Complexes of vanadium(III) with L-alanine and L-aspartic acid

The equilibria of the complexation processes of V(3+) with L-alanine and L-aspartic acid in aqueous solution over a wide pH range (2-10) were studied by potentiometric and spectroscopic (UV-Vis, CD) methods. The results show that alanine forms complexes with V(3+) in the metal ion concentration range and at the ligand-to-metal ratios investigated, giving mononuclear species only. In ML(2) species, which dominate in the range pH 4-8, alanine acts as a bidendate ligand through O and N atoms. The complexation processes of V(3+) with aspartic acid are more complicated. In acidic solution (up to pH approximately 4) they are similar to those for alanine. In the higher pH region, however, there are complicated equilibria among mono- and various dinuclear species. These dinuclear species consist of carboxylic or mu-oxo bridges and differ from each other by the number of coordinated ligands and OH(-) groups. The solid phase of the V(III) complex with aspartic acid could be isolated from nonaqueous solution only. Spectroscopic (UV-Vis-IR) measurements and magnetic susceptibility data confirm the coordination of vanadium(III) by two carboxylic groups. Both V(III)-L-aspartic acid and V(III)-L-alanine complexes have a significant apoptotic effect on Hepatoma Morris 5123 cells.     

Interaction of Al(III) with the peptides AspAsp and AspAspAsp

The interactions of Al(III) with the dipeptide AspAsp and the tripeptide AspAspAsp in aqueous solutions were studied by pH-potentiometry and multinuclear 1H- and 13C- nuclear magnetic resonance (NMR) spectroscopy. Their numerous negatively charged COO(-) functions allow these ligands to bind Al(III) even in weakly acidic solutions. Various mononuclear 1:1 complexes are formed in different protonation states. 13C-NMR spectroscopy unambiguously proved participation of the COO(-) functions in a monodentate or chelating mode in Al(III) binding, however, the terminal-NH(2) group seems to be excluded from the coordination. Depending on the metal ion to ligand ratio precipitation occurs at pH approximately 5 to 6. This indicates that the COO(-) groups at the low level of preorganization in such small peptides are not sufficient to keep the Al(III) ion in solution and to prevent the precipitation of Al(OH)(3) at physiological pH. To achieve this, a more specific arrangement of the side-chain donors seems necessary.     

Structural features of aluminium(III) complexes with bioligands in glutamate dehydrogenase reaction system--a review

Aluminium(III) complexes are essential for understanding the toxicity, bioavailability and transport mechanisms of aluminium in environmental and biological systems. Since elucidation of the exact structures of these weakly coordinated systems is very difficult, the structures of Al(III) complexes in glutamate dehydrogenase reactions system were investigated recently from the following four aspects: (1) Constitutional studies: The keto-enol tautomerism of the complexes between aluminium(III) ion and alpha-ketoglutarate ligands in acidic aqueous solutions was studied. It is clearly demonstrated that Al(III) can promote the keto-enol tautomerization of alpha-ketoglutarate. (2) Configurational studies: Compared with L-Glu, the complex stability of D-Glu-Al is stronger, especially for the tridentate species. The result was further supported by computational results in the molecular mechanics model with the UFF forcefield. It is implied that Al(III) complexation may favor the racemization from L- to D-amino acids. (3) Conformational studies: At biologically relevant pH and concentrations of Al(III) and NADH, Al(III) was found to increase the percentage of folded forms of NADH, which results in reducing the activity of the coenzyme NADH in the hollow-dehydrogenase reactions system. However, the conformations of NAD(+) and Al-NAD(+) are dependent upon the solvents and other ligands in the complexes. (4) Biological effects: The effects of Al(III) on the activity of the glutamate dehydrogenase-catalyzed reactions were studied by monitoring the differential-pulse polarography reduction current of NAD(+). At the physiologically relevant pH values (pH 6.5 and 7.5), the activity of the GDH enzyme was strongly dependent on the concentration of the Al(III) in the assayed mixture solutions.     

Aqueous and solid complexes of iron(III) with hyaluronic acid. Potentiometric titrations and infrared spectroscopy studies

The coordination of iron(III) ion to hyaluronic acid (Hyal) in aqueous solutions and solid state was accomplished by potentiometric titrations and infrared spectroscopy. The potentiometric titration studies provided the binding constants for the complexes found in the systems and the speciation of these species according to the variation of pH values. The complexes found presented a complexing ability through both the chelating moieties of Hyal (via the N-glucosamine and D-glucoronic acid), showing no special preference for either one while in solid state, but when in aqueous solution the complexation via the N-glucosamine moiety was the preferred, forming two complexed species, ML and ML(2) (log K(ML)=8.2 and log K(ML2)=7.9). The presence of a mu-oxo complex via the D-glucoronic acid was also detected in both aqueous (log K=6.7) and solid states via the N-glucosamine and D-glucoronic acid simultaneously linked to two Hyal chains. A structure for this latter complex was suggested. The results indicated that these complexes could be used in eliminating the excess iron(III) in living organisms.     

