The vanadium environment in blood cells of Ascidia ceratodes is divergent at all organismal levels: an XAS and EPR spectroscopic study

K-edge X-ray absorption and EPR spectroscopies were used to test the variation in blood cell vanadium between and within specimens of the tunicate Ascidia ceratodes from Bodega Bay, California. Intracellular vanadium was speciated by fitting the XAS spectra of whole blood cells with linear combinations of the XAS spectra of models. Blood cell samples representing one specimen each, respectively, revealed 92.5 and 38.7% of endogenous vanadium as [V(H(2)O)(6)](3+), indicating dissimilar distributions. Conversely, vanadium distributions within blood cell samples respectively representing one and six specimens proved very similar. The derived array of V(III) complexes was consistent with multiple intracellular regions that differ both in pH and c(sulfate), both within and between specimens. No systematic effect on vanadium distribution was apparent on mixing blood cells. EPR and XAS results indicated at least three forms of endogenous vanadyl ion, two of which may be dimeric. An inverse linear correlation was found between soluble and complexed forms of vanadyl ion, implying co-regulation. The EPR A value of endogenous vanadyl ion [A(0)=(1.062+/-0.008)x10(-2) cm(-1)] was marginally different from that representing Monterey Bay A. ceratodes [A(0)=(1.092+/-0.006) x10(-2) cm(-1)]. Comparisons indicate that Bodega Bay A. ceratodes maintain V(III) in a more acidic intracellular environment on average than do those from Monterey Bay, showing variation across populations. Blood cell vanadium thus noticeably diverges at all organismal levels among A. ceratodes.     

The uptake and fate of vanadyl ion in ascidian blood cells and a detailed hypothesis for the mechanism and location of biological vanadium reduction. A visible and X-ray absorption spectroscopic study

Vanadium K-edge X-ray absorption spectroscopy (XAS) has been used to track the uptake and fate of VO(2+) ion in blood cells from Ascidia ceratodes, following exposure to dithiothreitol (DTT) or to DTT plus VO(2+). The full range of endogenous vanadium was queried by fitting the XAS of blood cells with the XAS spectra of model vanadium complexes. In cells exposed only to DTT, approximately 0.4% of a new V(III) species was found in a site similar to Na[V(edta)(H(2)O)]. With exposure to DTT and VO(2+), average intracellular [VO(aq)](2+) increased from 3% to 5%, and 6% of a new complexed form of vanadyl ion appeared evidencing a ligand array similar to [VO(edta)](2-). At the same time, the relative ratio of blood cell [V(H(2)O)(6)](3+) increased at the expense of [V(H(2)O)(5)(SO(4))](+) in a manner consistent with a significant increase in endogenous acidity. In new UV/Visible experiments, VO(2+) could be reduced to 7-coordinate [V(nta)(H(2)O)(3)] or [V(nta)(ida)](2-) with cysteine methyl ester in pH 6.5 solution. Ascorbate reduced [VO(edta)](2-) to 7-coordinate [V(edta)(H(2)O)](-), while [VO(trdta)](2-) was unreactive. These results corroborate the finding that the reductive EMF of VO(2+) is increased by the availability of a 7-coordinate V(III) product. Finally, a new and complete hypothesis is proposed for an ascidian vanadate reductase. The structure of the enzyme active site, the vanadate-vanadyl-vanadic reduction mechanism, the cellular locale, and elements of the regulatory machinery governing the biological reduction of vanadate and vanadyl ion by ascidians are all predicted. Together these constitute the new field of vanadium redox enzymology.     

Oxidation of L-serine and L-threonine by bis(hydrogen periodato)argentate(III) complex anion: a mechanistic study

