The equilibrium between the octahedral and square pyramidal form and the influence of an axial ligand on the molecular properties of V(IV)O complexes: a spectroscopic and DFT study

Experimental and Theoretical Investigation of the Mechanism of the Reduction of O2 from Air to O22- by VIVO2+-N,N,N-Amidate Compounds and Their Potential Use in Fuel Cells

The two-electron reductive activation of O2 to O22- is of particular interest to the scientific community mainly due to the use of peroxides as green oxidants and in powerful fuel cells. Despite of the great importance of vanadium(IV) species to activate the two-electron reductive activation of O2, the mechanism is still unclear. Reaction of VIVO2+ species with the tridentate-planar N,N,N-carboxamide (ΗL) ligands in solution (CH3OH:H2O) under atmospheric O2, at room temperature, resulted in the quick formation of [VV(═O)(η2-O2)(κ3-L)(H2O)] and cis-[VV(═O)2(κ3-L)] compounds. Oxidation of the VIVO2+ complexes with the sterically hindered tridentate-planar N,N,N-carboxamide ligands by atmospheric O2 gave only cis-[VV(═O)2(κ3-L)] compounds. The mechanism of formation of [VV(═O)(η2-O2)(κ3-L)(H2O)] (I) and cis-[VV(═O)2(κ3-L)] (II) complexes vs time, from the interaction of [VIV(═O)(κ3-L)(Η2Ο)2]+ with atmospheric O2, was investigated with 51V, 1H NMR, UV-vis, cw-X-band EPR, and 18O2 labeling IR and resonance Raman spectroscopies revealing the formation of a stable intermediate (Id). EPR, MS, and theoretical calculations of the mechanism of the formation of I and II revealed a pathway, through a binuclear [VIV(═O)(κ3-L)(H2O)(η1,η1-O2)VIV(═O)(κ3-L)(H2O)]2+ intermediate. The results from cw-EPR, 1H NMR spectroscopies, cyclic voltammetry, and the reactivity of the complexes [VIV(═O)(κ3-L)(Η2Ο)2]+ toward O2 reduction fit better to an intermediate with a binuclear nature. Dynamic experiments in combination with computational calculations were undertaken to fully elucidate the mechanism of the O2 reduction to O22- by [VIV(═O)(κ3-L)(Η2Ο)2]+. The galvanic cell {Zn|VIII,VII||Id, [VIVO(κ3-L)(H2O)2]+|O2|C(s)} was manufactured, demonstrating the important applicability of this new chemistry to Zn|H2O2 fuel cells technology generating H2O2 in situ from the atmospheric O2.

A Combined Experimental and Theoretical Investigation of Oxidation Catalysis by cis-[VIV(O)(Cl/F)(N4)]+ Species Mimicking the Active Center of Metal-Enzymes

Reaction of VIVOCl2 with the nonplanar tetradentate N4 bis-quinoline ligands yielded four oxidovanadium(IV) compounds of the general formula cis-[VIV(O)(Cl)(N4)]Cl. Sequential treatment of the two nonmethylated N4 oxidovanadium(IV) compounds with KF and NaClO4 resulted in the isolation of the species with the general formula cis-[VIV(O)(F)(N4)]ClO4. In marked contrast, the methylated N4 oxidovanadium(IV) derivatives are inert toward KF reaction due to steric hindrance, as evidenced by EPR and theoretical calculations. The oxidovanadium(IV) compounds were characterized by single-crystal X-ray structure analysis, cw EPR spectroscopy, and magnetic susceptibility. The crystallographic characterization showed that the vanadium compounds have a highly distorted octahedral coordination environment and the d(VIV-F) = 1.834(1) Å is the shortest to be reported for (oxido)(fluorido)vanadium(IV) compounds. The experimental EPR parameters of the VIVO2+ species deviate from the ones calculated by the empirical additivity relationship and can be attributed to the axial donor atom trans to the oxido group and the distorted VIV coordination environment. The vanadium compounds act as catalysts toward alkane oxidation by aqueous H2O2 with moderate ΤΟΝ up to 293 and product yields of up to 29% (based on alkane); the vanadium(IV) is oxidized to vanadium(V), and the ligands remain bound to the vanadium atom during the catalysis, as determined by 51V and 1H NMR spectroscopies. The cw X-band EPR studies proved that the mechanism of the catalytic reaction is through hydroxyl radicals. The chloride substitution reaction in the cis-[VIV(O)(Cl)(N4)]+ species by fluoride and the mechanism of the alkane oxidation were studied by DFT calculations.