A report describing the effect of di-2-pyridyl ligand architecture on the coordination properties with gold(III)

In an effort to develop novel gold-based chemotherapies, gold(III) coordination complexes possessing a series of di-2-pyridyl ligands were targeted as synthetic products. It was found that di-2-pyridyl ligands linked by different groups exhibited varying coordination to gold(III). Di-2-pyridyl sulfide (DPS) exhibited bidentate binding to gold(III), and formed a complex ion with a gold tetrachloride counter ion {[(DPS)AuCl₂]AuCl₄; compound 3}; di-2-pyridyl ether (DPO) formed a neutral monodentate coordination complex with gold(III) {[(DPO)(AuCl₃)]; compound 4}; and attempts to make a gold(III) complex with di-2-pyridyl ketone (DPK) were unsuccessful, as a complex ion possessing the protonated ligand and a gold tetrachloride anion was isolated {[HDPK][AuCl₄]; compound 5}. Compounds 3-5 were structurally characterized using X-ray crystallography, which confirmed the different coordination environments around the gold(III) metal centers.     

The effect of V(III)-adenine complex on yeast as a model of eukaryotic cells

The dinuclear micro-okso vanadium (III) complex compound H(4)V(2)OCl(4)(Ad)(2) synthesized in our laboratory was investigated as a potential cytotoxic agent against yeast cells. The results of these studies could be helpful in the explanation of the mechanism governing the V (III) compound action on yeast as a simple model of eukaryotic cells. The important factors influencing the toxicity of this complex compound are: the stage of the yeast life cycle, the rate of growth and the pH of reaction mixture. The lethal effect was distinctly stronger when the reaction mixture was slightly acidic (pH = 4). In such solutions V(III) mononuclear species with adenine was relatively stable, and during the time of experiment possibly only a slow oxidation process to V(IV) occurred. In the solutions with pH = 7, several hydrolytic, perhaps mixed-valence, polynuclear species were present and their action on the yeast cells was rather weak. The increased lethal activity of this compound in acidic solutions may be useful in specific treatment against cancer cells whose cytoplasm and/or closest surrounding has lower pH value. The next important result was an observation that the killing activity of this compound was enhanced for yeast cells being in log phase. Also these which had a slower rate of growth (possessing some auxotrophic mutations) were more resistant than those growing faster. The extent of yeast mutagenesis caused by the complex compound is negligible, as the number of mutants found in experiments was within the limit of experimental error. These results are promising and the investigated complex can be considered as a potential anti cancer agent.     

The permeability and cytotoxicity of insulin-mimetic vanadium (III,IV,V)-dipicolinate complexes

Vanadium (III,IV,V)-dipicolinate complexes with different redox properties were selected to investigate the structure-property relationship of insulin-mimetic vanadium complexes for membrane permeability and gastrointestinal (GI) stress-related toxicity using the Caco-2 cell monolayer model. The cytotoxicity of the vanadium complexes was assayed with 3-(4,5-dimethylthiazoyl-2-yl) 2,5-diphenyltetrazolium bromide (MTT) assays and the effect on monolayer integrity was measured by the trans-epithelial electric resistance (TEER). The three vanadium complexes exhibited intermediate membrane permeability (P(app) = 1.4-3.6x10(-6) cm/s) with low cellular accumulation level (<1%). The permeability of all compounds was independent of the concentration of vanadium complexes and excess picolinate ligands. Both V(III) and V(V)-dipicolinate complexes induced 3-4-fold greater reactive oxygen and nitrogen species (RONS) production than the V(IV)-dipicolinate complex; while the vanadium (III)-dipicolinate was 3-fold less damaging to tight junction of the Caco-2 cell monolayer. Despite the differences in apparent permeability, cellular accumulation, and capacity to induce reactive oxygen and nitrogen species (RONS) levels, the three vanadium complexes exhibited similar cytotoxicity (IC50 = 1.7-1.9 mM). An ion pair reagent, tetrabutylammonium, increased the membrane apparent permeability by 4-fold for vanadium (III and IV)-dipicolinate complexes and 16-fold for vanadium (V)-dipicolinate as measured by decrease in TEER values. In addition, the ion pair reagent prevented damage to monolayer integrity. The three vanadium (III,IV,V)-dipicolinate complexes may pass through caco-2 monolayer via a passive diffusion mechanism. Our results suggest that formation of ion pairs may influence compound permeation and significantly reduce the required dose, and hence the GI toxicity of vanadium-dipicolinate complexes.     

