A functional model for quercetin 2,4-dioxygenase: Geometric and electronic structures and reactivity of a nickel(II) flavonolate complex

Appended Lewis Acids Enable Dioxygen Reactivity and Catalytic Oxidations with Ni(II)

We disclose a suite of Ni(II) complexes featuring secondary sphere Lewis acids of varied Lewis acidity and tether lengths. Several of these complexes feature atypical behavior of Ni(II): reactivity with O2 that occurs only in the presence of a tethered Lewis acid. In situ UV-vis spectroscopy revealed that, although adducts are stable at -40 °C, complexes containing 9-borabicyclo[3.3.1]nonane (9-BBN) Lewis acids underwent irreversible oxidative deborylation when warmed to room temperature. We computationally and experimentally identified that oxidative instability of appended 9-BBN moieties can be mitigated using weaker Lewis acids such as pinacolborane (BPin). These insights enabled the realization of catalytic reactions: hydrogen atom abstraction from phenols and room temperature oxygen atom transfer to PPh3.

The case for an oxidopyrylium intermediate in the mechanism of quercetin dioxygenases

The quercetin dioxygenases (QDOs) are unusual metalloenzymes in that they display ring-opening dioxygenase activity with several different first-row transition metal ions which do not undergo redox changes during turnover. The QDOs are also unique in that the substrate binds as an η1-flavonolate rather than the η2 -bidentate mode seen in all reported model complexes. The flavonol substrates were early examples of excited state intramolecular proton transfer (ESIPT) phenomena, in which photoexcitation causes an H-atom exchange between the adjacent hydroxyl and ketone, generating an oxidopyrylium emissive state. These oxidopyryliums undergo ring-opening dioxygenations analogous to the enzymatic reactions. Our hypothesis is that lability of the divalent metal ion may allow access to a reactive oxidopyrylium intermediate via coordination switching from the oxy to ketone position, which allows reaction with O2. In this report, we use a straight-forward methylation strategy to generate a panel of flavonol and thioflavonol derivatives modeling several η1- and η2-coordination modes. Methylation of 3-hydroxythioflavone generates an air stable η1 hydroxopyrylium salt, which undergoes rapid ring-opening dioxygenation by deprotonation or photoexcitation. By comparison, the η1-methoxyflavonol does not react with O2 under any condition. We find that any of the studied flavonol derivatives, η1 or η2, which demonstrates ESIPT-like oxidopyrylium emissions undergo QDO-like ring-opening reactions with dioxygen. The implications of these results concerning the mechanism of QDOs and related dioxygenases is discussed.

Flavonol dioxygenation catalysed by cobalt(II) complexes supported with 3N(COO) and 4N donor ligands: a comparative study to assess the carboxylate effects on quercetin 2,4-dioxygenase-like reactivity

Two new cobalt(II)-acetato complexes, [CoII(L3NCOO)(OAc)]·0.5H2O (1OAc·0.5H2O) and [CoII(L4N)(OAc)](PF6) (2OAc(PF6)), were synthesised using ligands L3NCOO- (Li+L3NCOO- = lithium 2-(benzyl((6'-methyl-[2,2'-bipyridin]-6-yl)methyl)amino)acetate) and L4N (N-benzyl-1-(6'-methyl-[2,2'-bipyridin]-6-yl)-N-(pyridin-2-ylmethyl)methanamine), respectively, to mimic the functional activity of cobalt(II)-quercetin-2,4-dioxygenase (CoII-2,4-QD). Additionally, Co(II)-flavonolato ternary complexes, [CoII(L3NCOO)(fla)]·H2O (1fla·H2O) and [CoII(L4N)(fla)](PF6) (2fla(PF6)), were synthesised as enzyme-substrate models. All four complexes were thoroughly characterised by elemental analyses and spectroscopic methods. Structural characterisation was performed for 1OAc·0.5H2O, 2OAc(PF6)·CH2Cl2 and 2fla+ with a perchlorate counter anion, 2fla(ClO4)·1.5H2O. Furthermore, density functional theory (DFT) calculations, time-dependent DFT (TD-DFT) and molecular orbital (MO) analysis were performed for the flavonolato adducts 1fla and 2fla+. The catalytic activities of complexes 1OAc·0.5H2O and 2OAc(PF6) in the oxygenative degradation of flavonol (multiple-turnover reactions) were investigated at 70 °C in DMF to determine the effect of the carboxylate substituent over a pyridyl donor residue on reactivity. Complex 1OAc·0.5H2O showed a higher catalytic rate than complex 2OAc(PF6). The same reactivity order was observed for single-turnover dioxygenation reactions with ternary complexes (1fla > 2fla+). The formation constants (Kf) of 1fla and 2fla+ species are comparable, implying that catalyst-substrate adduct formation occurs in similar amounts for both catalytic reactions. Therefore, the Kf values have a similar impact on reactivities. However, the oxidation potential of the bound fla-/fla˙ couple in 1fla is considerably lower than that in 2fla+. DFT calculations predicted that the negatively charged carboxylate group of ligand L3NCOO- determines the higher reactivity of 1fla with dioxygen by decreasing the oxidation potential of the bound fla-/fla˙ couple. During the dioxygenation process, the reactive Co(II)-bound flavonoxy radical was generated via single-electron transfer from the coordinated fla- to dioxygen, simultaneously forming a superoxide ion. The anionic carboxylate group improves the stability of the bound flavonoxy radical by providing substantial electron density to the electron-deficient fla˙ through the Co(II) centre, allowing the reactive fla˙ species to accumulate at an optimal concentration for effective catalysis. EPR spectroscopy successfully detected the cobalt-bound fla˙ species formed through the dioxygenation of 1fla. NBT2+ and EPR spin-trapping experiments confirmed superoxide formation during the dioxygenation process. So, the present work describes CoII-2,4-QD model studies and clarifies the function of carboxylate in quercetinase-like reactivity.

