Formation of New Non-oxido Vanadium(IV) Species in Aqueous Solution and in the Solid State by Tridentate (O, N, O) Ligands and Rationalization of Their EPR Behavior

Molecular Structures and Redox Properties of Homoleptic Aluminum(III) Complexes with Azobisphenolate (azp) Ligands

To elucidate the oxidation behavior of the 2,2′-azobisphenolate (azp) ligand, a series of homoleptic 1:2 AlIII complexes of four azp derivatives (L1) with 5,5′-dichloro-, 5,5′-dimethyl-, 5,5′-di-t-butyl-, 3,3′,5,5′-tetramethyl-substituents and of one imino derivative (L2) were synthesized and obtained as TPP[Al(L)2]·solvent (TPP = tetraphenylphosphonium ion). The X-ray crystal structure analyses showed that the two ONO-tridentate ligands were meridionally coordinated to a central AlIII ion in an almost perpendicular manner to give a homoleptic octahedral coordination structure in all the AlIII complexes. The proton nuclear magnetic resonance spectra suggested that all the AlIII complexes retained the homoleptic coordination structure in solution. From the cyclic voltammetry measurements in dichloromethane solutions, all the AlIII complexes with the azp ligands showed two partially reversible oxidation waves, and an additional reversible or partially reversible reduction wave. The substitution effects on the first oxidation and reduction peak potentials were revealed in the AlIII complexes with the azp ligands. On the other hand, the imino complex showed a partially reversible oxidation wave accompanying a film deposition. The density functional theory (DFT) calculations indicated that the molecular orbital (MO) coefficients of the frontier MOs in the AlIII complexes were present on the ligands and were absent on the AlIII ion. These results confirmed that the azp ligands are susceptible to oxidation and can give a relatively stable oxidation species depending upon substituent effects.

Vanadium(IV) Complexes with Methyl-Substituted 8-Hydroxyquinolines: Catalytic Potential in the Oxidation of Hydrocarbons and Alcohols with Peroxides and Biological Activity

Methyl-substituted 8-hydroxyquinolines (Hquin) were successfully used to synthetize five-coordinated oxovanadium(IV) complexes: [VO(2,6-(Me)₂-quin)₂] (1), [VO(2,5-(Me)₂-quin)₂] (2) and [VO(2-Me-quin)₂] (3). Complexes 1-3 demonstrated high catalytic activity in the oxidation of hydrocarbons with H₂O₂ in acetonitrile at 50 °C, in the presence of 2-pyrazinecarboxylic acid (PCA) as a cocatalyst. The maximum yield of cyclohexane oxidation products attained was 48%, which is high in the case of the oxidation of saturated hydrocarbons. The reaction leads to the formation of a mixture of cyclohexyl hydroperoxide, cyclohexanol and cyclohexanone. When triphenylphosphine is added, cyclohexyl hydroperoxide is completely converted to cyclohexanol. Consideration of the regio- and bond-selectivity in the oxidation of n-heptane and methylcyclohexane, respectively, indicates that the oxidation proceeds with the participation of free hydroxyl radicals. The complexes show moderate activity in the oxidation of alcohols. Complexes 1 and 2 reduce the viability of colorectal (HCT116) and ovarian (A2780) carcinoma cell lines and of normal dermal fibroblasts without showing a specific selectivity for cancer cell lines. Complex 3 on the other hand, shows a higher cytotoxicity in a colorectal carcinoma cell line (HCT116), a lower cytotoxicity towards normal dermal fibroblasts and no effect in an ovarian carcinoma cell line (order of magnitude HCT116 > fibroblasts > A2780).

Thermodynamic Study of Oxidovanadium(IV) with Kojic Acid Derivatives: A Multi-Technique Approach

The good chelating properties of hydroxypyrone (HPO) derivatives towards oxidovanadium(IV) cation, VIVO2+, constitute the precondition for the development of new insulin-mimetic and anticancer compounds. In the present work, we examined the VIVO2+ complex formation equilibria of two kojic acid (KA) derivatives, L4 and L9, structurally constituted by two kojic acid units linked in position 6 through methylene diamine and diethyl-ethylenediamine, respectively. These chemical systems have been characterized in solution by the combined use of various complementary techniques, as UV-vis spectrophotometry, potentiometry, NMR and EPR spectroscopy, ESI-MS spectrometry, and DFT calculations. The thermodynamic approach allowed proposing a chemical coordination model and the calculation of the complex formation constants. Both ligands L4 and L9 form 1:1 binuclear complexes at acidic and physiological pHs, with various protonation degrees in which two KA units coordinate each VIVO2+ ion. The joined use of different techniques allowed reaching a coherent vision of the complexation models of the two ligands toward oxidovanadium(IV) ion in aqueous solution. The high stability of the formed species and the binuclear structure may favor their biological action, and represent a good starting point toward the design of new pharmacologically active vanadium species.