pH Change in Culture Medium Induces Reorganization of Intranuclear Actin in Two-Cell Mouse Embryos

Actin is a permanent component of the cell nucleus involved in many nuclear processes. However, some nuclear functions of actin remain insufficiently explored. The role played by various extracellular stimuli in regulation of nuclear actin still remains enigmatic. Deviation of basic parameters of culture medium from optimal values is a member of the group of extracellular stimuli that are very important for mammalian embryos cultured in vitro. Change in culture medium pH from the level optimal for embryo homeostasis is one such signals. The purpose of this study was to investigate the intranuclear actin distribution in nuclei of two-cell mouse embryos under stress conditions induced by changes in extracellular pH. The pattern of actin localization has been tracked after short-term culturing of the embryos at optimal (pH 7.2), increased (pH 7.8), or decreased (pH 6.5) pH conditions. Analysis was carried out with confocal microscopy using methods of direct fluorescent and indirect immunofluorescent identification of actin. It has been shown that the change of culture medium pH from the optimum value is the signal that alters intranuclear actin distribution in nuclei of the embryonic cells. Culture of two-cell mouse embryos in suboptimal pH conditions (pH 6.5 and pH 7.8) induced alterations in the intranuclear actin localization, which, in particular, were expressed in accumulation of monomeric actin and the appearance of phalloidin-stainable actin in the nuclei. These changes, in our opinion, show some signs of similarity with stress-induced changes in nuclear-actin distribution, which, as has been reported earlier by a number of researchers, have been observed in the nuclei of somatic cells.     

A 59Co NMR relaxation probe of macromolecule anionic binding sites

⁵⁹Co NMR linewidth measurements are reported which show that hexacyanocobaltate-(III) ion may be used as a sensitive probe of protein interactions with anions in aqueous solutions. Applications are demonstrated to bovine serum albumin where the probe ion binding is monitored as a function of pH and is displaced from the protein sites by hexacyanoferrate(III) ion. The general utility of complex metal ions is suggested as a generally useful approach to the analysis of ion-macromolecule interactions.     

Hemes and hemeproteins, 2: The pH-dependent equilibria of microperoxidase-8 and characterization of the coordination sphere of Fe(III)

Titration of the monomeric heme octapeptide from horse heart cytochrome c, microperoxidase-8 (MP-8) from pH 1 to pH 13 in 20% (v/v) methanol-water solutions, mu = 0.1, at 25 degrees C shows three reversible concentration-independent pKs (4.43 +/- 0.09; 8.90 +/- 0.03; 10.48 +/- 0.09) which are ascribed to successive proton loss from the conjugate acid of His (and its coordination to Fe(III)), bound H2O, and from bound His to form an imidazolate complex, respectively. The equilibrium constant for coordination of imidazole between pH 5.5 and 7.0 is independent of pH (logK = 4.45) which proves that His-18 is coordinated to Fe(III) in aqueous solution.     

The orientation of the ligands in iron(III)-bleomycin intercalated with DNA

EPR data show that Fe(III)-bleomycin intercalates with DNA, or that the Fe(III) coordination sphere has a fixed geometrical configuration with respect to the DNA helical axis. An analysis of the data from oriented DNA fibers, drawn from a viscous gel, shows that the angle between the fiber axis and the normal to a plane containing the Fe(III) ion and ligands ranges between 15 and 30 degrees. The principal g values for the low-spin Fe(III)-bleomycin-DNA complex at pH 7.5 are 2.45, 2.18 and 1.87.     

The orientation of the ligands in iron(III)-bleomycin intercalated with DNA

EPR data show that Fe(III)-bleomycin intercalates with DNA, or that the Fe(III) coordination sphere has a fixed geometrical configuration with respect to the DNA helical axis. An analysis of the data from oriented DNA fibers, drawn from a viscous gel, shows that the angle between the fiber axis and the normal to a plane containing the Fe(III) ion and ligands ranges between 15 and 30 degrees. The principal g values for the low-spin Fe(III)-bleomycin-DNA complex at pH 7.5 are 2.45, 2.18 and 1.87     

Vanadium(IV/V) speciation of pyridine-2,6-dicarboxylic acid and 4-hydroxy-pyridine-2,6-dicarboxylic acid complexes: potentiometry,EPR spectroscopy and comparison across oxidation states

