α-Hydroxy coordination of mononuclear vanadyl citrate, malate and S-citramalate with N-heterocycle ligand, implying a new protonation pathway of iron-vanadium cofactor in nitrogenase

Protonated and deprotonated vanadyl imidazole tartrates for the mimics of the vanadium coordination in the FeV-cofactor of V-nitrogenase

Chiral imidazole-based oxidovanadium tartrates (H2im)2[Δ,Λ-VIV2O2(R,R-H2tart)(R,R-tart)(Him)2]·Him (1, H4tart = tartaric acid, Him = imidazole) and [Λ,Λ-VIV2O2(R,R-tart)(Him)6]·4H2O (2) and their corresponding enantiomers (H2im)2[Λ,Δ-VIV2O2(S,S-H2tart)(S,S-tart)(Him)2]·Him (3) and [Δ,Δ-VIV2O2(S,S-tart)(Him)6]·4H2O (4) were obtained in alkaline solutions. Interestingly, the tartrates chelate with vanadium bidentately through α-alkoxy/α-hydroxy and α-carboxy groups and imidazole coordinates monodentately through nitrogen atom. It is worth noting that complexes 1 and 3 contain both protonated α-hydroxy and deprotonated α-alkoxy groups simultaneously, which have short V-Oα-alkoxy distances [1.976(4)av Å in 1-4] and long V-Oα-hydroxy distances [2.237(3)av Å in 1 and 2.230(2)av Å in 3]. There is an interesting strong intramolecular hydrogen bond [O(11)⋯O(1) 2.731(5) Å] between the two parts in 1 and 3. The protonated V-O distances are closer to the average bond distance in reported FeV-cofactors (FeV-cos, V-Oα-alkoxy 2.156av Å) in VFe proteins, which corresponds to the feasible protonation of coordinated α-hydroxy in R-homocitrate in V-nitrogenase, showing the homocitrate in the mechanistic model for nitrogen reduction as a secondary proton donor. Furthermore, vibrational circular dichroism (VCD) and IR spectra of 1-4 pointed out the disparity between the characteristic vibrations of the C-O and C-OH groups clearly. EPR experiment and theoretical calculations support +4 oxidation states for vanadium in 1-4. Solution 13C {1H} NMR spectra and CV analyses exhibited the solution properties for 1 and 2, respectively, which indicates that there should be a rapid exchange equilibrium between the protonated and deprotonated species in solutions.

Heteroleptic oxidovanadium(IV)-malate complex improves glucose uptake in HepG2 and enhances insulin action in streptozotocin-induced diabetic rats

Diabetes mellitus, a complex and heterogeneous disease associated with hyperglycemia, is a leading cause of mortality and reduces life expectancy. Vanadium complexes have been studied for the treatment of diabetes. The effect of complex [VO(bpy)(mal)]·H₂O (complex A) was evaluated in a human hepatocarcinoma (HepG2) cell line and in streptozotocin (STZ)-induced diabetic male Wistar rats conditioned in seven groups with different treatments (n = 10 animals per group). Electron paramagnetic resonance and ⁵¹V NMR analyses of complex A in high-glucose Dulbecco's Modified Eagle Medium (DMEM) revealed the oxidation and hydrolysis of the oxidovanadium(IV) complex over a period of 24 h at 37 °C to give low-nuclearity vanadates "V1" (H₂VO₄^-), "V2" (H₂V₂O₇^2-), and "V4" (V₄O₁₂^4-). In HepG2 cells, complex A exhibited low cytotoxic effects at concentrations 2.5 to 7.5 μmol L^-1 (IC₅₀ 10.53 μmol L^-1) and increased glucose uptake (2-NBDG) up to 93%, an effect similar to insulin. In STZ-induced diabetic rats, complex A at 10 and 30 mg kg^-1 administered by oral gavage for 12 days did not affect the animals, suggesting low toxicity or metabolic impairment during the experimental period. Compared to insulin treatment alone, complex A (30 mg kg^-1) in association with insulin was found to improve glycemia (30.6 ± 6.3 mmol L^-1 vs. 21.1 ± 8.6 mmol L^-1, respectively; p = 0.002), resulting in approximately 30% additional reduction in glycemia. The insulin-enhancing effect of complex A was associated with low toxicity and was achieved via oral administration, suggesting the potential of complex A as a promising candidate for the adjuvant treatment of diabetes.