New VIV, VIVO, VVO, and VVO2 Systems: Exploring their Interconversion in Solution, Protein Interactions, and Cytotoxicity

The synthesis and characterization of one oxidoethoxidovanadium(V) [V^VO(L¹)(OEt)] (1) and two nonoxidovanadium(IV) complexes, [V^IV(L^2-3)₂] (2 and 3), with aroylhydrazone ligands incorporating naphthalene moieties, are reported. The synthesized oxido and nonoxido vanadium complexes are characterized by various physicochemical techniques, and their molecular structures are solved by single crystal X-ray diffraction (SC-XRD). This revealed that in 1 the geometry around the vanadium atom corresponds to a distorted square pyramid, with a O₄N coordination sphere, whereas that of the two nonoxido V^IV complexes 2 and 3 corresponds to a distorted trigonal prismatic arrangement with a O₄N₂ coordination sphere around each "bare" vanadium center. In aqueous solution, the V^VO moiety of 1 undergoes a change to V^VO₂ species_, yielding [V^VO₂(L¹)]^- (1'), while the nonoxido V^IV-compounds 2 and 3 are partly converted into their corresponding V^IVO complexes, [V^IVO(L^2-3)(H₂O)] (2' and 3'). Interaction of these V^VO₂, V^IVO, and V^IV systems with two model proteins, ubiquitin (Ub) and lysozyme (Lyz), is investigated through docking approaches, which suggest the potential binding sites: the interaction is covalent for species 2' and 3', with the binding to Glu16, Glu18, and Asp21 for Ub, and His15 for Lyz, and it is noncovalent for species 1', 2, and 3, with the surface residues of the proteins. The ligand precursors and complexes are also evaluated for their in vitro antiproliferative activity against ovarian (A2780) and prostate (PC3) human cancer cells and in normal fibroblasts (V79) to check the selectivity of the compounds for cancer cells.

Simultaneous formation of non-oxidovanadium(iv) and oxidovanadium(v) complexes incorporating phenol-based hydrazone ligands in aerobic conditions

A family of non-oxidovanadium(iv) complexes incorporating multidentate hydrazone ligands were synthesized through a thermodynamically unfavourable process along with oxidovanadium(v) species.

Chemistry of mixed-ligand oxidovanadium(IV) complexes of aroylhydrazones incorporating quinoline derivatives: Study of solution behavior, theoretical evaluation and protein/DNA interaction

A series of eight hexacoordinated mixed-ligand oxidovanadium(IV) complexes [VO(L^x)(L^N-N)] (1-8), where L^x = L¹ - L⁴ are four differently substituted ONO donor aroylhydrazone ligands and L^N-N are N,N-donor bases like 2,2'-bipyridine (bipy) (1, 3, 5 and 7) and 1,10-phenanthroline (phen) (2, 4, 6 and 8), have been reported. All synthesized complexes have been characterized by various physicochemical techniques and molecular structures of 1 and 6 were determined by X-ray crystallography. With a view to evaluate the biological activity of the V^IVO species, the behavior of the systems V^IVO²⁺/L^x, V^IVO²⁺/L^x/bipy and V^IVO²⁺/L^x/phen was studied as a function of pH in a mixture of H₂O/DMSO 50/50 (v/v). DFT calculations allowed finding out the relative stability of the tautomeric forms of the ligands, and predicting the structure of vanadium complexes and their EPR parameters. To study their interaction with proteins, firstly the ternary systems V^IVO²⁺/L^1,2 with 1-methylimidazole, which is a good model for histidine binding, were examined. Subsequently the interaction of the complexes with lysozyme (Lyz), cytochrome c (Cyt) and bovine serum albumin (BSA) was studied. The results indicate that the complexes showed moderate binding affinity towards BSA, while no interaction takes place with lysozyme and cytochrome c. This could be explained with the higher number of accessible coordinating and polar residues for BSA than for Lyz and Cyt. Further, the complexes were also evaluated for their DNA binding propensity through UV-vis absorption titration and fluorescence spectral studies. These results were consistent with BSA binding affinity and showed moderate binding affinity towards CT-DNA.

Interaction of Vanadium(IV) Species with Ubiquitin: A Combined Instrumental and Computational Approach

The interaction of V^IVO²⁺ ion and five V^IVOL₂ compounds with potential pharmacological application, where L indicates maltolate (ma), kojate (koj), acetylacetonate (acac), 1,2-dimethyl-3-hydroxy-4(1 H)-pyridinonate (dhp), and l-mimosinate (mim), with ubiquitin (Ub) was studied by EPR, ESI-MS, and computational (docking and DFT) methods. The free metal ion V^IVO²⁺ interacts with Glu, Asp, His, Thr, and Leu residues, but the most stable sites (named 1 and 2) involve the coordination of (Glu16, Glu18) and (Glu24, Asp52). In the system with V^IVOL₂ compounds, the type of binding depends on the vanadium concentration. When the concentration is in the mM range, the binding occurs with cis-VOL₂(H₂O), L = ma, koj, dhp, and mim, or with VO(acac)₂: in the first case, the equatorial coordination of His68, Glu16, Glu18, or Asp21 residues yields species with formula n[VOL₂]-Ub where n = 2-3, while with VO(acac)₂ only noncovalent surface interactions are revealed. When the concentration of V is on the order of micromolar, the mono-chelated species VOL(H₂O)₂⁺ with L = ma, koj, acac, dhp, and mim, favored by the hydrolysis, interact with Ub, and adducts with composition n[VOL]-Ub ( n = 1-2) are observed with the contemporaneous coordination of (Glu18, Asp21) or (Glu16, Glu18), and (Glu24, Asp52) or (Glu51, Asp52) donors. The results of this work suggest that the combined application of spectroscopic, spectrometric, and computational techniques allow the complete characterization of the ternary systems formed by a V compound and a model protein such as ubiquitin. The same approach can be applied, eventually changing the spectroscopic/spectrometric techniques, to study the interaction of other metal species with other proteins.