Oxidation of l-serine and l-threonine by a silver(III) complex anion, [Ag(HIO(6))(2)](5-), has been studied in aqueous alkaline medium. The oxidation products of the amino acids have been identified as ammonia, glyoxylic acid and aldehyde (formaldehyde for serine and acetaldehyde for threonine). Kinetics of the oxidation reactions has been followed by the conventional spectrophotometry in the temperature range of 20.0-35.0 degrees C and the reactions display an overall second-order behavior: first-order with respect to both Ag(III) and the amino acids. Analysis of influences of [OH(-)] and [periodate] on the second-order rate constants k' reveals an empirical rate expression: k(')=(k(a)+k(b)[OH(-)])K(1)/([H(2)IO(6)(3-)](e)+K(1)), where [H(2)IO(6)(3-)](e) is equilibrium concentration of periodate, and where k(a)=6.1+/-0.5M(-1)s(-1), k(b)=264+/-6M(-2)s(-1), and K(1)=(6.5+/-1.3)x10(-4)M for serine and k(a)=12.6+/-1.7M(-1)s(-1), k(b)=(5.5+/-0.2)x10(2)M(-2)s(-1), and K(1)=(6.2+/-1.5)x10(-4)M for threonine at 25.0 degrees C and ionic strength of 0.30M. Activation parameters associated with k(a) and k(b) have also been derived. A reaction mechanism is proposed to involve two pre-equilibria, leading to formation of an Ag(III)-periodato-amino acid ternary complex. The ternary complex undergoes a two-electron transfer from the coordinated amino acid to the metal center via two parallel pathways: one pathway is spontaneous and the other is assisted by a hydroxide ion. Potential applications of the Ag(III) complex as a reagent for modifications of peptides and proteins are implicated.     

Model investigations for vanadium-protein interactions: vanadium(III) compounds with dipeptides and their oxovanadium(IV) analogues

The reaction of VCl(3) with 1,10-phenanthroline and a series of dipeptides (H(2)dip), having aliphatic as well as aromatic side chains, in methyl alcohol and in the presence of triethylamine affords vanadium(III) compounds of the general formula [V(III)(dip)(MeOH)(phen)]Cl. Aerial oxidation/hydrolysis of the vanadium(III) species gives their oxovanadium(IV) analogues of the general formula [V(IV)O(dip)(phen)]. X-ray crystallographic characterization of the [V(IV)O(dip)(phen)] compounds (where dip(2-)=Gly- L-Ala, Gly- L-Val and Gly- L-Phe) revealed that the vanadium atom possesses a severely distorted octahedral coordination and is ligated to a tridentate dip(2-) ligand at the N(amine) atom, the deprotonated N(peptide) atom and one of the O(carboxylate) atoms, as well as an oxo group and two phenanthroline nitrogen atoms. Circular dichroism characterization of the V(III)/V(IV)O(2+)-dipeptide compounds revealed a strong signal for the V(IV)O(2+) species in the visible range of the spectrum, with a characteristic pattern which may be exploited to identify the N(am), N(pep) and O(car) ligation of a peptide or a protein to V(IV)O(2+) center, and a weak Cotton effect of opposite sign to their vanadium(III) analogues. The visible spectra of the V(III)-dipeptide compounds revealed two d-d bands with high intensity, thus indicating that the covalency of the metal-donor atoms is significant, i.e. the vanadium d orbitals are significantly mixed with the ligand orbitals, and this is confirmed by the low values of their Racah B parameters. The high-intensity band of the V(IV)O(2+)-dipeptide compounds at approximately 460 nm implies also a strong covalency of the metal with the equatorial donor atoms and this was supported by the EPR spectra of these compounds. Moreover, the V(III)/V(IV)O(2+)-dipeptide complexes were characterized by EPR and IR spectroscopies as well as conductivity and magnetic susceptibility measurements.     

Cooperativity and intermediates in the equilibrium reactions of Fe(II,III) with ethanethiolate in N-methylformamide solution

The reaction of FeCl(2) or FeCl(3) with sodium ethanethiolate (SEt) in N-methylformamide (NMF) has been reevaluated to rectify a previous Fe(II) oxidation artifact. On titrating Fe(II) with EtS(-) concentrations up to 12 mol Eq, new features in the UV/vis spectrum (epsilon(344)=(3.1+/-0.2)x10(3) M(-1) cm(-1); epsilon(486)=(4.5+/-0.1)x10(2) M(-1) cm(-1)) indicated that the first observable step was the formation of a single complex different from the known tetrahedral tetrathiolate, [Fe(SEt)(4)](2-) . As the EtS(-) concentration increased past 12.5 mol Eq the UV/vis spectrum gradually transformed to that of [Fe(SEt)(4)](2-) (lambda(max)=314 nm). A Hill-formalism fit to the titration data of the initially formed complex indicated cooperative ligation by three ethanethiolate ions, with K(coop)=(1.7+/-0.1)x10(3) M(-3) and Hill "n"=2.4+/-0.1 (r=0.997). The 3:1 EtS(-)-Fe(II) complex is proposed to be [Fe(2)(SEt)(6)](2-). Titration of Fe(III) with EtS(-) showed direct cooperative formation of [Fe(SEt)(4)](-) [epsilon(340)=(3.4+/-0.5)x10(3) M(-1) cm(-1)] with a Hill-formalism K(coop)=(4.3+/-0.1)x10(2) M(-4) and a Hill coefficient "n"=3.7+/-0.2 (r=0.996). Further ligation past [Fe(SEt)(4)](-) was observed at EtS(-) concentrations above 35 mol Eq. The Fe(III) Hill constants are at variance with our previous report. However, the UV/vis spectrum of Fe(III) in NMF solution was found to change systematically over time, consistent with a slow progressive deprotonation of [Fe(nmf)](3+). The observed time-to-time differences in the equilibrium chemistry of Fe(III) with ethanethiolate in NMF thus reflect variation in the microscopic solution composition of FeCl(3) in alkaline NMF solvent. These results are related to the chemistry of nitrogenase FeMo cofactor in alkaline NMF solution.     