Exploring Anticancer Activities and Structure-Activity Relationships of Binuclear Oxidovanadium(IV) Complexes

Dimeric mixed-ligand oxidovanadium complexes [V₂O₂(1,3-pdta)(bpy)₂]·9H₂O (1) and [V₂O₂(1,3-pdta)(phen)₂]·6H₂O (2) feature a symmetric binuclear structure bridged by 1,3-pdta, which is different from our previous reported asymmetric binuclear complex [V₂O₂(edta)(phen)₂]·11H₂O (3).In this study, a wide range of analytical techniques were carried out to fully characterize the complexes 1 and 2 and further investigate their structural stabilities. Density functional theory calculations of 1 and 2 also suggest that they might have good reactivity with biomolecules as anticancer agents. To assess and screen the antitumor activities of compounds 1-3 together with their four corresponding monomeric complexes [VO(ida)(phen)], [VO(ida)(bpy)], [VO(OH)(phen)₂]Cl, and [VO(Hedta)]^-, we have performed in vitro experiments with hepatocellular carcinoma HepG2 and SMMC-7721 cell lines by MTT analyses. Complex 2 was found to have the highest inhibitory potency against the growth of HepG2 and SMMC-7721 cells (IC₅₀ = 2.07 ± 0.72 μM for HepG2; 13.00 ± 3.06 μM for SMMC-7721) compared to other compounds. The structure-activity relationship studies showed that the antitumor effect of compound 2 is higher than that of other compounds. After studying the monomeric compounds of 1-3, their effects were also ranked. Moreover, complex 2 displayed stronger binding affinity toward calf thymus DNA (K_b = 5.71 × 10⁴ M^-1) and cleavage activities than the other complexes (K_b = 1.34 × 10⁴ M^-1 for 1 and 5.22 × 10⁴ M^-1 for 3, respectively). We further extended the cellular mechanisms of drug action and found that 2 could block DNA synthesis and cell division of HepG2 and 7721 cells and further induce apoptosis by flow cytometry assays. In short, these results indicate that binuclear oxidovanadium compounds could have potential as simple, effective, and safe antitumor agents.

Large Hydrogen Isotope Fractionation Distinguishes Nitrogenase-Derived Methane from Other Methane Sources

Biological nitrogen fixation is catalyzed by the enzyme nitrogenase. Two forms of this metalloenzyme, the vanadium (V)- and iron (Fe)-only nitrogenases, were recently found to reduce small amounts of carbon dioxide (CO2) into the potent greenhouse gas methane (CH4). Here, we report carbon (13C/12C) and hydrogen (2H/1H) stable isotopic compositions and fractionations of methane generated by V- and Fe-only nitrogenases in the metabolically versatile nitrogen fixer Rhodopseudomonas palustris The stable carbon isotope fractionation imparted by both forms of alternative nitrogenase are within the range observed for hydrogenotrophic methanogenesis (13αCO2/CH4 = 1.051 ± 0.002 for V-nitrogenase and 1.055 ± 0.001 for Fe-only nitrogenase; values are means ± standard errors). In contrast, the hydrogen isotope fractionations (2αH2O/CH4 = 2.071 ± 0.014 for V-nitrogenase and 2.078 ± 0.018 for Fe-only nitrogenase) are the largest of any known biogenic or geogenic pathway. The large 2αH2O/CH4 shows that the reaction pathway nitrogenases use to form methane strongly discriminates against 2H, and that 2αH2O/CH4 distinguishes nitrogenase-derived methane from all other known biotic and abiotic sources. These findings on nitrogenase-derived methane will help constrain carbon and nitrogen flows in microbial communities and the role of the alternative nitrogenases in global biogeochemical cycles.IMPORTANCE All forms of life require nitrogen for growth. Many different kinds of microbes living in diverse environments make inert nitrogen gas from the atmosphere bioavailable using a special enzyme, nitrogenase. Nitrogenase has a wide substrate range, and, in addition to producing bioavailable nitrogen, some forms of nitrogenase also produce small amounts of the greenhouse gas methane. This is different from other microbes that produce methane to generate energy. Until now, there was no good way to determine when microbes with nitrogenases are making methane in nature. Here, we present an isotopic fingerprint that allows scientists to distinguish methane from microbes making it for energy versus those making it as a by-product of nitrogen acquisition. With this new fingerprint, it will be possible to improve our understanding of the relationship between methane production and nitrogen acquisition in nature.