Integrated ESI-MS/EPR/computational characterization of the binding of metal species to proteins: vanadium drug–myoglobin application

An integrated strategy based on ESI-MS spectrometry, EPR spectroscopy and docking/QM computational methods is applied to the systems formed by V^IVO²⁺ ions and four potential V^IVOL₂ drugs and myoglobin. This approach is generizable to other metals and proteins.

Synthesis, structure and biological evaluation of mixed ligand oxidovanadium(iv) complexes incorporating 2-(arylazo)phenolates

Synthesis and characterization of mixed ligand oxidovanadium(iv) complexes [V^IVO(L^1–4)(L^NN)] incorporating arylazo ligands: evaluation of DNA/BSA interaction and cytotoxicity activity.

Effect of secondary interactions, steric hindrance and electric charge on the interaction of VIVO species with proteins

The effect of secondary interactions (hydrogen bonds and van der Waals contacts), steric hindrance and electric charge, on the binding of V^IV complexes formed by pipemidic and 8-hydroxyquinoline-5-sulphonic acids with ubiquitin and lysozyme is studied.

VIVO complexes with antibacterial quinolone ligands and their interaction with serum proteins

Quinolone derivatives are among the most commonly prescribed antibacterials in the world and could also attract interest as organic ligands in the design of metal complexes with potential pharmacological activity. In this study, five compounds, belonging to the first (nalidixic acid or Hnal), second (ciprofloxacin or Hcip, and norfloxacin or Hnor) and third generation (levofloxacin or Hlev, and sparfloxacin or Hspar) of quinolones, were used as ligands to bind the V^IVO²⁺ ion. In aqueous solution, mono- and bis-chelated species were formed as a function of pH, with cis-[VOH_xL₂(H₂O)]^x+ and [VOH_xL₂]^x+, x = 0-2, being the major complexes at pH 7.4. DFT calculations indicate that the most stable isomers are the octahedral OC-6-32 and the square pyramidal SPY-5-12, in equilibrium with each other. To the best of our knowledge, this is the first case that an equilibrium between a penta-coordinated square pyramidal complex and a hexa-coordinated octahedral complex is observed in solution for ligands forming six-membered chelated rings. Nalidixic acid forms the solid compound [VO(nal)₂(H₂O)], to which a cis-octahedral geometry was assigned. The interaction with 1-methylimidazole (MeIm) causes a shift of the equilibrium SPY-5 + H₂O ⇄ OC-6 toward the right after the formation of cis-[VOH_xL₂(MeIm)]^x+, where MeIm replaces an equatorial water ligand. The study of the systems containing [VO(nal)₂(H₂O)] and the serum proteins - albumin (HSA), apo-transferrin (apo-hTf) and holo-transferrin (holo-hTf) - indicates that HSA and holo-hTf form the mixed species {VO(nal)₂}_y(HSA) and {VO(nal)₂}_y(holo-hTf), where y = 1-3 denotes the number of VO(nal)₂ moieties bound to accessible histidines (His105, His367, His510 for HSA, and His25, His349, His606 for holo-hTf), whereas apo-hTf yields VO(nal)₂(apo-hTf) with the coordination of the His289 residue only. Docking calculations suggest that the specific conformation of apo-hTf and the steric hindrance of the cis-VO(nal)₂ moiety interfere with its interaction with all the surface His residues and the formation of a hydrogen bond network which could stabilize the binding sites.

Speciation in human blood of Metvan, a vanadium based potential anti-tumor drug

The first report on the anti-cancer activity of the compound Metvan, [V^IVO(Me₂phen)₂(SO₄)], where Me₂phen is 4,7-dimethyl-1,10-phenanthroline, dates back to 2001. Although it was immediately identified as one of the most promising multitargeted anti-cancer V compounds, no development on the medical experimentation was carried out. One of the possible reasons is the lack of information on its speciation in aqueous solution and its thermodynamic stability, factors which influence the transport in the blood and the final form which reaches the target organs. To fill this gap, in this work the speciation of Metvan in aqueous solution and human blood was studied by instrumental (EPR, electronic absorption spectroscopy, ESI-MS and ESI-MS/MS), analytical (pH-potentiometry) and computational (DFT) methods. The results suggested that Metvan transforms at physiological pH into the hydrolytic species cis-[VO(Me₂phen)₂(OH)]⁺ and that both citrate and proteins (transferrin and albumin in the blood serum, and hemoglobin in the erythrocytes) form mixed complexes, denoted [VO(Me₂phen)(citrH_-1)]^2- and VO-Me₂phen-Protein with the probable binding of His-N donors. The measurements with erythrocytes suggest that Metvan is able to cross their membrane forming mixed species VO-Me₂phen-Hb. The redox stability in cell culture medium was also examined, showing that ca. 60% is oxidized to V^V after 5 h. Overall, the speciation of Metvan in the blood mainly depends on the V concentration: when it is larger than 50 μM, [VO(Me₂phen)(citrH_-1)]^2- and VO-Me₂phen-Protein are the major species, while for concentrations lower than 10 μM, (VO)(hTf) is formed and Me₂phen is lost. Therefore, it is plausible that the pharmacological activity of Metvan could be due to the synergic action of free Me₂phen, and V^IVO and V^VO/V^VO₂ species.