Mononuclear nickel(ii)-flavonolate complexes of tetradentate tripodal 4N ligands as structural and functional models for quercetin 2,4-dioxygenase: structures, spectra, redox and dioxygenase activity

Three new nickel(ii)-flavonolate complexes of the type [Ni(L)(fla)](ClO4) 1-3, where L is the tripodal 4N ligand tris(pyrid-2-ylmethyl)amine (tpa, L1) or (pyrid-2-ylmethyl)bis(6-methylpyrid-2-ylmethyl)amine (6-Me2-tpa, L2) or tris(N-Et-benzimidazol-2-ylmethyl)amine (Et-ntb, L3), have been isolated as functional models for Ni(ii)-containing quercetin 2,4-dioxygenase. Single crystal X-ray structures of 1 and 3 reveal that Ni(ii) is involved in π-back bonding with flavonolate (fla-), as evident from enhancement in C[double bond, length as m-dash]O bond length upon coordination [H(fla), 1.232(3); 1, 1.245(7); 3, 1.262(8) Å]. More asymmetric chelation of fla- in 3 than in 1 [Δd = (Ni-Ocarbonyl - Ni-Oenolate): 1, 0.126; 3, 0.182 Å] corresponds to lower π-delocalization in 3 with electron-releasing N-Et substituent. The optimized structures of 1-3 and their geometrical isomers have been computed by DFT methods. The HOMO and LUMO, both localized on Ni(ii)-bound fla-, are highly conjugated bonding π- and antibonding π*-orbitals respectively. They are located higher in energy than the Ni(ii)-based MOs (HOMO-1, dx2-y2; HOMO-2/6, dz2), revealing that the Ni(ii)-bound fla- rather than Ni(ii) would undergo oxidation upon exposure to dioxygen. The results of computational studies, in combination with spectral and electrochemical studies, support the involvement of redox-inactive Ni(ii) in π-back bonding with fla-, tuning the π-delocalization in fla- and hence its activation. Upon exposure to dioxygen, all the flavonolate adducts in DMF solution decompose to produce CO and depside, which then is hydrolyzed to give the corresponding acids at 70 °C. The highest rate of dioxygenase reactivity of 3 (kO2: 3 (29.10 ± 0.16) > 1 (16.67 ± 0.70) > 2 (1.81 ± 0.04 × 10-1 M-1 s-1)), determined by monitoring the disappearance of the LMCT band in the range 440-450 nm, is ascribed to the electron-releasing N-Et substituent on bzim ring, which decreases the π-delocalization in fla- and enhances its activation.