Antioxidant and Anticancer Activities and Protein Interaction of the Oxidovanadium(IV) Naringin Complex

The complex of oxidovanadium(IV) with naringin (Narg) [VO(Narg)2] 8H2O (VONarg) was prepared according to the literature improving the synthetic procedure and physicochemical characterization. In addition, biological activities (cytotoxic, antioxidant, and BSA interaction) were determined. The metal coordinated through the 5-hydroxy and 4-carbonyl groups of rings A and C of naringin, respectively. The antioxidant activity of VONarg, determined in vitro, was higher than those of the flavonoid against superoxide and peroxyl reactive oxygen species (ROS) and DPPH radical. The cytotoxic properties were determined by a MTT assay on adenocarcinoma human alveolar basal epithelial cells (A549). VONarg exerted a 20% decrease in cancer cells viability at 24 h incubation, while naringin and oxidovanadium(IV) cation did not show cytotoxicity. Measurements with the normal HEK293 cell line showed that the inhibitory action of the complex is selective. VONarg generated intracellular reactive oxygen species (ROS), depletion of reduced glutathione and depolarization of mitochondrial membrane potential, typical for apoptotic pathway, producing cell death by oxidative stress mechanism. Moreover, naringin interacted with bovine serum albumin (BSA) through hydrophobic interactions in a spontaneous process, and VONarg showed greater affinity for the protein but can still be transported and delivered by it (Ka 104 L·mol−1 order).

Influence of temperature on the equilibria of oxidovanadium(IV) complexes in solution

The equilibria in the solution of three different oxidovanadium(IV) complexes, VO(dhp)₂ (dhp = 1,2-dimethyl-3-hydroxy-4(1H)-pyridinonato), VO(ma)₂ (ma = maltolato) and VO(pic)₂(H₂O) (pic = picolinato), were examined in the temperature range of 120-352 K through a combination of instrumental (EPR spectroscopy) and computational techniques (DFT methods). The results revealed that a general equilibrium exists: VOL₂ + H₂O ⇄ cis-VOL₂(H₂O) ⇄ trans-VOL₂(H₂O), where cis and trans refer to the relative position of H₂O and the oxido ligand. The equilibrium is more or less shifted to the right depending on the ligand, the temperature, the ionic strength and the coordinating properties of the solvent. With VO(dhp)₂, only the square pyramidal species exists at 298 K in aqueous solution, while at 120 K the cis- and trans-VO(dhp)₂(H₂O) species are also present. The complex of maltol exists almost exclusively in the form cis-VO(ma)₂(H₂O) in aqueous solution at 298 K, while the trans species can be revealed only at higher temperatures, where the EPR linewidth significantly decreases. The equilibria involving 1-methylimidazole (MeIm), a model for the side chain His coordination, are also influenced by temperature, with its coordination being favored by decreasing the temperature. The implications of these results in the study of the (vanadium complex)-protein systems are discussed and the interaction with myoglobin (Mb) is examined as a representative example.

Rationalizing the Decavanadate(V) and Oxidovanadium(IV) Binding to G-Actin and the Competition with Decaniobate(V) and ATP