Origin of the unusually strong and selective binding of vanadium by polyamidoximes in seawater

Amidoxime-functionalized polymeric adsorbents are the current state-of-the-art materials for collecting uranium (U) from seawater. However, marine tests show that vanadium (V) is preferentially extracted over U and many other cations. Herein, we report a complementary and comprehensive investigation integrating ab initio simulations with thermochemical titrations and XAFS spectroscopy to understand the unusually strong and selective binding of V by polyamidoximes. While the open-chain amidoxime functionalities do not bind V, the cyclic imide-dioxime group of the adsorbent forms a peculiar non-oxido V⁵⁺ complex, exhibiting the highest stability constant value ever observed for the V⁵⁺ species. XAFS analysis of adsorbents following deployment in environmental seawater confirms V binding solely by the imide-dioximes. Our fundamental findings offer not only guidance for future optimization of selectivity in amidoxime-based sorbent materials, but may also afford insight to understanding the extensive accumulation of V in some marine organisms.

Biorelevant reactions of the potential anti-tumor agent vanadocene dichloride

The interaction of the potential anti-tumor agent vanadocene dichloride ([Cp2VCl2] or VDC) with some relevant bioligands of the cytosol such as proteins (Hb), amino acids (glycine and histidine), NADH derivatives (NADH, NADPH, NAD(+) and NADP(+)), reductants (GSH and ascorbic acid), phosphates (HPO4(2-), P2O7(4-), cAMP, AMP, ADP and ATP) and carboxylate derivatives (lactate) and its uptake by red blood cells were studied. The results indicated that [Cp2VCl2] transforms at physiological pH into [Cp2V(OH)2] and that only HPO4(2-), P2O7(4-), lactate, ATP and ADP form mixed species with the [Cp2V](2+) moiety replacing the two hydroxide ions. EPR and electronic absorption spectroscopy, agarose gel electrophoresis and spin trapping measurements allow excluding any direct interaction and/or intercalation with DNA and the formation of reactive oxygen species (ROS) in Fenton-like reactions. Uptake experiments by erythrocytes suggested that VDC crosses the membrane and enters inside the cells, whereas 'bare' V(IV) transforms into V(IV)O species with loss of the two cyclopentadienyl rings. This transformation in the cellular environment could be related to the mechanism of action of VDC.

Elucidation of Binding Site and Chiral Specificity of Oxidovanadium Drugs with Lysozyme through Theoretical Calculations

This study presents an implementation of the protein-ligand docking program GOLD and a generalizable method to predict the binding site and orientation of potential vanadium drugs. Particularly, theoretical methods were applied to the study of the interaction of two V^IVO complexes with antidiabetic activity, [V^IVO(pic)₂(H₂O)] and [V^IVO(ma)₂(H₂O)], where pic is picolinate and ma is maltolate, with lysozyme (Lyz) for which electron paramagnetic resonance spectroscopy suggests the binding of the moieties VO(pic)₂ and VO(ma)₂ through a carboxylate group of an amino acid residue (Asp or Glu). The work is divided in three parts: (1) the generation of a new series of parameters in GOLD program for vanadium compounds and the validation of the method on five X-ray structures of V^IVO and V^V species bound to proteins; (2) the prediction of the binding site and enantiomeric preference of [VO(pic)₂(H₂O)] to lysozyme, for which the X-ray diffraction analysis displays the interaction of a unique isomer (i.e., OC-6-23-Δ) through Asp52 residue, and the subsequent refinement of the results with quantum mechanics/molecular mechanics methods; (3) the application of the same approach to the interaction of [VO(ma)₂(H₂O)] with lysozyme. The results show that convenient implementation of protein-ligand docking programs allows for satisfactorily reproducing X-ray structures of metal complexes that interact with only one coordination site with proteins and predicting with blind procedures relevant low-energy binding modes. The results also demonstrate that the combination of docking methods with spectroscopic data could represent a new tool to predict (metal complex)-protein interactions and have a general applicability in this field, including for paramagnetic species.