Isolated Mixed-Valence Iron Vanadium Malate and Its Metal Hydrates (M = Fe2+, Cu2+, Zn2+) with Reversible and Irreversible Adsorptions for Oxygen

Isolated octanuclear iron-vanadium malate (NH₄)₃(CH₃NH₃)₃[Fe^III₂V^IV₂V^V₄O₁₁(mal)₆]·7.5H₂O (1; H₃mal = malic acid) and its family of metal hydrates M'_3n[M^II(H₂O)₂]_1.5n[Fe^III₂V^IV₂V^V₄O₁₁(mal)₆]_n·xnH₂O (2 or 2-Fe, M' = NH₄⁺, M = Fe, x = 7.5; 3 or 3-Cu, M' = K⁺, M = Cu, x = 10; 4 or 4-Zn, M' = K⁺, M = Zn, x = 6.5) have been obtained by self-assembly in water. The cluster anion [Fe₂V₆O₁₁(mal)₆]^6- (1a) shows an interesting iron bicapped-triangular-prismatic structure, which is bridged by M²⁺ hydrates (M = Fe, Cu, Zn) to construct isostructural metal organic frameworks (MOFs) 2-4. The mixed-valence vanadium systems in 1-4 were determined by theoretical bond valence calculations (BVS) and charge balance. The magnetic susceptibilities are further elucidated as high spin for Fe³⁺ in 1a and bridging Fe²⁺ in 2-Fe, respectively. A strong ferromagnetic interaction was also observed for 2-Fe at 3 K. 2-Fe, 3-Cu, and 4-Zn have similar hydrophilic channels with diameters of 6.8, 6.5, and 6.6 Å, respectively, which show obvious affinity for O₂ in comparison with no adsorption of N₂, H₂, CO₂, and CH₄ at room temperature under different pressures. Moreover, 2-Fe and 4-Zn exhibit irreversible O₂ absorptions, which may be attributed to charge transfer between O₂ and open metal sites (OMSs) formed during vacuum heating pretreatment. UV-vis and EPR spectra show a change in electronic structure of 2-Fe after O₂ adsorption. The reversible adsorption observed in 3-Cu suggests a weak interaction between O₂ and Cu²⁺ due to the Jahn-Teller effect. The properties of gas adsorption provide an insight into the performances of small molecules in the channels constructed by synthetic octanuclear model compounds, which are related to the interactions between the gas substrate and the heterometal cluster in biology.

Bond-valence analyses of the crystal structures of FeMo/V cofactors in FeMo/V proteins

The bond-valence method has been used for valence calculations of FeMo/V cofactors in FeMo/V proteins using 51 crystallographic data sets of FeMo/V proteins from the Protein Data Bank. The calculations show molybdenum(III) to be present in MoFe₇S₉C(Cys)(HHis)[R-(H)homocit] (where H₄homocit is homocitric acid, HCys is cysteine and HHis is histidine) in FeMo cofactors, while vanadium(III) with a more reduced iron complement is obtained for FeV cofactors. Using an error analysis of the calculated valences, it was found that in FeMo cofactors Fe1, Fe6 and Fe7 can be unambiguously assigned as iron(III), while Fe2, Fe3, Fe4 and Fe5 show different degrees of mixed valences for the individual Fe atoms. For the FeV cofactors in PDB entry 5n6y, Fe4, Fe5 and Fe6 correspond to iron(II), iron(II) and iron(III), respectively, while Fe1, Fe2, Fe3 and Fe7 exhibit strongly mixed valences. Special situations such as CO-bound and selenium-substituted FeMo cofactors and O(N)H-bridged FeV cofactors are also discussed and suggest rearrangement of the electron configuration on the substitution of the bridging S atoms.