Light-induced O2-dependent aliphatic carbon-carbon (C-C) bond cleavage in bipyridine-ligated Co(II) chlorodiketonate complexes

Mononuclear bipyridine (bpy)-ligated Co(II) chlorodiketonate complexes [(bpy)2Co(R-PhC(O)C(Cl)C(O)R-Ph)]ClO4 (R = -H (8), -CH3 (9), and -OCH3 (10)), were prepared, characterized and investigated for O2-dependent aliphatic C-C bond cleavage reactivity. Complexes 8-10 have a distorted psuedo-octahedral geometry. 1H NMR spectra of 8-10 in CD3CN show signals for the coordinated diketonate moiety, and signals suggesting ligand exchange reactivity leading to the formation of a small amount of [(bpy)3Co](ClO4)2 (11) in solution. While 8-10 are air stable at room temperature, illumination at 350 nm results in oxidative cleavage reactivity within the diketonate moiety leading to the formation of 1,3-diphenylpropanetrione, benzoic acid, benzoic anhydride, and benzil. Illumination of 8 under 18O2 results in a high level of 18O incorporation (>80%) in the benzoate anion. The product mixture, high level of 18O incorporation, and additional mechanistic studies suggest a reaction sequence wherein light-induced reactivity leads to the formation of a triketone intermediate that undergoes either oxidative C-C bond cleavage or benzoyl migration promoted by a bipyridine-ligated Co(II) or Co(III) fragment.

Flavonol dioxygenase chemistry mediated by a synthetic nickel superoxide

Nature exploits transition metal centers to enhance and tune the oxidizing power of natural oxidants such as O₂ and H₂O₂. The design and interrogation of synthetic metallocomplexes with similar reactivity to metalloproteins provides one strategy for gaining insight into the mechanistic underpinnings of oxygen-activating enzymes such as oxidases, oxygenases, and dioxygenases like Ni-quercetinase (Ni-QueD). Ni-QueD catalyzes the oxidative ring opening of the polyphenol quercetin, a natural product with antioxidant properties. Herein, we report the synthesis and characterization of Ni(13-DOB), a Ni(II) species complexed by an N4-macrocycle that has been characterized by single crystal X-ray crystallography. Ni(13-DOB) forms a Ni-superoxide intermediate (Ni(13-DOB)O₂^•-) upon treatment with H₂O₂ and Et₃N, as verified by resonance Raman spectroscopy. We demonstrate through UV/vis and LCMS that Ni(13-DOB)O₂^•- is capable of the 1-electron oxidation of flavonols, including both 3-hydroxyflavone (3-HF, the simplest flavonol) and quercetin itself. Incorporation of two O-atoms into the flavonol radical via superoxide from Ni(13-DOB)O₂^•- precedes oxidative cleavage of the flavonol scaffold in each case, consistent with quercetinase ring cleavage by Ni-QueD in Streptomyces sp. FLA. Conversion of 3-HF into 2-hydroxybenzoylbenzoic acid was accomplished with catalytic turnover of Ni(13-DOB) at ambient temperature, as confirmed by HPLC timecourses and GCMS analysis of isotopic labeling studies. The Ni(13-DOB)-mediated oxidative cleavage of quercetin to the corresponding biomimetic phenolic ester was also verified through ¹⁸O-isotopic labeling studies. Through the HPLC characterization of both on- and off-pathway products of flavonol dioxygenation by Ni(13-DOB)O₂^•-, the stringent reaction pathway control provided by enzyme active sites is highlighted.

An artificial metalloprotein with metal-adaptive coordination sites and Ni-dependent quercetinase activity

Engineering non-native metal active sites into proteins using canonical amino acids offers many advantages but is hampered by significant challenges. The TIM barrel protein, imidazole glycerol phosphate synthase from the hyperthermophilic organism Thermotoga maritima (tHisF), is well-suited for the construction of artificial metalloenzymes by this approach. To this end, we have generated a tHisF variant (tHisF^EHH) with a Glu/His/His motif for metal ion coordination. Crystal structures of Zn^II:tHisF^EHH and Ni^II:tHisF^EHH reveal that both metal ions bind to the engineered histidines. However, the two metals bind at distinct sites with different geometries, demonstrating the adaptability of tHisF. Only Zn^II additionally ligates the Glu residue and adopts a tetrahedral geometry. The pseudo-octahedral Ni^II site comprises the two His and a native Ser residue. Ni^II:tHisF^EHH catalyzes the oxidative cleavage of the flavanols quercetin and myricetin, providing an unprecedented example of an artificial metalloprotein with quercetinase activity.