Electron transfer kinetics and mechanistic study of the thionicotinamide coordinated to the pentacyanoferrate(III)/(II) complexes: a model system for the in vitro activation of thioamides anti-tuberculosis drugs

The mechanism of activation thioamide-pyridine anti-tuberculosis prodrugs is poorly described in the literature. It has recently been shown that ethionamide, an important component of second-line therapy for the treatment of multi-drug-resistant tuberculosis, is activated through an enzymatic electron transfer (ET) reaction. In an attempt to shed light on the activation of thioamide drugs, we have mimicked a redox process involving the thionicotinamide (thio) ligand, investigating its reactivity through coordination to the redox reversible [Fe(III/II)(CN)(5)(H(2)O)](2-/3-) metal center. The reaction of the Fe(III) complex with thionicotinamide leads to the ligand conversion to the 3-cyanopyridine species coordinated to a Fe(II) metal center. The rate constant, k(et)=10 s(-1), was determined for this intra-molecular ET reaction. A kinetic study for the cross-reaction of thionicotinamide and [Fe(CN)(6)](3-) was also carried out. The oxidation of thionicotinamide by [Fe(CN)(6)](3-) leads to formation of mainly 3-cyanopyridine and [Fe(CN)(6)](4-) with a k(et)=(5.38+/-0.03) M(-1)s(-1) at 25 degrees C, pH 12.0. The rate of this reaction is strongly dependent on pH due to an acid-base equilibrium related to the deprotonation of the R-SH functional group of the imidothiol form of thionicotinamide. The kinetic results reinforced the assignment of an intra-molecular mechanism for the ET reaction of [Fe(III)(CN)(5)(H(2)O)](2-) and the thioamide ligand. These results can be valuable for the design of new thiocarbonyl-containing drugs against resistant strains of Mycobacterium tuberculosis by a self-activating mechanism.     

Kinetics and mechanism of the reduction of CrVI and CrV by D-lactobionic acid

The oxidation of D-lactobionic acid by Cr(VI) yields the 2-ketoaldobionic acid and Cr(3+) as final products when a 20-times or higher excess of the aldobionic acid over Cr(VI) is used. The redox reaction takes place through a complex multistep mechanism, which involves the formation of intermediate Cr(IV) and Cr(V) species. Cr(IV) reacts with lactobionic acid much faster than Cr(V) and Cr(VI) do, and cannot be directly detected. However, the formation of CrO(2)(2+), observed by the first time for an acid saccharide/Cr(VI) system, provides indirect evidence for the intermediacy of Cr(IV) in the reaction path. Cr(VI) and the intermediate Cr(V) react with lactobionic acid at comparable rates, being the complete rate laws for the Cr(VI) and Cr(V) consumption expressed by: -d[Cr(VI)]/dt=[k(I)+k(II)[H(+)]][lactobionicacid][Cr(VI)], where k(I)=(4.1+/-0.1) x 10(-3) M(-1) s(-1) and k(II)=(2.1+/-0.1) x 10(-2) M(-2) s(-1); and -d[Cr(V)]/dt=[k(III)[H(+)]+(k(IV)+k(V)[H(+)])[lactobionicacid]] [Cr(V)], where k(III)=(1.8+/-0.1) x 10(-3) M(-1) s(-1), k(IV)=(1.1+/-0.1) x 10(-2) M(-1) s(-1) and k(V)=(1.0+/-0.1) x 10(-2) M(-2) s(-1), at 33 degrees C. The Electron Paramagnetic Resonance (EPR) spectra show that five-co-ordinate oxo-Cr(V) bischelates are formed at pH 1-5 with the aldobionic acid bound to Cr(V) through the alpha-hydroxyacid group.     

