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.

Unveiling VIV O2+ Binding Modes to Human Serum Albumins by an Integrated Spectroscopic-Computational Approach.

Human serum albumin (HSA) is involved in the transport of metal ions and potential metallodrugs. Depending on the metal, several sites are available, among which are N-terminal (NTS) and multi-metal binding sites (MBS). Despite the large number of X-ray determinations for albumins, only one structure with Zn2+ is available. In this work, the binding to HSA of the VIV O2+ ion was studied by an integrated approach based on spectroscopic and computational methods, which allowed the systems to be characterized even in the absence of X-ray analysis. The behavior depends on the type of albumin, defatted (HSAd ) or fatted (HSAf ). With HSAd 'primary' and 'secondary' sites were revealed, NTS with (His3, His9, Asp13, Asp255) and MBS with (His67, His247, Asp249, Asn99 or H2 O); with increasing the ratio VIV O2+ /HSAd , 'tertiary' sites, with one His-N and other donors (Asp/Glu-O or carbonyl-O) are populated. With HSAf , fatty acids (FAs) cause a rotation of the subdomains IA and IIA, which results in the formation of a dinuclear ferromagnetic adduct (VIV O)2 D (HSAf ) with a µ1,1 -Asp249 and the binding of His247, Glu100, Glu252, and His67 or Asn99. FAs hinder also the binding of VIV O2+ to the MBS.

Biospeciation of Potential Vanadium Drugs of Acetylacetonate in the Presence of Proteins.

Among vanadium compounds with potential medicinal applications, [VIVO(acac)2] is one of the most promising for its antidiabetic and anticancer activity. In the organism, however, interconversion of the oxidation state to +III and +V and binding to proteins are possible. In this report, the transformation of VIII(acac)3, VIVO(acac)2, and VVO2(acac) 2 - after the interaction with two model proteins, lysozyme (Lyz) and ubiquitin (Ub), was studied with ESI-MS (ElectroSpray Ionization-Mass Spectroscopy), EPR (Electron Paramagnetic Resonance), and computational (docking) techniques. It was shown that, in the metal concentration range close to that found in the organism (15-250 µM), VIII(acac)3 is oxidized to VIVO(acac)+ and VIVO(acac)2, which-in their turn-interact with proteins to give n[VIVO(acac)]-Protein and n[VIVO(acac)2]-Protein adducts. Similarly, the complex in the +IV oxidation state, VIVO(acac)2, dissociates to the mono-chelated species VIVO(acac)+ which binds to Lyz and Ub. Finally, VVO2(acac) 2 - undergoes complete dissociation to give the 'bare' VVO 2 + ion that forms adducts n[VVO2]-Protein with n = 1-3. Docking calculations allowed the prediction of the residues involved in the metal binding. The results suggest that only the VIVO complex of acetylacetonate survives in the presence of proteins and that its adducts could be the species responsible of the observed pharmacological activity, suggesting that in these systems VIVO2+ ion should be used in the design of potential vanadium drugs. If VIII or VVO2 potential active complexes had to be designed, the features of the organic ligand must be adequately modulated to obtain species with high redox and thermodynamic stability to prevent oxidation and dissociation.

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

The interaction with erythrocytes of four [VVO2L2]- 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: [VVO2(dhp)2]- > [VVO2(przNH2)2]- > [VVO2(pic)2]- > [VVO2(picCN)2]-. With picCN and pic, V exists at physiological pH as H2VVO4-, with przNH2 as a mixture of H2VVO4- and [VVO2(przNH2)2]- and with dhp as [VVO2(dhp)2]-. In the systems with pic and picCN, H2VVO4- 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 VIVOL2 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.

Polymeric nanoparticles encapsulating white tea extract for nutraceutical application.

With the aim to obtain controlled release and to preserve the antioxidant activity of the polyphenols, nanoencapsulation of white tea extract into polymeric nanoparticles (NPs) based on poly(ε-caprolactone) (PCL) and alginate was successfully performed. NPs were prepared by nanoprecipitation method and were characterized in terms of morphology and chemical properties. Total polyphenols and catechins contents before and after encapsulation were determined. Moreover, in vitro release profiles of encapsulated polyphenols from NPs were investigated in simulated gastrointestinal fluids. The antioxidant activity and stability of encapsulated extract were further evaluated. Interestingly, NPs released 20% of the polyphenols in simulated gastric medium, and 80% after 5 h at pH 7.4, showing a good capacity to control the polyphenols delivery. Furthermore, DPPH(•) assay confirmed that white tea extract retained its antioxidant activity and NPs protected tea polyphenols from degradation, thus opening new perspectives for the exploitation of white tea extract-loaded NPs for nutraceutical applications.

Effect of chitosan concentration on PLGA microcapsules for controlled release and stability of resveratrol.

The polyphenols as nutraceutical and therapeutic agents are gaining growing interest for their beneficial effects and potential in human health. In order to protect their scaffolds and functionality, and to improve the bioavailability, the microencapsulation can represent a promising strategy. This study reports on the formulation of the natural resveratrol (RSV) into microcapsules (MCs) prepared by using different concentrations of chitosan (CS) and poly(D,L-lactic-co-glycolic acid) (PLGA) as polymeric matrix. MCs were prepared by W/O/W double emulsion method and characterized in terms of morphology, size, encapsulation efficiency, physicochemical and thermal properties. RSV release behavior from MCs was evaluated under simulated gastrointestinal fluids, and the long term stability was monitored at different storage conditions. MCs resulted to have spherical shape and different morphology, with size ranging from 11 to 20 µm, and encapsulation efficiencies of 40-52%, depending on the CS concentration. Moreover, MCs containing CS exhibited a significant lower release of RSV than those containing only PLGA. Furthermore, all tested formulations were able to ensure a good retention and stability of encapsulated RSV until 6 months. In summary, CS/PLGA MCs can be proposed as an attractive delivery system to control the release and long term protection of RSV.

Behavior of the potential antitumor V(IV)O complexes formed by flavonoid ligands. 1. Coordination modes and geometry in solution and at the physiological pH.

The coordination modes and geometry assumed in solution by the potent antitumor oxidovanadium(IV) complexes formed by different flavonoids were studied by spectroscopic (Electron Paramagnetic Resonance, EPR) and computational (Density Functional Theory, DFT) methods. A series of bidentate flavonoid ligands (L) with increasing structural complexity was examined, which can involve (CO, O(-)) donors and formation of five- and six-membered chelate rings, or (O(-), O(-)) donors and five-membered chelate rings. The geometry corresponding to these coordination modes can be penta-coordinated, [VOL2], or cis-octahedral, cis-[VOL2(H2O)]. The results show that, at physiological pH, ligands provided with (CO, O(-)) donor set yield cis-octahedral species with "maltol-like" coordination when five-membered chelate rings are formed (as with 3-hydroxyflavone), while penta-coordinated structures with "acetylacetone-like" coordination are preferred when the chelate rings are six-membered (as with chrysin). When both the binding modes are possible, as with morin, the "acetylacetone-like" coordination is observed. For the ligands containing a catecholic donor set, such as 7,8-dihydroxyflavone, baicalein, fisetin, quercetin and rutin, the formation of square pyramidal complexes with (O(-), O(-)) "catechol-like" coordination and five-membered chelate rings is preferred at physiological pH. The determination of the different coordination modes and geometry is important to define the biotransformation in the blood and the interaction of these complexes with the biological membranes.