Exploring the effect of hydroxylic and non-hydroxylic solvents on the reaction of [VIVO(β-diketonate)2] with 2-aminobenzoylhydrazide in aerobic and anaerobic conditions

Refluxing [V^IVO(β-diketonate)₂], namely [V^IVO(acetylacetonate)₂] and [V^IVO(benzoylacetonate)₂], separately with an equivalent or excess amount of 2-aminobenzoylhydrazide (ah) in laboratory grade (LG) CH₃OH in aerobic conditions afforded non-oxidovanadium(iv) and oxidovanadium(v) complexes of the type [V^IV(L¹)₂] (1), [V^VO(L¹)(OCH₃)]₂ (3) and [V^IV(L²)₂] (2), and [V^VO(L²)(OCH₃)] (4), respectively. (L¹)^2- and (L²)^2- represent the dianionic forms of 2-aminobenzoylhydrazone of acetylacetone (H₂L¹) and benzoylacetone (H₂L²), respectively, (general abbreviation, H₂L), which was formed by the in situ condensation of ah with the respective coordinated [β-diketonate] in medium-to-good yield. The yield of different resulting products was dependent upon the ratio of ah to [V^IVO(β-diketonate)₂]. For example, the yield of 1 and 2 complexes increased significantly associated with a decrease in the amount of 3 and 4 with an increase in the molar ratio of ah. Upon replacing CH₃OH by a non-hydroxylic solvent, LG CHCl₃, the above reaction yielded only oxidovanadium(v) complexes of the type [V^VO(L¹)(OH)]₂ (5), [V^VO(L²)(OH)] (6) and [VO₃(L)₂] (7, 8) whereas, upon replacing CHCl₃ by another non-hydroxylic solvent, namely LG CH₃CN, only the respective [VO₃(L)₂] (7, 8) complex was isolated in 72-78% yield. However, upon performing the above reactions in the absence of air using dry CH₃OH or dry CHCl₃, only the respective [V^IV(L)₂] complex was obtained, suggesting that aerial oxygen was the oxidising agent and the type of pentavalent product formed was dependent upon the nature of solvent used. Complexes 3 and 4 were converted, respectively, to 7 and 8 on refluxing in LG CHCl₃via the respective unstable complex 5 and 6. The DFT calculated change in internal energy (ΔE) for the reactions 2[V^VO(L²)(OCH₃)] + 2H₂O → 2[V^VO(L²)(OH)] + 2CH₃OH and 2[V^VO(L²)(OH)] → [VO₃(L²)₂] + H₂O was, respectively, +3.61 and -7.42 kcal mol^-1, suggesting that the [V^VO(L²)(OH)] species was unstable and readily transformed to the stable [VO₃(L²)₂] complex. Upon one-electron reduction at an appropriate potential, each of 7 and 8 generated mixed-valence [(L)V^VO-(μ-O)-OV^IV(L)]^- species, which showed valence-delocalisation at room temperature and localisation at 77 K. Some of the complexes showed a wide range of toxicity in a dose-dependent manner against lung cancer cells comparable with that observed with cis-platin.

A trigonal prismatic anionic iron(iii) complex of a radical o-iminobenzosemiquinonate derivative: structural and spectral analyses

The complex [Fe^III(L²⁻˙)₂]⁻is a rare example of a trigonal prismatic Fe(iii) complex with twoS= 1/2 radical isoquinone ligands showing strong antiferromagnetic coupling.

Nonoxido V(IV) Complexes: Prediction of the EPR Spectrum and Electronic Structure of Simple Coordination Compounds and Amavadin

Density functional theory (DFT) calculations of the (51)V hyperfine coupling (HFC) tensor A have been completed for 20 "bare" V(IV) complexes with different donor sets, electric charges, and coordination geometries. Calculations were performed with ORCA and Gaussian software, using functionals BP86, TPSS0, B1LYP, PBE0, B3LYP, B3P, B3PW, O3LYP, BHandHLYP, BHandH, and B2PLYP. Among the basis sets, 6-311g(d,p), 6-311++g(d,p), VTZ, cc-pVTZ, def2-TZVPP, and the "core properties" CP(PPP) were tested. The experimental Aiso and Ai (where i = x or z, depending on the geometry and electronic structure of V(IV) complex) were compared with the values calculated by DFT methods. The results indicated that, based on the mean absolute percentage deviation (MAPD), the best functional to predict Aiso or Ai is the double hybrid B2PLYP. With this functional and the basis set VTZ, it is possible to predict the Aiso and Az of the EPR spectrum of amavadin with deviations of -1.1% and -2.0% from the experimental values. The results allowed us to divide the spectra of nonoxido V(IV) compounds in three types-called "type 1", "type 2", and "type 3", characterized by different composition of the singly occupied molecular orbital (SOMO) and relationship between the values of Ax, Ay, and Az. For "type 1" spectra, Az ≫ Ax ≈ Ay and Az is in the range of (135-155) × 10(-4) cm(-1); for "type 2" spectra, Ax ≈ Ay ≫ Az and Ax ≈ Ay are in the range of (90-120) × 10(-4) cm(-1); and for the intermediate spectra of "type 3", Az > Ay > Ax or Ax > Ay > Az, with Az or Ax values in the range of (120-135) × 10(-4) cm(-1). The electronic structure of the V(IV) species was also discussed, and the results showed that the values of Ax or Az are correlated with the percent contribution of V-dxy orbital in the SOMO. Similarly to V(IV)O species, for amavadin the SOMO is based mainly on the V-dxy orbital, and this accounts for the large experimental value of Az (153 × 10(-4) cm(-1)).

