Medicinal applications of vanadium complexes with Schiff bases

Many transition metal complexes have been explored for their therapeutic properties after the discovery of cisplatin. Schiff bases have an efficient complexation tendency with the transition metals and several medicinal properties have been reported. However, fewer studies have reported the medicinal utility of vanadium and its Schiff base complexes. This paper provides a comprehensive overview of vanadium complexes with Schiff bases along with their mechanistic insight. Vanadium complexes in + 4 and + 5 oxidation states have exhibited well-defined geometry and found to be thermodynamically stable. The studies have reported the G0/G1 phase cell cycle arrest and decreased delta psi m, inducing mitochondrial membrane depolarization in cancer cell lines along with the alterations in the metabolism of the cancer cells upon dosing with the vanadium complexes. Cancer cell invasion and growth are also found to be markedly reduced by peroxo complexes of vanadium. The studies included in the review paper have been taken from leading indexing databases and focus was laid on recent reports in literature. The biological potential of vanadium complexes of Schiff bases opens new horizons for future interdisciplinary studies and investigation focussed on understanding the biochemistry of these complexes, along with designing new complexes which have better bioavailability, solubility and low or non-toxicity.

Multiple and Variable Binding of Pharmacologically Active Bis(maltolato)oxidovanadium(IV) to Lysozyme

The interaction with proteins of metal-based drugs plays a crucial role in their transport, mechanism, and activity. For an active ML_n complex, where L is the organic carrier, various binding modes (covalent and non-covalent, single or multiple) may occur and several metal moieties (M, ML, ML₂, etc.) may interact with proteins. In this study, we have evaluated the interaction of [V^IVO(malt)₂] (bis(maltolato)oxidovanadium(IV) or BMOV, where malt = maltolato, i.e., the common name for 3-hydroxy-2-methyl-4H-pyran-4-onato) with the model protein hen egg white lysozyme (HEWL) by electrospray ionization mass spectrometry, electron paramagnetic resonance, and X-ray crystallography. The multiple binding of different V-containing isomers and enantiomers to different sites of HEWL is observed. The data indicate both non-covalent binding of cis-[VO(malt)₂(H₂O)] and [VO(malt)(H₂O)₃]⁺ and covalent binding of [VO(H₂O)_3–4]²⁺ and cis-[VO(malt)₂] and other V-containing fragments to the side chains of Glu35, Asp48, Asn65, Asp87, and Asp119 and to the C-terminal carboxylate. Our results suggest that the multiple and variable interactions of potential V^IVOL₂ drugs with proteins can help to better understand their solution chemistry and contribute to define the molecular basis of the mechanism of action of these intriguing molecules.

The interaction of [V^IVO(malt)₂] (BMOV, malt = maltolato) with hen egg white lysozyme (HEWL) reveals the multiple binding of different V-containing isomers and enantiomers to different sites and non-covalent and covalent binding of cis-[VO(malt)₂(H₂O)], [VO(malt)(H₂O)₃]⁺, [VO(H₂O)₃₋₄]²⁺, and cis-[VO(malt)₂] to Glu, Asp, and Asn residues.

Spectroscopic/Computational Characterization and the X-ray Structure of the Adduct of the VIVO-Picolinato Complex with RNase A

The structure, stability, and enzymatic activity of the adduct formed upon the reaction of the V-picolinato (pic) complex [VIVO(pic)2(H2O)], with an octahedral geometry and the water ligand in cis to the V═O group, with the bovine pancreatic ribonuclease (RNase A) were studied. While electrospray ionization-mass spectrometry, circular dichroism, and ultraviolet-visible absorption spectroscopy substantiate the interaction between the metal moiety and RNase A, electron paramagnetic resonance (EPR) allows us to determine that a carboxylate group, stemming from Asp or Glu residues, and imidazole nitrogen from His residues are involved in the V binding at acidic and physiological pH, respectively. Crystallographic data demonstrate that the VIVO(pic)2 moiety coordinates the side chain of Glu111 of RNase A, by substituting the equatorial water molecule at acidic pH. Computational methods confirm that Glu111 is the most affine residue and interacts favorably with the OC-6-23-Δ enantiomer establishing an extended network of hydrogen bonds and van der Waals stabilizations. By increasing the pH around neutrality, with the deprotonation of histidine side chains, the binding of the V complex to His105 and His119 could occur, with that to His105 which should be preferred when compared to that to the catalytically important His119. The binding of the V compound affects the enzymatic activity of RNase A, but it does not alter its overall structure and stability.