Novel bidentate oxovanadium(IV) glycolate, α-hydroxybutyrate and citrate with terpyridine and their conversions to nitrosyl products

A series of monomeric α-hydroxycarboxylato oxovanadium(IV) complexes [VO(H₂cit)(tpy)]·H₂O (1) (H₄cit = citric acid, tpy = 2,2':6',2-terpyridine), [VO(glyc)(tpy)]·5.5H₂O (2) (H₂glyc = glycolic acid) and [VO(α-hbut)(tpy)]·3H₂O (3) (α-H₂hbut = α-hydroxybutyratic acid) have been obtained from the reactions of vanadyl sulfate with α-hydroxycarboxylates and terpyridine in acidic solutions. These complexes feature bidentate citrate, glycolate or α-hydroxybutyrate respectively. The ligand chelates to vanadium atom through α-hydroxy (in 1) or α-alkoxy (in 2 and 3) and α-carboxy groups, while β-carboxy groups of citrate in 1 are free to participate strong hydrogen bonds with neighboring citrate. With comparable chelation, 1 shares a similar V-O_α-hydroxy distance [2.168(1) Å] with that observed in FeV-cofactor [2.17 Å] [1]. Moreover, nitrosyl vanadium complexes [V(NO)(glyc)(tpy)]·3H₂O (4) and [V(NO)(α-hbut)(tpy)]·4H₂O (5) were obtained via reductions of synthetic solutions of 2 and 3 with hydroxylamine respectively. The terminal oxygen atoms were substituted by linear nitrosyl groups in 4 and 5. They were fully characterized by UV-vis, IR, EPR spectra, X-ray structural analyses and theoretical bond valence calculations.

α-Lys424 Participates in Insertion of FeMoco to MoFe Protein and Maintains Nitrogenase Activity in Klebsiella oxytoca M5al

Our previous investigation of substrates reduction catalyzed by nitrogenase suggested that α-Ile⁴²³ of MoFe protein possibly functions as an electron transfer gate to Mo site of active center-“FeMoco”. Amino acid residue α-Lys⁴²⁴ connects directly to α-Ile⁴²³, and they are located in the same α-helix (α423-431). In the present study, function of α-Lys⁴²⁴ was investigated by replacing it with Arg (alkaline, like Lys), Gln (neutral), Glu (acidic), and Ala (neutral) through site-directed mutagenesis and homologous recombination. The mutants were, respectively, termed 424R, 424Q, 424E, and 424A. Studies of diazotrophic cell growth, cytological, and enzymatic properties indicated that none of the substitutions altered the secondary structure of MoFe protein, or normal expression of nifA, nifL, and nifD. Substitution of alkaline amino acid (i.e., 424R) maintained acetylene (C₂H₂) and proton (H⁺) reduction activities at normal levels similar to that of wild-type (WT), because its FeMoco content did not reduce. In contrast, substitution of acidic or neutral amino acid (i.e., 424Q, 424E, 424A) impaired the catalytic activity of nitrogenase to varying degrees. Combination of MoFe protein structural simulation and the results of a series of experiments, the function of α-Lys⁴²⁴ in ensuring insertion of FeMoco to MoFe protein was further confirmed, and the contribution of α-Lys⁴²⁴ in maintaining low potential of the microenvironment causing efficient catalytic activity of nitrogenase was demonstrated.