Metabolism of added orthovanadate to vanadyl and high-molecular-weight vanadates by Saccharomyces cerevisiae

The effect of vanadium oxides on living systems may involve the in vivo conversion of vanadate and vanadyl ions. The addition of 5 mM orthovanadate (VO4(3-), V(V)), a known inhibitor of the (Na,K)-ATPase, to yeast cells stopped growth. In contrast, the addition of 5 mM vanadyl (VO2+, V(IV) stimulated growth. Orthovanadate addition to whole cells is known to stimulate various cellular processes. In yeast, both ions inhibited the plasma membrane Mg2+ ATPase and were transported into the cell as demonstrated with [48V]VO4(3-) and VO2+. ESR spectroscopy has been used to measure the cell-associated paramagnetic vandyl ion, while 51V NMR has detected cell-associated diamagnetic vanadium (e.g. V(V)). Cells were exposed to both toxic (5 mM) and nontoxic (1 mM) concentrations of vanadate in the culture medium. ESR showed that under both conditions, vanadate became cell associated and was converted to vanadyl which then accumulated in the cell culture medium. 51V NMR studies showed the accumulation of new cell-associated vanadium resonances identified as dimeric vanadate and decavanadate in cells exposed to toxic amounts of medium vanadate (5 mM). These vanadate compounds did not accumulate in cells exposed to 1 mM vanadate. These studies confirm that the inhibitory form of vanadium usually observed in in vitro experiments is vanadate, in one or more of its hydrated forms. These data also support the hypothesis that the stimulatory form of vanadium usually observed in whole cell experiments is the vanadyl ion or one or more of its liganded derivatives.     

Early transition metal complexes of dinucleating Pacman ligands: X-ray crystal structures of mixed-valence V/V complexes

The use of a Schiff-base calixpyrrole (L) as a dinucleating ligand for early transition metals is described. Salt elimination reactions between the crystallographically-characterised [K₄(THF)₃(PhMe)(L)] and titanium(III) and vanadium(III) halides form the new dinuclear complexes [(MCl)₂(L)] (M = Ti, V). Adventitious, and partial, oxidation of [(VCl)₂(L)] resulted in the formation of the unusual mixed-valence vanadyl complexes [(VO)(S)(VCl)(L)] (S = THF or pyridine), which both adopt desired Pacman geometries in the solid state in which the oxo ligand is accommodated within the dinuclear molecular cleft.     

The [Ru(Hedta)NO](0.1-) system: structure, chemical reactivity and biological assays

The [Ru(II)(Hedta)NO(+)] complex is a diamagnetic species crystallizing in a distorted octahedral geometry, with the Ru-N(O) length 1.756(4) A and the RuNO angle 172.3(4) degrees . The complex contains one protonated carboxylate (pK(a)=2.7+/-0.1). The [Ru(II)(Hedta)NO(+)] complex undergoes a nitrosyl-centered one-electron reduction (chemical or electrochemical), with E(NO+/NO)=-0.31 V vs SCE (I=0.2 M, pH 1), yielding [Ru(II)(Hedta)NO](-), which aquates slowly: k(-NO)=2.1+/-0.4x10(-3) s(-1) (pH 1.0, I=0.2 M, CF(3)COOH/NaCF(3)COO, 25 degrees C). At pHs>12, the predominant species, [Ru(II)(edta)NO](-), reacts according to [Ru(II)(edta)NO](-)+2OH(-)-->[Ru(II)(edta)NO(2)](3-), with K(eq)=1.0+/-0.4 x 10(3) M(-2) (I=1.0 M, NaCl; T=25.0+/-0.1 degrees C). The rate-law is first order in each of the reactants for most reaction conditions, with k(OH(-))=4.35+/-0.02 M(-1)s(-1) (25.0 degrees C), assignable mechanistically to the elementary step comprising the attack of one OH(-) on [Ru(II)(edta)NO](-), with subsequent fast deprotonation of the [Ru(II)(edta)NO(2)H](2-) intermediate. The activation parameters were DeltaH(#)=60+/-1 kJ/mol, DeltaS(#)=-31+/-3 J/Kmol, consistent with a nucleophilic addition process between likely charged ions. In the toxicity up-and-down tests performed with Swiss mice, no death was observed in all the doses administered (3-9.08 x 10(-5) mol/kg). The biodistribution tests performed with Wistar male rats showed metal in the liver, kidney, urine and plasma. Eight hours after the injection no metal was detected in the samples. The vasodilator effect of [Ru(II)(edta)NO](-) was studied in aortic rings without endothelium, and was compared with sodium nitroprusside (SNP). The times of maximal effects of [Ru(II)(edta)NO](-) and SNP were 2 h and 12 min, respectively, suggesting that [Ru(II)(edta)NO](-) releases NO slowly to the medium in comparison with SNP.     