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.

Misinterpretations in Evaluating Interactions of Vanadium Complexes with Proteins and Other Biological Targets

In aqueous media, VIV- and VV-ions and compounds undergo chemical changes such as hydrolysis, ligand exchange and redox reactions that depend on pH and concentration of the vanadium species, and on the nature of the several components present. In particular, the behaviour of vanadium compounds in biological fluids depends on their environment and on concentration of the many potential ligands present. However, when reporting the biological action of a particular complex, often the possibility of chemical changes occurring has been neglected, and the modifications of the complex added are not taken into account. In this work, we highlight that as soon as most vanadium(IV) and vanadium(V) compounds are dissolved in a biological media, they undergo several types of chemical transformations, and these changes are particularly extensive at the low concentrations normally used in biological experiments. We also emphasize that in case of a biochemical interaction or effect, to determine binding constants or the active species and/or propose mechanisms of action, it is essential to evaluate its speciation in the media where it is acting. This is because the vanadium complex no longer exists in its initial form.

ESI-MS Study of the Interaction of Potential Oxidovanadium(IV) Drugs and Amavadin with Model Proteins

In this study, the binding to lysozyme (Lyz) of four important VIV compounds with antidiabetic and/or anticancer activity, [VIVO(pic)2(H2O)], [VIVO(ma)2], [VIVO(dhp)2], and [VIVO(acac)2], where pic-, ma-, dhp-, and acac- are picolinate, maltolate, 1,2-dimethyl-3-hydroxy-4(1H)-pyridinonate, and acetylacetonate anions, and of the vanadium-containing natural product amavadin ([VIV(hidpa)2]2-, with hidpa3- N-hydroxyimino-2,2'-diisopropionate) was investigated by ElectroSpray Ionization-Mass Spectrometry (ESI-MS). Moreover, the interaction of [VIVO(pic)2(H2O)], chosen as a representative VIVO2+ complex, was examined with two additional proteins, myoglobin (Mb) and ubiquitin (Ub), to compare the data. The examined vanadium concentration was in the range 15-150 μM, i.e., very close to that found under physiological conditions. With pic-, dhp-, and hidpa3-, the formation of adducts n[VIVOL2]-Lyz or n[VIVL2]-Lyz is favored, while with ma- and acac- the species n[VIVOL]-Lyz are detected, with n dependent on the experimental VIV/protein ratio. The behavior of the systems with [VIVO(pic)2(H2O)] and Mb or Ub is very similar to that of Lyz. The results suggested that under physiological conditions, the moiety cis-VIVOL2 (L = pic-, dhp-) is bound by only one accessible side-chain protein residue that can be Asp, Glu, or His, while VIVOL+ (L = ma-, acac-) can interact with the two equatorial and axial sites. If the VIV complex is thermodynamically stable and does not have available coordination positions, such as amavadin, the protein cannot interact with it through the formation of coordination bonds and, in such cases, noncovalent interactions are predicted. The formation of the adducts is dependent on the thermodynamic stability and geometry in aqueous solution of the VIVO2+ complex and affects the transport, uptake, and mechanism of action of potential V drugs.