Preliminary Assignment of Protonated and Deprotonated Homocitrates in Extracted FeMo-Cofactors by Comparisons with Molybdenum(IV) Lactates and Oxidovanadium Glycolates

A similar pair of protonated and deprotonated mononuclear oxidovanadium glycolates [VO(Hglyc)(phen)(H₂O)]Cl·2H₂O (1) and [VO(glyc)(bpy)(H₂O)] (2) and a mixed-(de)protonated oxidovanadium triglycolate (NH₄)₂[VO(Hglyc)₂(glyc)]·H₂O (3) were isolated and examined. The ≡C-O(H) (≡C-OH or ≡C-O) groups coordinated to vanadium were spectroscopically and structurally identified. The glycolate in 1 features a bidentate chelation through protonated α-hydroxy and α-carboxy groups, whereas the glycolate in 2 coordinates through deprotonated α-alkoxy and α-carboxy groups. The glycolates in 3 coordinate to vanadium through α-alkoxy or α-hydroxy and α-carboxy groups and thus have both protonated ≡C-OH and deprotonated ≡C-O bonds simultaneously. Structural investigations revealed that the longer protonated V-O_α-hydroxy bonds [2.234(2) Å and 2.244(2) Å] in 1 and 3 are close to those of FeV-cofactor (FeV-co) 2.17 Ź (FeMo-co 2.17 Ų), while deprotonated V-O_α-alkoxy bonds [2, 1.930(2); 3, 1.927(2) Å] were obviously shorter. This shows a similar elongated trend as the Mo-O distances in the previously reported deprotonated vs protonated molybdenum lactates (Wang, S. Y. et al. Dalton Trans. 2018, 47, 7412-7421) and these vanadium and molybdenum complexes have the same local V/Mo-homocitrate structures as those of FeV/Mo-cos of nitrogenases. The IR spectra of these oxidovanadium and the previously synthesized molybdenum complexes including different substituted ≡C-O(H) model compounds show red-shifts for ≡C-OH vs ≡C-O alternation, which further assign the two IR bands of extracted FeMo-co at 1084 and 1031 cm^-1 to ≡C-O and ≡C-OH vibrations, respectively. Although the structural data or IR spectra for some of the previously synthesized Mo/V complexes and extracted FeMo-co were measured earlier, this is the first time that the ≡C-O(H) coordinated peaks are assigned. The overall structural and IR results well suggest the coexistence of homocitrates coordinated with α-alkoxy (deprotonated) and α-hydroxy (protonated) groups in the extracted FeMo-co.

Molybdenum imidazole citrate and bipyridine homocitrate in different oxidation states – balance between coordinated α-hydroxy and α-alkoxy groups

Oxo and thiomolybdenum(iv/vi) imidazole hydrocitrates K₂{Mo^IV₃O₄(im)₃[Mo^VIO₃(Hcit)]₂}·3im·4H₂O (1), (Him)₂{Mo^IV₃SO₃(im)₃[Mo^VIO₃(Hcit)]₂}·im·6H₂O (2), molybdenum(v) bipyridine homocitrate trans-[(Mo^VO)₂O(H₂homocit)₂(bpy)₂]·4H₂O (3) and molybdenum(vi) citrate (Et₄N)[Mo^VIO₂Cl(H₂cit)]·H₂O (4) (H₄cit = citric acid, H₄homocit = homocitric acid, im = imidazole and bpy = 2,2′-bipyridine) with different oxidation states were prepared. 1 and 2 are the coupling products of [Mo^VIO₃(Hcit)]³⁻ anions and incomplete cubane units [Mo^IV₃O₄]⁴⁺ ([Mo^IV₃SO₃]⁴⁺) with monodentate imidazoles, respectively, where tridentate citrates coordinate with α-hydroxy, α-carboxy and β-carboxy groups, forming pentanuclear skeleton structures. The molybdenum atoms in 1 and 2 show unusual +4 and +6 valences based on charge balances, theoretical bond valence calculations and Mo XPS spectrum. The coordinated citrates in 1 and 2 are protonated with α-hydroxy groups, while 3 and 4 with higher oxidation states of +5 and +6 are deprotonated with α-alkoxy group even under strong acidic condition, respectively. This shows the relationship between the oxidation state and protonation of the α-alkoxy group in citrate or homocitrate, which is related to the protonation state of homocitrate in FeMo-cofactor of nitrogenase. The homocitrate in 3 chelates to molybdenum(v) with bidentate α-alkoxy and monodentate α-carboxy groups. Molybdenum(vi) citrate 4 is only protonated with coordinated and uncoordinated β-carboxy groups. The solution behaviours of 1 and 2 are discussed based on ¹H and ¹³C NMR spectroscopies and cyclic voltammograms, showing no decomposition of the species.