The experimental data collected over the past 15 years on the interaction of decavanadate(V) (V10O286-; V10), a polyoxometalate (POM) with promising anticancer and antibacterial action, with G-actin, were rationalized by using several computational approaches (docking, density functional theory (DFT), and molecular dynamics (MD)). Moreover, a comparison with the isostructural and more stable decaniobate(V) (Nb10O286-; Nb10) was carried out. Four binding sites were identified, named α, β, γ, and δ, the site α being the catalytic nucleotide site located in the cleft of the enzyme at the interface of the subdomains II and IV. It was observed that the site α is preferred by V10, whereas Nb10 is more stable at the site β; this indicates that, differently from other proteins, G-actin could contemporaneously bind the two POMs, whose action would be synergistic. Both decavanadate and decaniobate induce conformational rearrangements in G-actin, larger for V10 than Nb10. Moreover, the binding mode of oxidovanadium(IV) ion, VIVO2+, formed upon the reduction of decavanadate(V) by the -SH groups of accessible cysteine residues, is also found in the catalytic site α with (His161, Asp154) coordination; this adduct overlaps significantly with the region where ATP is bound, accounting for the competition between V10 and its reduction product VIVO2+ with ATP, as previously observed by EPR spectroscopy. Finally, the competition with ATP was rationalized: since decavanadate prefers the nucleotide site α, Ca2+-ATP displaces V10 from this site, while the competition is less important for Nb10 because this POM shows a higher affinity for β than for site α. A relevant consequence of this paper is that other metallodrug-protein systems, in the absence or presence of eventual inhibitors and/or competition with molecules of the organism, could be studied with the same approach, suggesting important elements for an explanation of the biological data and a rational drug design.

Structure of the human gonadotropin-releasing hormone receptor GnRH1R reveals an unusual ligand binding mode

Gonadotrophin-releasing hormone (GnRH), also known as luteinizing hormone-releasing hormone, is the main regulator of the reproductive system, acting on gonadotropic cells by binding to the GnRH1 receptor (GnRH1R). The GnRH-GnRH1R system is a promising therapeutic target for maintaining reproductive function; to date, a number of ligands targeting GnRH1R for disease treatment are available on the market. Here, we report the crystal structure of GnRH1R bound to the small-molecule drug elagolix at 2.8 Å resolution. The structure reveals an interesting N-terminus that could co-occupy the enlarged orthosteric binding site together with elagolix. The unusual ligand binding mode was further investigated by structural analyses, functional assays and molecular docking studies. On the other hand, because of the unique characteristic of lacking a cytoplasmic C-terminal helix, GnRH1R exhibits different microswitch structural features from other class A GPCRs. In summary, this study provides insight into the ligand binding mode of GnRH1R and offers an atomic framework for rational drug design.

Trinuclear vanadium(iv) and vanadium(v) complexes derived from 2,4,6-triacetylphloroglucinol and study of their peroxidase mimicking activity

Solid state and solution studies of trinuclear V^IVO- and V^VO₂-complexes and their peroxidase mimicking activity, through oxidation of dopamine to aminochrome, are reported.

New VIVO-complexes for oxidative desulfurization of refractory sulfur compounds in fuel: synthesis, structure, reactivity trend and mechanistic studies

A series of 5-coordinate oxidovanadium(iv) complexes based on 2-(2'-hydroxyphenyl)imidazole (HPIMH), with substituent groups of different electronegativities on the phenolic para position (HPIMX; X = -H, -Br, -OMe and -NO2), were synthesized and characterized. Three of these complexes were characterized by single crystal X-ray diffraction, [VIVO(PIMH)2], [VIVO(PIMBr)2] and [VIVO(PIMNO2)2], as well as a dioxidovanadium(v) compound ([VVO2(PIMH)(PIMH2)]). The complexes were tested for their catalytic activities in the oxidation of dibenzothiophene (DBT), the major refractory organosulfur compound found in fuel. The nitro substituted compound [VIVO(PIMNO2)2] had the highest catalytic oxidation activity followed by: [VIVO(PIMH)2] > [VIVO(PIMBr)2] > [VIVO(PIMMeO)2]. The decrease in activity is attributed to the different electronegativities of the substituent groups, which influence the electron density on the metal center, the V[double bond, length as m-dash]O bond distances and infrared stretching bands. Geometry index (τ) values calculated from single crystal X-ray diffraction (SC-XRD) data and DFT studies provided further insights on the trend in activity observed. SC-XRD, EPR, 51V NMR and UV-Vis spectroscopies, and DFT studies were instrumental in studying the mechanism of the catalyzed reaction and proposal of intermediate species. Both radical and non-radical pathways are plausible for the catalytic oxidation and participation of reactive oxygen species in both pathways is also postulated.