Vanadium and insulin: Partners in metabolic regulation

Since the 1970s, the biological role of vanadium compounds has been discussed as insulin-mimetic or insulin-enhancer agents. The action of vanadium compounds has been investigated to determine how they influence the insulin signaling pathway. Khan and coworkers proposed key proteins for the insulin pathway study, introducing the concept "critical nodes". In this review, we also considered critical kinases and phosphatases that participate in this pathway, which will permit a better comprehension of a critical node, where vanadium can act: a) insulin receptor, insulin receptor substrates, and protein tyrosine phosphatases; b) phosphatidylinositol 3'-kinase, 3-phosphoinositide-dependent protein kinase and mammalian target of rapamycin complex, protein kinase B, and phosphatase and tensin homolog; and c) insulin receptor substrates and mitogen-activated protein kinases, each node having specific negative modulators. Additionally, leptin signaling was considered because together with insulin, it modulates glucose and lipid homeostasis. Even in recent literature, the possibility of vanadium acting against metabolic diseases or cancer is confirmed although the mechanisms of action are not well understood because these critical nodes have not been systematically investigated. Through this review, we establish that vanadium compounds mainly act as phosphatase inhibitors and hypothesize on their capacity to affect kinases, which are critical to other hormones that also act on common parts of the insulin pathway.

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.

Exploring oxidovanadium(iv) homoleptic complexes with 8-hydroxyquinoline derivatives as prospective antitrypanosomal agents

[V^IVO(L-H)₂] and [V^VO(OCH₃)(L-H)₂] compounds of 8-hydroxyquinoline derivatives L showed activity againstTrypanosoma cruziandLeishmania infantumand high selectivities. Metallomics and interaction with BSA, apo-HTF and DNA were studied.

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.

Role of Ligands in the Uptake and Reduction of V(V) Complexes in Red Blood Cells

The interaction with erythrocytes of four [V^VO₂L₂]^- complexes, with L = picolinate (pic), 5-cyanopicolinate (picCN), 3-aminopyrazine-2-carboxylate (przNH₂), and 1,2-dimethyl-3-hydroxy-4(1 H)-pyridinonate (dhp), was studied. The thermodynamic stability at physiological pH is: [V^VO₂(dhp)₂]^- > [V^VO₂(przNH₂)₂]^- > [V^VO₂(pic)₂]^- > [V^VO₂(picCN)₂]^-. With picCN and pic, V exists at physiological pH as H₂V^VO₄^-, with przNH₂ as a mixture of H₂V^VO₄^- and [V^VO₂(przNH₂)₂]^- and with dhp as [V^VO₂(dhp)₂]^-. In the systems with pic and picCN, H₂V^VO₄^- and the ligands cross the erythrocyte membrane independently, with dhp the uptake occurs by diffusion, whereas with przNH₂ both the mechanisms are active. Inside erythrocytes stable V^IVOL₂ complexes are formed, indicating that there is no relationship with the stability and redox state of the administered compounds and that, if the metal ion changes its oxidation state in the cytosol as V does, unstable complexes in the extracellular medium could become stable inside the cells and contribute to the pharmacological action.

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.

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.

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)).

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.

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.

Coordination chemistry may explain pharmacokinetics and clinical response of vanadyl sulfate in type 2 diabetic patients

Vanadium, abbreviated V, is an early transition metal that readily forms coordination complexes with a variety of biological products such as proteins, metabolites, membranes and other structures. The formation of coordination complexes stabilizes metal ions, which in turn impacts the biodistribution of the metal. To understand the biodistribution of V, V in oxidation state iv in the form of vanadyl sulfate (25, 50, 100 mg V daily) was given orally for 6 weeks to 16 persons with type 2 diabetes. Elemental V was determined using Graphite Furnas Atomic Absorption Spectrometry against known concentrations of V in serum, blood or urine. Peak serum V levels were 15.4 ± 6.5, 81.7 ± 40 and 319 ± 268 ng ml(-1) respectively, and mean peak serum V was positively correlated with dose administered (r = 0.992, p = 0.079), although large inter-individual variability was found. Total serum V concentration distribution fit a one compartment open model with a first order rate constant for excretion with mean half times of 4.7 ± 1.6 days and 4.6 ± 2.5 days for the 50 and 100 mg V dose groups respectively. At steady state, 24 hour urinary V output was 0.18 ± 0.24 and 0.97 ± 0.84 mg in the 50 and 100 mg V groups respectively, consistent with absorption of 1 percent or less of the administered dose. Peak V in blood and serum were positively correlated (r = 0.971, p < 0.0005). The serum to blood V ratio for the patients receiving 100 mg V was 1.7 ± 0.45. Regression analysis showed that glycohemoglobin was a negative predictor of the natural log(ln) peak serum V (R(2) = 0.40, p = 0.009) and a positive predictor of the euglycemic-hyperinsulinemic clamp results at high insulin values (R(2) = 0.39, p = 0.010). Insulin sensitivity measured by euglycemic-hyperinsulinemic clamp was not significantly correlated with ln peak serum V. Globulin and glycohemoglobin levels taken together were negative predictors of fasting blood glucose (R(2) = 0.49, p = 0.013). Although V accumulation in serum was dose-dependent, no correlation between total serum V concentration and the insulin-like response was found in this first attempt to correlate anti-diabetic activity with total serum V. This study suggests that V pools other than total serum V are likely related to the insulin-like effect of this metal. These results, obtained in diabetic patients, document the need for consideration of the coordination chemistry of metabolites and proteins with vanadium in anti-diabetic vanadium complexes.