Behavior of the potential antitumor V(IV)O complexes formed by flavonoid ligands. 3. Antioxidant properties and radical production capability

The radical production capability and the antioxidant properties of some V(IV)O complexes formed by flavonoid ligands were examined. In particular, the bis-chelated species of quercetin (que), [VO(que)2](2-), and morin (mor), [VO(mor)2], were evaluated for their capability to reduce the stable radical 1,1-diphenyl-2-picrylhydrazyl (DPPH) and produce the hydroxyl radical (•)OH by Fenton-like reactions, where the reducing agent is V(IV)O(2+). The results were compared with those displayed by other V(IV)O complexes, such as [VO(H2O)5](2+), [VO(acac)2] (acac=acetylacetonate) and [VO(cat)2](2-) (cat=catecholate). The capability of the V(IV)O flavonoids complexes to reduce DPPH is much larger than that of the V(IV)O species formed by non-antioxidant ligands and it is due mainly to the flavonoid molecule. Through the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trapping assay of the hydroxyl radical it was possible to demonstrate that in acidic solution V(IV)O(2+) has an effectiveness in producing (•)OH radicals comparable to that of Fe(2+). When V(IV)O complexes of flavonoids were taken into account, the amount of hydroxyl radicals produced in Fenton-like reactions depends on the specific structure of the ligand and on their capability to reduce H2O2 to give (•)OH. Both the formation of reactive oxygen species (ROS) under physiological conditions by V(IV)O complexes of flavonoid ligands and their radical scavenging capability can be put in relationship with their antitumor effectiveness and it could be possible to modulate these actions by changing the features of the flavonoid coordinated to the V(IV)O(2+) ion, such as the entity, nature and position of the substituents and the number of phenolic groups.

Chemistry of Monomeric and Dinuclear Non-Oxido Vanadium(IV) and Oxidovanadium(V) Aroylazine Complexes: Exploring Solution Behavior

A series of mononuclear non-oxido vanadium(IV) [V(IV)(L(1-4))2] (1-4), oxidoethoxido vanadium(V) [V(V)O(L(1-4))(OEt)] (5-8), and dinuclear μ-oxidodioxidodivanadium(V) [V(V)2O3(L(1))2] (9) complexes with tridentate aroylazine ligands are reported [H2L(1) = 2-furoylazine of 2-hydroxy-1-acetonaphthone, H2L(2) = 2-thiophenoylazine of 2-hydroxy-1-acetonaphthone, H2L(3) = 1-naphthoylazine of 2-hydroxy-1-acetonaphthone, H2L(4) = 3-hydroxy-2-naphthoylazine of 2-hydroxy-1-acetonaphthone]. The complexes are characterized by elemental analysis, by various spectroscopic techniques, and by single-crystal X-ray diffraction (for 2, 3, 5, 6, 8, and 9). The non-oxido V(IV) complexes (1-4) are quite stable in open air as well as in solution, and DFT calculations allow predicting EPR and UV-vis spectra and the electronic structure. The solution behavior of the [V(V)O(L(1-4))(OEt)] compounds (5-8) is studied confirming the formation of at least two different types of V(V) species in solution, monomeric corresponding to 5-8, and μ-oxidodioxidodivanadium [V(V)2O3(L(1-4))2] compounds. The μ-oxidodioxidodivanadium compound [V(V)2O3(L(1))2] (9), generated from the corresponding mononuclear complex [V(V)O(L(1))(OEt)] (5), is characterized in solution and in the solid state. The single-crystal X-ray diffraction analyses of the non-oxido vanadium(IV) compounds (2 and 3) show a N2O4 binding set and a trigonal prismatic geometry, and those of the V(V)O complexes 5, 6, and 8 and the μ-oxidodioxidodivanadium(V) (9) reveal that the metal center is in a distorted square pyramidal geometry with O4N binding sets. For the μ-oxidodioxidodivanadium species in equilibrium with 5-8 in CH2Cl2, no mixed-valence complexes are detected by chronocoulometric and EPR studies. However, upon progressive transfer of two electrons, two distinct monomeric V(IV)O species are detected and characterized by EPR spectroscopy and DFT calculations.

Behavior of the potential antitumor V(IV)O complexes formed by flavonoid ligands. 2. Characterization of sulfonate derivatives of quercetin and morin, interaction with the bioligands of the plasma and preliminary biotransformation studies

The biotransformation in the plasma and red blood cells of two potential antitumor V(IV)O complexes formed by flavonoid ligands (quercetin or que and morin or mor) and their sulfonic derivatives (quercetin-5'-sulfonic acid or que(S) and morin-5'-sulfonic acid or mor(S)) was studied by spectroscopic (EPR, Electron Paramagnetic Resonance) and computational (DFT, Density Functional Theory) methods. Que and que(S) form with V(IV)O stable complexes, and in the systems with apo-transferrin (apo-hTf) and albumin (HSA) VO(que)2 and VO(que(S))2 remain unchanged. VO(mor)2 and VO(mor(S))2 undergo displacement reactions to give the partial formation of (VO)x(HSA) and (VO)(apo-hTf)/(VO)2(apo-hTf); moreover, mor(S) forms with apo-transferrin and albumin mixed species VO-mor(S)-apo-hTf and VO-mor(S)-HSA. In the systems with apo-hTf and HSA anisotropic EPR spectra at room temperature are detected in which the protein is not directly coordinated to V(IV)O(2+) ion. This is explained assuming that the bis-chelated complexes interact strongly with the proteins through a network of hydrogen bonds with the polar groups present on the protein surface. It is suggested that this "indirect" transport of V(IV)O species could be common to all the species containing ligands which can interact with the blood proteins. Uptake experiments by red blood cells were also carried out, using vanadium concentration of 5.0×10(-4)M and incubation time in the range 0-160min. VO(que)2/VO(que(S))2 and VO(mor)2/VO(mor(S))2 cross the erythrocytes membrane and in the cytosol VO(que)2/VO(que(S))2 do not transform, whereas VO(mor)2/VO(mor(S))2 give the partial formation of mixed species with hemoglobin (Hb) and other V(IV)O complexes.