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.

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.

Naphthoylhydrazones: coordination to metal ions and biological screening

V^IVO, Cu^II and Zn^II complexes from three new naphthoylhydrazones were screened towards their ability to bind albumin and their cytotoxicity.

DFT Protocol for EPR Prediction of Paramagnetic Cu(II) Complexes and Application to Protein Binding Sites

With the aim to provide a general protocol to interpret electron paramagnetic resonance (EPR) spectra of paramagnetic copper(II) coordination compounds, density functional theory (DFT) calculations of spin Hamiltonian parameters g and A for fourteen Cu(II) complexes with different charges, donor sets, and geometry were carried out using ORCA software. The performance of eleven functionals was tested, and on the basis of the mean absolute percent deviation (MAPD) and standard deviation (SD), the ranking of the functionals for Az is: B3LYP > B3PW91 ~ B3P86 > PBE0 > CAM-B3LYP > TPSSh > BH and HLYP > B2PLYP > MPW1PW91 > ω-B97x-D >> M06; and for gz is: PBE0 > BH and HLYP > B2PLYP > ω-B97x-D > B3PW91~B3LYP~B3P86 > CAM-B3LYP > TPSSh~MPW1PW91 >> M06. With B3LYP the MAPD with respect to A z exp t l is 8.6% with a SD of 4.2%, while with PBE0 the MAPD with respect to g z exp t l is 2.9% with a SD of 1.1%. The results of the validation confirm the fundamental role of the second order spin-orbit contribution to Az. The computational procedure was applied to predict the values of gz and Az of the adducts formed by Cu(II) with albumin and two fragments of prion protein, 106–126 and 180–193.

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.

Synthesis, structure, solution behavior, reactivity and biological evaluation of oxidovanadium(iv/v) thiosemicarbazone complexes

The synthesis and characterization of an oxidovanadium(iv) [VIVO(L)(acac)] (1) and of two dioxidovanadium(v) [VVO2(L')] (2) and [VVO2(L)] (2a) complexes of the Schiff base formed from the reaction of 4-(p-fluorophenyl) thiosemicarbazone with pyridine-2-aldehyde (HL) are described. The oxidovanadium(iv) species [VIVO(L)(acac)] (1) was synthesized by the reaction of VIVO(acac)2 with the thiosemicarbazone HL in refluxing ethanol. The recrystallization of [VIVO(L)(acac)] (1) in DMF, CH3CN or EtOH gave the same product i.e. the dioxidovanadium(v) complex [VVO2(L)] (2a); however, upon recrystallization of 1 in DMSO a distinct compound [VVO2(L')] (2) was formed, wherein the original ligand L- is transformed to a rearranged one, L'-. In the presence of DMSO the ligand in complex 1 is found to undergo methylation at the carbon centre attached to imine nitrogen (aldimine) and transformed to the corresponding VVO2-species through in situ reaction. The synthesized HL and the metal complexes were characterized by elemental analysis, IR, UV-Vis, NMR and EPR spectroscopy. The molecular structure of [VVO2(L')] (2) was determined by single crystal X-ray crystallography. The methylation of various other ligands and complexes prepared from different vanadium precursors under similar reaction conditions was also attempted and it was confirmed that the imine methylation observed is both ligand and metal precursor specific. Complexes 1 and 2 show in vitro insulin-like activity against insulin responsive L6 myoblast cells, higher than VIVO(acac)2, with complex 1 being more potent. In addition, the in vitro cytotoxicity studies of HL, and of complexes 1 and 2 against the MCF-7 and Vero cell lines were also done. The ligand is not cytotoxic and complex 2 is significantly more cytotoxic than 1. DAPI staining experiments indicate that an increase in the time of incubation and an increase of concentration of the complexes lead to the increase in cell death.

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.

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.