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 oxovanadium(iv)–salphen complexes with bovine serum albumin and their cytotoxicity against cancer

An oxovanadium(iv)–salphen complex acts as a probe for bovine serum albumin and shows cytotoxicity against cancer cells.

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.

Metal speciation in health and medicine represented by iron and vanadium

The influence of metals in biology has become more and more apparent within the past century. Metal ions perform essential roles as critical scaffolds for structure and as catalysts in reactions. Speciation is a key concept that assists researchers in investigating processes that involve metal ions. However, translation of the essential area across scientific fields has been plagued by language discrepancies. To rectify this, the IUPAC Commission provided a framework in which speciation is defined as the distribution of species. Despite these attempts, contributions from inorganic chemists to the area of speciation have not fully materialized in part because the past decade's contributions focused on technological advances, which are not yet to the stage of measuring speciation distribution in biological solutions. In the following, we describe how speciation influences the area of metals in medicine and how speciation distribution has been characterized so far. We provide two case studies as an illustration, namely, vanadium and iron. Vanadium both has therapeutic importance and is known as a cofactor for metalloenzymes. In addition to being a cation, vanadium(V) has analogy with phosphorus and as such is a potent inhibitor for phosphorylases. Because speciation can change the metal's existence in cationic or anionic forms, speciation has profound effects on biological systems. We also highlight how speciation impacts iron metabolism, focusing on the rather low abundance of biologically relevant iron cation that actually exists in biological fluids. fluids. Furthermore, we point to recent investigations into the mechanism of Fenton chemistry, and that the emerging results show pH dependence. The studies suggest formation of Fe(IV)-intermediates and that the generally accepted mechanism may only apply at low pH. With broader recognition toward biological speciation, we are confident that future investigations on metal-based systems will progress faster and with significant results. Studying metal complexes to explore the properties of a potential "active species" and further uncovering the details associated with their specific composition and geometry are likely to be important to the action.

Interaction of vanadium(IV) with human serum apo-transferrin

The interaction of V(IV)O-salts as well as of a few V(IV)O(carrier)n complexes with human serum transferrin (hTF) is studied focusing on the determination of the nature and stoichiometry of the binding of V(IV)O(2+) to hTF, as well as whether the conformation of hTF upon binding to V(IV)O(2+) or to its complexes is changed. Circular dichroism (CD) spectra measured for solutions containing V(IV)O(2+) and apo-hTF, and V(IV)O-maltol and apo-hTF, clearly indicate that hTF-V(IV)O-maltol ternary species form with a V(IV)O:maltol stoichiometry of 1:1. For V(IV)O salts and several V(IV)O(carrier)n complexes (carrier ligand=maltolato, dhp, picolinato and dipicolinato) (Hdhp=1,2-dimethyl-3-hydroxy-4-pyridinone) the maximum number of V(IV)O(2+) bound per mole of hTF is determined to be ~2 or lower in all cases. The binding of V(IV)O to apo-hTF most certainly involves several amino acid residues of the Fe-binding site, and as concluded by urea gel electrophoresis experiments, the formation of (V(IV)O)2hTF species may occur with the closing of the hTF conformation as is the case in (Fe(III))2hTF, which is an essential feature for the transferrin receptor recognition.