Daniele Sanna

EPR and potentiometric reinvestigation of copper(II) complexation with simple oligopeptides and related compounds

Abstract The coordination modes of Cu(II) to Di-, Tri-, and Tetra-glycine and related ligands were investigated, within the entire measurable pH range and over a wide range of ligand excess, by means of electron paramagnetic resonance, electron spectroscopy, and in some cases, pH potentiometry. The results, besides allowing the identification of the complex species involved and the attribution of distinctive spectral data set to the various structures, provide significant insight in relation to: i) the coordination ability of the zwitterionic AH form of simple oligopeptides to yield COO − - or 2COO − -complexes or species with mixed [(NH 2 , CO)(COO − )] or [(NH 2 , N − , COO − )(COO − )] coordination; ii) the influence of the size of the (NH 2 , CO), (NH 2 , N − ), and (N − , COO − ) chelated rings on the stability and structure of the complex species; iii) the bis-complex formation processes taking place with Gly-β-ala, β-Alagly, Tri-, and Tetraglycine.

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.

Oxovanadium(IV) complexes of citric and tartaric acids in aqueous solution

Abstract The complex-formation processes taking place in the systems containing VO(IV) and d-, l-, dl- or meso -tartaric acid or citric acid in aqueous solution were re-examined by combined use of pH potentiometry and spectroscopic (EPR and electronic absorption) methods. The results allowed a full characterization of the dinuclear species existing in the VO(IV)— d-, l- and dl -tartrate systems and indicated a different complexation behaviour in the corresponding meso -tartrate system. As its conformation is unfavourable for dimer formation, the meso ligand yields predominantly cyclic trinuclear species, one of them exhibiting distinctive EPR features. In the VO(IV)-citrate system, dinuclear species with structures different from those of the VO(IV) tartrates are suggested.

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 copper(II) with the prion peptide fragment HuPrP(76-114) encompassing four histidyl residues within and outside the octarepeat domain.

Complex formation processes between the 39-mer residue peptide fragment of human prion protein, HuPrP(76-114), and copper(II) ions have been studied by potentiometric, UV-vis, circular dichroism (CD), electron paramagnetic resonance, and electrospray ionization mass spectrometry methods. This peptide consists of 39 amino acid residues and contains two histidines (His77 and His85) belonging to the octarepeat domain and two histidines (His96 and His111) outside this domain. It was found that HuPrP(76-114) is able to bind 4 equiv of metal ions and all histidyl residues are independent, except nonequivalent metal binding sites in the oligonuclear species. Imidazole nitrogen donor atoms are the primary and exclusive metal binding sites below pH 5.5 in the form of various macrochelates. The macrochelation slightly suppresses, but cannot prevent, the deprotonation and metal ion coordination of amide functions, resulting in the formation of (N(im),N(-)), (N(im),N(-),N(-)), and (N(im),N(-),N(-),N(-))-coordinated copper(II) complexes in the pH range from 5.5 to 9. CD spectroscopy results gave clear evidence for the differences in the metal binding affinity of the histidyl sites according to the following order: His111 > His96 >> His77 approximately His85. Among the oligonuclear complexes, the formation of di- and tetranuclear species seems to be favored over the trinuclear ones, at pH values beyond the physiological ones. This phenomenon was not observed in the complex formation reactions of HuPrP(84-114), a peptide fragment containing only one histidyl residue from the octarepeat. As a consequence, the data support the existence of cooperativity in the metal binding ability of this peptide probably due to the presence of two octarepeat sequences of the dimeric octarepeat domain of HuPrP(76-114) at basic pH values.

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.

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.

Transport of the anti-diabetic VO2 + complexes formed by pyrone derivatives in the blood serum

The biotransformation in the blood serum of the two anti-diabetic agents [VO(ema)(2)] - or BEOV - and [VO(koj)(2)] formed by ethylmaltol (Hema) and kojic acid (Hkoj) was studied with EPR spectroscopy, pH-potentiometry and DFT calculations. For comparison, the behavior of the systems with tropolone (Htrop) was also analyzed. The interaction of [VO(ema)(2)] and [VO(koj)(2)] with the most important bioligands of the serum, lactic (Hlact) and citric acid (H(3)citr), human serum transferrin (hTf), human serum albumin (HSA) and immunoglobulin G (IgG) was examined and discussed. Among the several mixed species observed, cis-VO(carrier)(2)(hTf), cis-VO(carrier)(2)(HSA) and cis-VO(carrier)(2)(IgG), where carrier is ethylmaltolate or kojate, with a His-N of the protein coordinated in the equatorial position, are plausible candidates for the transport processes of the drug toward the target organs. The values of the logβ are in the range 19.6-19.8 for the species formed by ethylmaltol and 17.4-17.6 for those formed by kojic acid. The formation of such species was confirmed through pH-titrations of the model systems VO(2+)/carrier/1-MeIm and VO(2+)/carrier/Ac-his, where 1-MeIm and Ac-his are 1-methylimidazole and N-acetylhistamine, and DFT calculations of (51)V A(z) of the model species cis-[VO(carrier)(2)(1-MeIm)] and cis-[VO(carrier)(2)(Ac-his)]. The values of the stability constants for the mixed species observed were used to predict the biodistribution of VO(2+) ion between the blood serum components for concentrations of 1, 10 and 50 μM.