Eugenio Garribba

Ternary complex formation between VO(IV)–picolinic acid or VO(IV)–6-methylpicolinic acid and small blood serum bioligands

In order to assess the role of the low molecular mass bioligands of blood serum in vanadium binding, a study was made of the interactions of the complexes formed in the VO(IV)-picolinic acid and VO(IV)-6-methylpicolinic acid systems with various low molecular mass constituents of blood serum, such as oxalate, lactate, citrate and phosphate. The speciation of VO(IV) in these ternary systems and also in the binary VO(IV)-picolinic acid and VO(IV)-6-methylpicolinic acid systems was studied by pH-potentiometry at 25 degrees C and at an ionic strength I = 0.2 M (KCl). The binding modes of the complexes formed were determined by spectral (electronic absorption and EPR) methods. Picolinic acid and 6-methylpicolinic acid were found to form mono and bis complexes through the pyridine nitrogen and carboxylate oxygen, but the presence of the methyl group in 6-methylpicolinic acid surprisingly decreases the stability of its complexes significantly. The results obtained on the ternary systems reveal that mixed ligand complex formation is favoured in these systems, especially with citrate, and must therefore be taken into account in the speciation description of VO(IV) in blood serum.

Speciation of insulin-mimetic VO(IV)-containing drugs in blood serum

The biospeciations of three potential insulin-mimetic VO(IV) compounds, VO(maltolate)2, VO(picolinate)2 and VO(6-Me-picolinate)2, in blood serum were assessed via modelling calculations, using the stability constants reported in the literature for the binary insulin-mimetic complexes and their ternary complexes formed with the most important low molecular mass binders in the serum: oxalic acid, lactic acid, citric acid and phosphate. The binding capabilities of two high molecular mass serum proteins, albumin and transferrin, were also taken into account.

Coordination modes of hydroxamic acids in copper(II), nickel(II) and zinc(II) mixed-ligand complexes in aqueous solution

The stability constants and coordination modes of the mixed-ligand complexes formed by Cu(II), Ni(II), Zn(II), ethylenediamine (en), 2,2%-bipyridine (bpy), glycinate (Gly), disodium salt of 4,5-dihydroxybenzene 1,3-disulfonate (Tiron), diethylenetriamine (dien) or 2,2%:6,2ƒ-terpyridine (terpy) (ligand B) and acetohydroxamate (Aha), N-methylacetohydroxamate (MeAha) or N-phenylacetohydroxamate (PhAha) (ligand A) were determined in water (25°C, I 0.2 M KCl) by pH-metric, spectrophotometric, EPR and calorimetric methods. Mixed-ligand complexes with typical hydroxamate type chelation mode involving the NHO moiety are formed in all systems. However, further copper(II) induced deprotonation of the NHO moiety of Aha in the presence of en or bpy results in the formation of mixed-ligand complexes with hydroximato chelates at high pH. The results show the favoured coordination of a hydroxamate to metal(II)‐en and especially to a metal(II)‐bpy moiety. If ligand B is Gly, the increase of stability of the mixed-ligand complexes is as expected on statistical basis, whereas the formation of complexes involving O,O-coordinated hydroxamate and O,O-coordinated Tiron is unfavoured. The tridentate coordination of dien or terpy results in five-coordinated mixed-ligand copper(II) complexes in which, most probably, the hydroxamate moiety adopts an equatorial‐axial coordination mode. This quite unstable hydroxamate chelate can not hinder the hydrolysis of the complex above pH 8. Under very basic conditions acetohydroximato moieties (CONO 2 ) displace the rigid terpy ligand from the coordination sphere and complexes, [Cu(AhaH 1)2] 2 involving hydroximato chelates are formed.

Speciation and NMR relaxation studies of VO(IV) complexes with several O-donor containing ligands: oxalate, malonate, maltolate and kojate

Abstract Potentiometric, spectral and 1H NMR relaxation studies are reported on the complex formation of VO(IV) with the bidentate chelating ligands oxalic and malonic acids, of a dicarboxylic nature, and maltol and kojic acid, of the hydroxy-4-pyrone type. Complexes with stochiometries VOA and VOA2 were characterised in aqueous solution. Two main features were established for the complex systems. It was found that five-membered ring chelation favours the cis arrangement in the bis complexes of maltol and kojic acid to a higher extent than in those of oxalic and malonic acids. Moreover, previous investigations of VO(IV) binding to maltol or its analogues did not consider the formation of hydroxo species. Formation of dihydroxo bridged dimeric complexes of stoichiometry (VOAH−1)2 and monomeric hydroxo species VOA2H−1 or VOA2(OH), are now taken into account in order to explain the equilibrium and spectroscopic results. 1H NMR relaxation measurements strongly suggest an equilibrium between the two isomers of the bis complexes.

Solution speciation and spectral studies on oxovanadium(IV) complexes of pyridinecarboxylic acids

Abstract Complex formation between oxovanadium(IV) and several pyridinecarboxylic acids (picolinic acid, 3-hydroxypicolinic acid and 2-hydroxynicotinic acid) was studied in aqueous solution by pH-potentiometric and spectroscopic (electron paramagnetic resonance and electronic absorption) techniques. The results demonstrated that picolinic acid forms mono and bis complexes with the VO(IV) ion, coordinated through the pyridine nitrogen and the carboxylate group. The pyridine nitrogen is also involved in the complexation of VO(IV) by 3-hydroxypicolinic acid and 2-hydroxynicotinic acid in the acidic pH range, but as the pH is increased deprotonation of the phenolic hydroxy group occurs and complexes with salicylate-type (COO − ,O − )-coordination are formed. In the VO(IV)–3-hydroxypicolinic acid system, the ligand coordinates to the metal ion in a tridentate fashion via the (N,COO − ,O − ) donor set, forming a tetrameric complex (VOA) 4 in the pH range 6–8.