Oxo and thiomolybdenum(iv/vi) citrates, molybdenum(v) homocitrate and molybdenum(vi) citrate were obtained, showing the influence of coordinated α-hydroxy and α-alkoxy groups with different oxidation states.

Decavanadate Inhibits Mycobacterial Growth More Potently Than Other Oxovanadates

51V NMR spectroscopy is used to document, using speciation analysis, that one oxometalate is a more potent growth inhibitor of two Mycobacterial strains than other oxovanadates, thus demonstrating selectivity in its interaction with cells. Historically, oxometalates have had many applications in biological and medical studies, including study of the phase-problem in X-ray crystallography of the ribosome. The effect of different vanadate salts on the growth of Mycobacterium smegmatis (M. smeg) and Mycobacterium tuberculosis (M. tb) was investigated, and speciation was found to be critical for the observed growth inhibition. Specifically, the large orange-colored sodium decavanadate (V10O 28 6 - ) anion was found to be a stronger inhibitor of growth of two mycobacterial species than the colorless oxovanadate prepared from sodium metavanadate. The vanadium(V) speciation in the growth media and conversion among species under growth conditions was monitored using 51V NMR spectroscopy and speciation calculations. The findings presented in this work is particularly important in considering the many applications of polyoxometalates in biological and medical studies, such as the investigation of the phase-problem in X-ray crystallography for the ribosome. The findings presented in this work investigate the interactions of oxometalates with other biological systems.

Synthesis, in vitro cytotoxicity, and structure-activity relationships (SAR) of multidentate oxidovanadium(iv) complexes as anticancer agents

Multidentate oxidovanadium(iv) complexes with different geometric configurations [VO(ox)(bpy)(H2O)] 1, [VO(ox)(phen)(H2O)] 2, [VO(ida)(bpy)]·2H2O 3, (phen)[VO(ida)(phen)]·4H2O 4, and (Hphen)[VO(H2O)(nta)]·2H2O 5 [ox = oxalic acid, bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, ida = iminodiacetic acid, nta = nitrilotriacetic acid] have been obtained from the reactions of oxidovanadium sulfate or vanadium pentoxide with oxalates, amino-polycarboxylates and N-heterocyclic ligands in neutral solution by the hydrothermal method, and have been fully characterized by elemental, thermogravimetric analyses and single crystal X-ray diffraction, as well as a wide range of spectroscopic techniques such as FT-IR, UV/Vis, NMR, ESI-MS. The anti-tumor properties of oxidovanadium compounds 1-5 were further evaluated in human HepG2 and SMMC-7721 hepatocellular carcinoma cell lines in vitro. The profiles of cytotoxicity, cell cycle distribution, as well as cell apoptosis upon test compound exposure, were determined by MTT and flow cytometry assays. Compound 2 exhibited a much higher anti-tumor activity than others. The IC50 values of 2 were 5.34 ± 0.034 μM and 29.07 ± 0.017 μM in SMMC-7721 and HepG2 cells after 48 h treatment, respectively. Furthermore, compound 2 could significantly arrest the cell cycle in the S and G2/M phases and further induce cell apoptosis in a dose-dependent manner. The structure-activity relationship (SAR) studies revealed that structural elements, for example, metal components, variations of coordination mode, labile water molecules, chelated ligands etc., probably exert an essential cooperative effect on the antitumor activity. In short, these findings not only provide an accessible model system to exploit V-based complexes as potential simple, safe and effective multifunctional antitumor agents, but also open up a rational approach to shed new light on the selection and optimization of ideal drug candidates.

Comparison of hydroxycarboxylato imidazole molybdenum(iv) complexes and nitrogenase protein structures: indirect evidence for the protonation of homocitrato FeMo-cofactors

Glycolato and lactato imidazole molybdenum(iv) complexes are used for structural comparison with FeMo-cofactors of MoFe-protein structures.