our ligand is not only as tructural model but, upon coordination of at hiolate substrate and reaction with O 2 ,p roduces al ow-spinF e III -peroxido intermediate and ad oubly oxygenated sulfinic acid as aproduct.…”