Evaluation of stability of vanadium(IV) and (V) complexes under similar conditions is critical for the interpretation and assessment of bioactivity of various vanadium species. Detailed understanding of the chemical properties of these complexes is necessary to explain differences observed their activity in biological systems. These studies are carried out to link the chemistry of both vanadium(IV) and (V) complexes of two ligands, 2,6-pyridinedicarboxylic acid (dipicolinic acid, H(2)dipic) and 4-hydroxy-2,6-pyridinedicarboxylic acid (H(2)dipic-OH). Solution speciation of the two 2,6-pyridinedicarboxylic acids with vanadium(IV) and vanadium(V) ions was determined by pH-potentiometry at I=0.2 M (KCl) ionic strength and at T=298 K. The stability and the metal affinities of the ligands were compared. Vanadium(V) complexes were found to form only tridentate coordinated 1:1 complexes, while vanadium(IV) formed complexes with both 1:1 and 1:2 stoichiometries. The formation constant reflects hindered coordination of a second ligand molecule, presumably because of the relatively small size of the metal ion. The most probable binding mode of the complexes was further explored using ambient and low temperature EPR spectroscopy for vanadium(IV) and 51V NMR spectroscopy for vanadium(V) systems. Upon complex formation the pyridinol-OH in position 4 deprotonates with pK approximately 3.7-4.1, which is approximately 6 orders of magnitude lower than that of the free ligand. The deprotonation enhances the ligand metal ion affinity compared to the parent ligand dipicolinic acid. In the light of the speciation and stability data of the metal complexes, the efficiency of the two ligands in transporting the metal ion in the two different oxidation states are assessed and discussed.     

Siderophore mediated iron(III) uptake in Gliocladium virens. 1. Properties of cis-fusarinine, trans-fusarinine, dimerum acid, and their ferric complexes

Gliocladium virens (ATCC 24290) produces two monohydroxamates (cis- and trans-fusarinine) and a dihydroxamate (dimerum acid) as the major siderophores in the culture filtrate. This fungus also produces minor quantities of three trihydroxamates (the deferri forms of ferricrocin, coprogen B, and coprogen). Structural features of the free ligands and the metal complexed forms of cis-fusarinine (cF), trans-fusarinine (tF), and dimerum acid (DA) have been investigated using electronic (visible), circular dichroism (CD), and NMR spectroscopy. In aqueous solution, in the pH range of 6.5-8.0, all of the ferric complexes of cF (and tF) exist as 3:1 chelates. Fe(cF)3 [or Fe(tF)3] forms both lambda and delta coordination isomers, but the former in a slight excess. DA forms a 3:2 ferric complex in the pH range of 5.0-8.0. Iron coordination in Fe2(DA)3 is predominantly delta. DA ligands in Ga2(DA)3 exist as two different conformers at a ratio of 2:1. In mixed solution cF, tF, and DA form a large number of homogeneous and heterogeneous Fe(III) chelates.     

Relaxometric characterization of human hemalbumin

Hemalbumin [i.e., Fe(III)-protoporphyrin IX-human serum albumin; Fe(III)heme-HSA] is an important intermediate in the recovery of heme iron following hemolysis. Relaxometric data are consistent with the occurrence of a hexacoordinated high-spin Fe(III) center with no water in the inner coordination sphere. The relatively high relaxation enhancement observed for an aqueous solution of Fe(III)heme-HSA (r1p=4.8 mM(-1)s(-1) at 20 MHz, pH 7, and 25 C) is ascribed to the occurrence of a strong contribution from water molecules in the second coordination sphere. Structural analysis of the putative binding region has been performed by a Monte Carlo simulated annealing procedure, which allowed us to identify His105 and Tyr148 as axial ligands. The role of a tyrosinate as the sixth Fe(III)heme ligand is supported by the pH-dependent analysis. Interestingly, when Fe(III) is replaced by Mn(III), the occurrence of a fast exchanging water molecule at pH values close to neutrality is detected. As the pH is increased, the Mn(III) containing system behaves analogously to Fe(III)heme-HSA. At higher pH, the phenolate ligand is eventually displaced by OH- from both Fe(III) and Mn(III) centers. Support for the proposed bonding scheme has been gained also from competitive binding assays for the sixth coordination site by fluoride, azide, and imidazole ligands.     