Interaction of [VIV O(acac)2 ] with Human Serum Transferrin and Albumin

[VO(acac)₂ ] is a remarkable vanadium compound and has potential as a therapeutic drug. It is important to clarify how it is transported in blood, but the reports addressing its binding to serum proteins have been contradictory. We use several spectroscopic and mass spectrometric techniques (ESI and MALDI-TOF), small-angle X-ray scattering and size exclusion chromatography (SEC) to characterize solutions containing [VO(acac)₂ ] and either human serum apotransferrin (apoHTF) or albumin (HSA). DFT and modeling protein calculations are carried out to disclose the type of binding to apoHTF. The measured circular dichroism spectra, SEC and MALDI-TOF data clearly prove that at least two VO-acac moieties may bind to apoHTF, most probably forming [V^IV O(acac)(apoHTF)] complexes with residues of the HTF binding sites. No indication of binding of [VO(acac)₂ ] to HSA is obtained. We conclude that V^IV O-acac species may be transported in blood by transferrin. At very low complex concentrations speciation calculations suggest that [(VO)(apoHTF)] species form.

Speciation of potential anti-diabetic vanadium complexes in real serum samples

In this work the speciation in real serum samples of five V^IVO complexes with potential application in the therapy of diabetes was studied through EPR spectroscopy as a function of V concentration (45.4, 90.9 and 454.5μM) and time (0-180min). [VO(dhp)₂], [VO(ma)₂], [VO(acac)₂], [VO(pic)₂(H₂O)], and [VO(mepic)₂], where Hdhp indicates 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone, Hma maltol, Hacac acetylacetone, Hpic picolinic acid, and Hmepic 6-methylpicolinic acid, were examined. The distribution of V^IVO²⁺ among the serum bioligands was calculated from the thermodynamic stability constants in the literature and compared with the experimental results. EPR results, which confirm the prediction, depend on the strength of the ligand L and geometry assumed by the bis-chelated species at physiological pH, cis-octahedral or square pyramidal. With dhp, the strongest chelator, the system is dominated by [VO(dhp)₂] and/or cis-VO(dhp)₂(Protein); with intermediate strength chelators, i.e. maltolate, acetylacetonate and picolinate, by cis-VO(ma)₂(Protein), [VO(acac)₂] or [VO(pic)(citrH_-1)]^3-/[VO(pic)(lactH_-1)]^- (citr=citrate and lact=lactate) when the V concentration overcomes 100-200μM and by (VO)(hTf)/(VO)₂(hTf) when concentration is lower than 100μM; with the weakest chelator, 6-methylpicolinate, (VO)(hTf)/(VO)₂(hTf), (VO)(HSA) (hTf = human serum transferrin and HSA = human serum albumin), and VO(mepic)(Protein)(OH) are the major species at concentration higher than 100-200μM, whereas hydrolytic processes are observed for lower concentrations. For [VO(dhp)₂], [VO(ma)₂], [VO(acac)₂] and [VO(pic)₂(H₂O)], the EPR spectra remain unaltered with elapsing time, while for mepic they change significantly because the hydrolyzed V^IVO species are complexed by the serum bioligands, in particular by lactate. The rate of oxidation in the serum is [VO(dhp)₂]>[VO(ma)₂]>[VO(acac)₂] and reflects the order of E_1/2 values.

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)).

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.

Vanadium(IV/V) complexes of Triapine and related thiosemicarbazones: Synthesis, solution equilibrium and bioactivity

The stoichiometry and thermodynamic stability of vanadium(IV/V) complexes of Triapine and two related α(N)-heterocyclic thiosemicarbazones (TSCs) with potential antitumor activity have been determined by pH-potentiometry, EPR and (51)V NMR spectroscopy in 30% (w/w) dimethyl sulfoxide/water solvent mixtures. In all cases, mono-ligand complexes in different protonation states were identified. Dimethylation of the terminal amino group resulted in the formation of vanadium(IV/V) complexes with considerably higher stability. Three of the most stable complexes were also synthesized in solid state and comprehensively characterized. The biological evaluation of the synthesized vanadium complexes in comparison to the metal-free ligands in different human cancer cell lines revealed only minimal influence of the metal ion. Thus, in addition the coordination ability of salicylaldehyde thiosemicarbazone (STSC) to vanadium(IV/V) ions was investigated. The exchange of the pyridine nitrogen of the α(N)-heterocyclic TSCs to a phenolate oxygen in STSC significantly increased the stability of the complexes in solution. Finally, this also resulted in increased cytotoxicity activity of a vanadium(V) complex of STSC compared to the metal-free ligand.