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.

On the transport of vanadium in blood serum.

The complexation of the VO(2+) ion in several systems that can model the physiological conditions of its transport in blood serum was studied using electron paramagnetic resonance (EPR) spectroscopy. Particularly, the ternary systems formed by (i) VO(2+) and two high-molecular-mass components of blood serum, human serum apo-transferrin (hTf) and human serum albumin (HSA); (ii) VO(2+), hTf, and bL; and (iii) VO(2+), HSA, and bL, where bL is one of the six most important low-molecular-mass bioligands of the blood serum (bL = lactate, citrate, oxalate, phosphate, glycine, or histidine), were examined. The results indicate that, in aqueous solution, transferrin is a stronger binder than albumin, and at the physiological ratio, most of the VO(2+) ion is present as (VO)(2)hTf, and a small amount as (VO)(2)(d)HSA, the dinuclear species formed by albumin where the two metal ions are interacting and the spin state S is 1. Among the bL ligands, only lactate and citrate are able to bind VO(2+) in the presence of transferrin or albumin, the others not interacting at all. Finally, the quaternary systems formed by (i) VO(2+), hTf, HSA, and lactate and (ii) VO(2+), hTf, HSA, and citrate were studied. In these cases, the results suggest that the predominant species is (VO)(2)hTf, followed by the mixed complexes VO(2+)-hTf-lactate or VO(2+)-hTf-citrate, whereas (VO)(2)(d)HSA and [(VO)(2)(citrH(-1))(2)](4-) are minor components at physiological pH. The conclusions of this study give new insights on how the VO(2+) ion distributes among the blood serum components and is transported in the plasma toward the target sites in the organism.

Interaction of VO2+ ion and some insulin-enhancing compounds with immunoglobulin G.

Complexation of VO(2+) ion with the most abundant class of human immunoglobulins, immunoglobulin G (IgG), was studied using EPR spectroscopy. Differently from the data in the literature which report no interaction of IgG with vanadium, in the binary system VO(2+)/IgG at least three sites with comparable strength were revealed. These sites, named 1, 2, and 3, seem to be not specific, and the most probable candidates for metal ion coordination are histidine-N, aspartate-O or glutamate-O, and serinate-O or threoninate-O. The mean value for the association constant of (VO)(x)IgG, with x = 3-4, is log β = 10.3 ± 1.0. Examination of the ternary systems formed by VO(2+) with IgG and human serum transferrin (hTf) and human serum albumin (HSA) allows one to find that the order of complexing strength is hTf ≫ HSA ≈ IgG. The behavior of the ternary systems with IgG and one insulin-enhancing agent, like [VO(6-mepic)(2)], cis-[VO(pic)(2)(H(2)O)], [VO(acac)(2)], and [VO(dhp)(2)], where 6-mepic, pic, acac, and dhp indicate the deprotonated forms of 6-methylpicolinic and picolinic acids, acetylacetone, and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone, is very similar to the corresponding systems with albumin. In particular, at the physiological pH value, VO(6-mepic)(IgG)(OH), cis-VO(pic)(2)(IgG), and cis-VO(dhp)(2)(IgG) are formed. In such species, IgG coordinates nonspecifically VO(2+) through an imidazole-N belonging to a histidine residue exposed on the protein surface. For cis-VO(dhp)(2)(IgG), log β is 25.6 ± 0.6, comparable with that of the analogous species cis-VO(dhp)(2)(HSA) and cis-VO(dhp)(2)(hTf). Finally, with these new values of log β, the predicted percent distribution of an insulin-enhancing VO(2+) agent between the high molecular mass (hTf, HSA, and IgG) and low molecular mass (lactate) components of the blood serum at physiological conditions is calculated.

Assessing the Dependence of 51V Az Value on the Aromatic Ring Orientation of VIVO2+ Pyridine Complexes

Characterization of V(IV) biomolecules relies strongly on electron paramagnetic resonance (EPR) spectroscopy, particularly the application of the additivity relationship of A(z) values. It has been shown experimentally that the A(z) values of V(IV)O(2+) imidazole species have a critical angular dependence. Density-functional theory (DFT) calculations elucidate the dependence of (51)V A(z) value on the orientation of the aromatic ring in V(IV)O(2+) pyridine complexes, following closely the functional dependence observed for V(IV)O(2+) imidazole species, [A(z)(pyr) = 42.23 + 1.80 x sin(2theta - 90)], with A(z) measured in 10(4) cm(-1). A DFT re-examination of V(IV)O(2+) imidazole complexes gives an equation very similar [A(z)(imid) = 42.35 + 2.34 x sin(2theta - 90)] to that experimentally found. These results generalize the application of the additivity relationship for V(IV)O(2+) complexes containing aromatic nitrogen ligands such as pyridine or imidazole. The increase of the absolute value of A(z), |A(z)|, when the dihedral angle theta between the V=O and N(pyr)-C or N(imid)-C bonds varies from a parallel to a perpendicular orientation is due to an increase of the d vanadium orbital contribution and to a decrease of the pi aromatic system participation in the singly occupied molecular orbital.

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.