A spectrophotometric determination of the binding constants of cyanide by hemin in aqueous ethanol solutions

Binding constants for the coordination of cyanide by monomeric hemin in aqueous ethanol solution were determined in the pH range 8–12 by fitting spectrophotometric data to a binding isotherm using a non-linear least-squares method. Two cyanide ligands are coordinated by Fe(III) without evidence for the intermediate mono-ligated species. The binding constants are strongly pH-dependent because of protonation of the ligand and competition for the metal ion by hydroxide. From the pH dependence of the observed binding constants, a pH-independent value of log K = 17.62±0.03 for the binding of two CN⁻ ligands by monomeric hemin in alkaline solution at 25 °C, μ=0.100 M, is obtained.     

Interactions of aluminium(III) with glycerolphosphates and glycerophosphorylcholine

The complexation of aluminium(III) with glycerol-1-phosphate (G1P) and glycerol-2-phosphate (G2P) in aqueous solutions has been studied as a function of pH, by pH-potentiometry, 31P NMR spectroscopy and ESI mass spectrometry. Various mononuclear complexes (MLH(2)(3+), MLH(2+), ML(+), ML(2)H, ML(2)(-)) and polynuclear species (M(3)L(3)H(-1)(2+), M(3)L(2)H(-n)((n-5)-) with n=5, 6, 7, M(2)L(2)H(-1)(+) ) are formed in the system where the full protonated ligands are noted LH(2). NMR experiments clearly show that G1P and G2P already interact with Al(III) at pH 1. The potentiometric results are confirmed by ESI measurements and 31P NMR studies. No metal ion-induced deprotonation and coordination of the alcoholic-OH functions seem to occur during the complexation. The situation is very different for the glycerophosphorylcholine ligand (GPC identical with LH). Only the complex ML(3+) is formed in aqueous solution with a relatively low formation constant (K=5 at 37 degrees C). This species is clearly identified in 31P and 27Al NMR spectra. The complexation study as a function of the temperature allowed us to determine the thermodynamic parameters of the complex formation. The complexation is not governed by the reaction enthalpy that is found to be positive but by the entropy that is largely positive.     

ATP(adenosine triphosphate)-vanadyl complex

Owing to the significance of inhibitory effect of vanadium ion to Na, K-ATPase, a complex formation between ATP and vanadyl ion was investigated over a wide pH range. Formations of two types of complex are observed : a blue complex formed in acidic and neutral pH regions and a green complex at higher than pH 11. On the basis of the results on potentiometric titration, optical and EPR spectra and empirical bonding coefficients calculated from the EPR parameters, two characteristic types of coordination environment are proposed for the ATP-vanadyl complex : a blue 1:1 complex is a relatively weak complex including a phosphate-vanadyl coordination mode, whereas a green 2:1 complex is much stronger complex including a vanadyl-oxygen coordination contributed from a deprotonated hydroxyl group of the ribose moiety of ATP.     

Mixed ligation to vanadium(III): Synthesis,spectra and structures of bis(acetylacetonato)(o-phenanthroline)vanadium(III) fluoroborate and bis(acetylacetonato)(o-phenanthroline)vanadium(III) perchlorate

Two mixed ligand vanadium(III) complexes bis(acetylacetonato)(phenanthroline)vanadium(III) fluoroborate (1) and bis(acetylacetonato)(phenanthroline)vanadium(III) perchorate (2) have been prepared and characterized by UV–Vis, IR, ¹H NMR spectroscopic techniques and single crystal X-ray diffraction. The electronic spectra are as expected for V(III) in an octahedral environment. The ¹H NMR spectra are typical of paramagnetic V(III) species. The complexes have crystallized with dichloromethane solvate and are isomorphous. The coordination sphere is composed of vanadium in a distorted octahedral environment, ligated to two bidentate chelating acetylacetonate ligands through the oxygen atoms and two phenanthroline nitrogens.     

Iron-bleomycin-deoxyribonucleic acid system. Evidence of deoxyribonucleic acid interaction with the alpha-amino group of the beta-aminoalanine moiety

The Fe(III) complex of bleomycin (BLM) is, at pH 4, in the high-spin form. At pH 7, the coordination of the alpha-amino group of the beta-aminoalanine moiety of BLM converts it to a low-spin species: BLM X Fe(III) X alpha NH2. The conversion of the high-spin species to the low-spin one can also take place at pH 4 (i) by addition of ligands L such as N3-, S2O3(2-), and SCN- or (ii) through interaction with DNA. Moreover, the addition, at pH 7, of DNA to BLM X Fe(III) that has been previously complexed with one of these ligands L displaces this latter from its position. These results suggest that (i) the ligand L occupies the same site of coordination as the alpha-amino group and (ii) an interaction occurs between the beta-aminoalanine moiety of BLM and DNA that lowers the pKd of the alpha-amino group, promoting its coordination to iron.