Characterization and biotransformation in the plasma and red blood cells of V(IV)O(2+) complexes formed by ceftriaxone

The coordination mode and geometry in aqueous solution of oxidovanadium(IV) complexes formed by a third-generation cephalosporin, ceftriaxone (H3cef), were studied by spectroscopic (EPR, electron paramagnetic resonance), pH-potentiometric and computational (DFT, density functional theory) methods. The behavior of the model systems containing 6-hydroxy-2-methyl-3-thioxo-3,4-dihydro-1,2,4-triazine-5(2H)-one (H2hmtdt) and 3-benzylthio-6-hydroxy-2-methyl-1,2,4-triazine-5(2H)-one (Hbhmt) was examined for comparison. The stability of the tautomers of ceftriaxone and 6-hydroxy-2-methyl-3-thioxo-3,4-dihydro-1,2,4-triazine-5(2H)-one in the neutral, mono- and bi-anionic form was calculated by DFT methods, both in the gas phase and in aqueous solution, and the electron density on the oxygen atoms of the hydroxytriazinone ring was related to the pKa of the ligands. The data demonstrate that ceftriaxone coordinates V(IV)O(2+) forming mono- and bis-chelated complexes with (Oket, O(-)) donor set and formation of five-membered chelate rings. The geometry of the bis-chelated complex, cis-[VO(Hcef)2(H2O)](2-), is cis-octahedral and this species can deprotonate, around physiological pH, to form the corresponding mono-hydroxido cis-[VO(Hcef)2(OH)](3-). The interaction of cis-[VO(Hcef)2(H2O)](2-) with apo-transferrin (apo-hTf) was studied and the results suggest that V(IV)O(2+) distributes between (VO)apo-hTf/(VO)2apo-hTf and cis-[VO(Hcef)2(H2O)](2-), whereas mixed complexes are not formed for charge and steric effects. The interaction of cis-[VO(Hcef)2(H2O)](2-) with red blood cells shows that ceftriaxone helps V(IV)O(2+) ion to cross the erythrocyte membrane. Inside the cell cis-[VO(Hcef)2(H2O)](2-) decomposes and the same species formed by inorganic V(IV)O(2+) are observed. The relationship between the biotransformation in the plasma and red blood cells and the potential pharmacological activity of V(IV)O(2+) species of ceftriaxone is finally discussed.

Synthesis and characterization of V(IV)O complexes of picolinate and pyrazine derivatives. Behavior in the solid state and aqueous solution and biotransformation in the presence of blood plasma proteins

Oxidovanadium(IV) complexes with 5-cyanopyridine-2-carboxylic acid (HpicCN), 3,5-difluoropyridine-2-carboxylic acid (HpicFF), 3-hydroxypyridine-2-carboxylic acid (H2hypic), and pyrazine-2-carboxylic acid (Hprz) have been synthesized and characterized in the solid state and aqueous solution through elemental analysis, IR and EPR spectroscopy, potentiometric titrations, and DFT simulations. The crystal structures of the complexes (OC-6-23)-[VO(picCN)2(H2O)]·2H2O (1·2H2O), (OC-6-24)-[VO(picCN)2(H2O)]·4H2O (2·4H2O), (OC-6-24)-Na[VO(Hhypic)3]·H2O (4), and two enantiomers of (OC-6-24)-[VO(prz)2(H2O)] (Λ-5 and Δ-5) have been determined also by X-ray crystallography. 1 presents the first crystallographic evidence for the formation of a OC-6-23 isomer for bis(picolinato) V(IV)O complexes, whereas 2, 4, and 5 possess the more common OC-6-24 arrangement. The strength order of the ligands is H2hypic ≫ HpicCN > Hprz > HpicFF, and this results in a different behavior at pH 7.40. In organic and aqueous solution the three isomers OC-6-23, OC-6-24, and OC-6-42 are formed, and this is confirmed by DFT simulations. In all the systems with apo-transferrin (VO)2(apo-hTf) is the main species in solution, with the hydrolytic V(IV)O species becoming more important with lowering the strength of the ligand. In the systems with albumin, (VO)(x)HSA (x = 5, 6) coexists with VOL2(HSA) and VOL(HSA)(H2O) when L = picCN, prz, with [VO(Hhypic)(hypic)](-), [VO(hypic)2](2-), and [(VO)4(μ-hypic)4(H2O)4] when H2hypic is studied, and with the hydrolytic V(IV)O species when HpicFF is examined. Finally, the consequence of the hydrolysis on the binding of V(IV)O(2+) to the blood proteins, the possible uptake of V species by the cells, and the possible relationship with the insulin-enhancing activity are discussed.