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.

Oxidovanadium(IV/V) complexes as new redox mediators in dye-sensitized solar cells: a combined experimental and theoretical study

Corrosiveness is one of the main drawbacks of using the iodide/triiodide redox couple in dye-sensitized solar cells (DSSCs). Alternative redox couples including transition metal complexes have been investigated where surprisingly high efficiencies for the conversion of solar to electrical energy have been achieved. In this paper, we examined the development of a DSSC using an electrolyte based on square pyramidal oxidovanadium(IV/V) complexes. The oxidovanadium(IV) complex (Ph4P)2[V(IV)O(hybeb)] was combined with its oxidized analogue (Ph4P)[V(V)O(hybeb)] {where hybeb(4-) is the tetradentate diamidodiphenolate ligand [1-(2-hydroxybenzamido)-2-(2-pyridinecarboxamido)benzenato}and applied as a redox couple in the electrolyte of DSSCs. The complexes exhibit large electron exchange and transfer rates, which are evident from electron paramagnetic resonance spectroscopy and electrochemistry, rendering the oxidovanadium(IV/V) compounds suitable for redox mediators in DSSCs. The very large self-exchange rate constant offered an insight into the mechanism of the exchange reaction most likely mediated through an outer-sphere exchange mechanism. The [V(IV)O(hybeb)](2-)/[V(V)O(hybeb)](-) redox potential and the energy of highest occupied molecular orbital (HOMO) of the sensitizing dye N719 and the HOMO of [V(IV)O(hybeb)](2-) were calculated by means of density functional theory electronic structure calculation methods. The complexes were applied as a new redox mediator in DSSCs, while the cell performance was studied in terms of the concentration of the reduced and oxidized form of the complexes. These studies were performed with the commercial Ru-based sensitizer N719 absorbed on a TiO2 semiconducting film in the DSSC. Maximum energy conversion efficiencies of 2% at simulated solar light (AM 1.5; 1000 W m(-2)) with an open circuit voltage of 660 mV, a short-circuit current of 5.2 mA cm(-2), and a fill factor of 0.58 were recorded without the presence of any additives in the electrolyte.

Synthesis, characterization, X-ray crystal structure, DFT calculation and antibacterial activities of new vanadium(IV, V) complexes containing chelidamic acid and novel thiourea derivatives

Three new thiourea ligands derived from the condensation of aroyl- and aryl-isothiocyanate derivatives with 2,6-diaminopyridine, named 1,1'-(pyridine-2,6-diyl)bis(3-(benzoyl)thiourea) (L1), 1,1'-(pyridine-2,6-diyl)bis(3-(2-chlorobenzoyl)thiourea) (L2) and 1,1'-(pyridine-2,6-diyl)bis(3-(4-chlorophenyl)thiourea) (L3), their oxido-vanadium(IV) complexes, namely [VO(L1('))(H2O)] (C1), [VO(L2('))(H2O)] (C2) and [VO(L3('))(H2O)] (C3), and also, dioxo-vanadium(V) complex containing 4-hydroxy-2,6-pyridine dicarboxylic acid (chelidamic acid, H2dipic-OH) and metformin (N,N-dimethylbiguanide, Met), named [H2Met][VO2(dipic-OH)]2·H2O (C4), were synthesized and characterized by elemental analysis, FTIR and (1)H NMR and UV-visible spectroscopies. Proposed structures for free thiourea ligands and their vanadium complexes were corroborated by applying geometry optimization and conformational analysis. Solid state structure of complex [H2Met][VO2(dipic-OH)]2·H2O (triclinic, Pī) was fully determined by single crystal X-ray diffraction analysis. In this complex, metformin is double protonated and acted as counter ion. The antibacterial properties of these compounds were investigated in vitro against standard Gram-positive and Gram-negative bacterial strains. The experiments showed that vanadium(IV) complexes had the superior antibacterial activities than novel thiourea derivatives and vanadium(V) complex against all Gram-positive and Gram-negative bacterial strains.

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.