Structural and redox requirements for the action of anti-diabetic vanadium compounds

This study presents the first systematic investigation of the anti-diabetic properties of non-oxido V(IV) complexes. In particular, the insulin-mimetic activity of [V(IV)(taci)2](4+), [V(IV)(inoH-3)2](2-), [V(IV)(dhab)2], [V(IV)(hyph(Ph))2], [V(IV)(cat)3](2-) and [V(IV)(pdbh)2]--where taci is 1,3,5-triamino-1,3,5-trideoxy-cis-inositol, ino is cis-inositol, H2dhab is 2,2'-dihydroxyazobenzene, H2hyph(Ph) is 3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazole, H2cat is catechol and H2pdbh is pentan-2,4-dione benzoylhydrazone--was evaluated in terms of free fatty acid (FFA) release. Among the six compounds examined, only [V(IV)(pdbh)2], [V(IV)(cat)3](2-) and [V(IV)(hyph(Ph))2], which at the physiological pH convert to the corresponding V(IV)O complexes, were found to exhibit a significant insulin-mimetic activity compared to VOSO4. In contrast, [V(taci)2](4+), [V(inoH-3)2](2-) and [V(dhab)2], which at pH 7.4 keep their 'bare' non-oxido structure, did not cause any inhibition of FFA. The results, therefore, suggest that a V(IV)O functionality is necessary for vanadium complexes to exhibit anti-diabetic effects. This agrees with the notion that the biotransformations of V compounds in the organism are more important than the nature of the species.

Interaction of antidiabetic vanadium compounds with hemoglobin and red blood cells and their distribution between plasma and erythrocytes

The interaction of V(IV)O(2+) ion with hemoglobin (Hb) was studied with the combined application of spectroscopic (EPR), spectrophotometric (UV-vis), and computational (DFT methods) techniques. Binding of Hb to V(IV)O(2+) in vitro was proved, and three unspecific sites (named α, β, and γ) were characterized, with the probable coordination of His-N, Asp-O(-), and Glu-O(-) donors. The value of log β for (VO)Hb is 10.4, significantly lower than for human serum apo-transferrin (hTf). In the systems with V(IV)O potential antidiabetic compounds, mixed species cis-VOL2(Hb) (L = maltolate (ma), 1,2-dimethyl-3-hydroxy-4(1H)-pyridinonate (dhp)) are observed with equatorial binding of an accessible His residue, whereas no ternary complexes are observed with acetylacetonate (acac). The experiments of uptake of [VO(ma)2], [VO(dhp)2], and [VO(acac)2] by red blood cells indicate that the neutral compounds penetrate the erythrocyte membrane through passive diffusion, and percent amounts higher than 50% are found in the intracellular medium. The biotransformation of [VO(ma)2], [VO(dhp)2], and [VO(acac)2] inside the red blood cells was proved. [VO(dhp)2] transforms quantitatively in cis-VO(dhp)2(Hb), [VO(ma)2] in cis-VO(ma)2(Hb), and cis-VO(ma)2(Cys-S(-)), with the equatorial coordination of a thiolate-S(-) of GSH or of a membrane protein, and [VO(acac)2] in the binary species (VO)xHb and two V(IV)O complexes with formulation VO(L(1),L(2)) and VO(L(3),L(4)), where L(1), L(2), L(3), and L(4) are red blood cell bioligands. The results indicate that, in the studies on the transport of a potential pharmacologically active V species, the interaction with red blood cells and Hb cannot be neglected, that a distribution between the erythrocytes and plasma is achieved, and that these processes can significantly influence the effectiveness of a V drug.

Interaction of insulin-enhancing vanadium compounds with human serum holo-transferrin

The interaction of VO(2+) ion and four insulin-enhancing compounds, [VO(ma)2], [VO(dhp)2], [VO(acac)2], and cis-[VO(pic)2(H2O)], where Hma, Hdhp, Hacac, and Hpic are maltol, 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone, acetylacetone, and picolinic acid, with holo-transferrin (holo-hTf) was studied through the combined application of electron paramagnetic resonance (EPR) and density functional theory (DFT) methods. Since in holo-hTf all of the specific binding sites of transferrin are saturated by Fe(3+) ions, VO(2+) can interact with surface sites (here named sites C), probably via the coordination of His-N, Asp-COO(-), and Glu-COO(-) donors. In the ternary systems with the insulin-enhancing compounds, mixed species are observed with Hma, Hdhp, and Hpic with the formation of VOL2(holo-hTf), explained through the interaction of cis-[VOL2(H2O)] (L = ma, dhp) or cis-[VOL2(OH)](-) (L = pic) with an accessible His residue that replaces the monodentate H2O or OH(-) ligand. The residues of His-289, His-349, His-473, and His-606 seem the most probable candidates for the complexation of the cis-VOL2 moiety. The lack of a ternary complex with Hacac was attributed to the square-pyramidal structure of [VO(acac)2], which does not possess equatorial sites that can be replaced by the surface His-N. Since holo-transferrin is recognized by the transferrin receptor, the formation of ternary complexes between VO(2+) ion, a ligand L(-), and holo-hTf may be a way to transport vanadium compounds inside the cells.