Cobalt and the iron acquisition pathway: competition towards interaction with receptor 1

During iron acquisition by the cell, complete homodimeric transferrin receptor 1 in an unknown state (R1) binds iron-loaded human serum apotransferrin in an unknown state (T) and allows its internalization in the cytoplasm. T also forms complexes with metals other than iron. Are these metals incorporated by the iron acquisition pathway and how can other proteins interact with R1? We report here a four-step mechanism for cobalt(III) transfer from CoNtaCO(3)(2-) to T and analyze the interaction of cobalt-loaded transferrin with R1. The first step in cobalt uptake by T is a fast transfer of Co(3+) and CO(3)(2-) from CoNtaCO(3)(2-) to the metal-binding site in the C-lobe of T: direct rate constant, k(1)=(1.1+/-0.1) x 10(6) M(-1) s(-1); reverse rate constant, k(-1)=(1.9+/-0.6) x 10(6) M(-1) s(-1); and equilibrium constant, K=1.7+/-0.7. This step is followed by a proton-assisted conformational change of the C-lobe: direct rate constant, k(2)=(3+/-0.3) x 10(6) M(-1) s(-1); reverse rate constant, k(-2)=(1.6+/-0.3) x 10(-2) s(-1); and equilibrium constant, K(2a)=5.3+/-1.5 nM. The two final steps are slow changes in the conformation of the protein (0.5 h and 72 h), which allow it to achieve its final thermodynamic state and also to acquire second cobalt. The cobalt-saturated transferrin in an unknown state (TCo(2)) interacts with R1 in two different steps. The first is an ultra-fast interaction of the C-lobe of TCo(2) with the helical domain of R1: direct rate constant, k(3)=(4.4+/-0.6)x10(10) M(-1) s(-1); reverse rate constant, k(-3)=(3.6+/-0.6) x 10(4) s(-1); and dissociation constant, K(1d)=0.82+/-0.25 muM. The second is a very slow interaction of the N-lobe of TCo(2) with the protease-like domain of R1. This increases the stability of the protein-protein adduct by 30-fold with an average overall dissociation constant K(d)=25+/-10 nM. The main trigger in the R1-mediated iron acquisition is the ultra-fast interaction of the metal-loaded C-lobe of T with R1. This step is much faster than endocytosis, which in turn is much faster than the interaction of the N-lobe of T with the protease-like domain. This can explain why other metal-loaded transferrins or a protein such as HFE-with a lower affinity for R1 than iron-saturated transferrin but with, however, similar or higher affinities for the helical domain than the C-lobe-competes with iron-saturated transferrin in an unknown state towards interaction with R1.     

Remarkably stereoselective photoinduced electron-transfer reaction between zinc myoglobin and optically active binaphthyl bisviologen

We have designed and synthesized new optically active bisviologens ([BNMV](4+)) containing a binaphthyl moiety to examine the stereoselective photoinduced electron-transfer (ET) reactions with zinc-substituted myoglobin (ZnMb) by flash photolysis. The photoexcited triplet state of ZnMb, (3)(ZnMb)*, was successfully quenched by [BNMV](4+) ions to form the radical pair of a ZnMb cation (ZnMb(.+)) and a reduced viologen ([BNMV](.3+)), followed by a thermal ET reaction to the ground state. The rate constants ( k(q)) for the ET quenching at 25 degrees C were obtained as k(q)( R)=(2.9+/-0.2)x10(7) M(-1) s(-1) and k(q)( S)=(2.2+/-0.2)x10(7) M(-1) s(-1), respectively. The ratio of k(q)( R)/ k(q)( S)=1.3 indicates that the ( R)-isomer of the chiral viologen preferentially quenches (3)(ZnMb)*. On the other hand, the rate constants ( k) for the thermal ET reaction from [BNMV](.3+) to ZnMb(-+) at 25 degrees C were k( R)=(1.2+/-0.1)x10(8) M(-1) s(-1) and k( S)=(0.47+/-0.03)x10(8) M(-1) s(-1), respectively, and the ratio remarkably increased to k( R)/ k( S)=2.6. The activation parameters, Delta H(not equal) and Delta S(not equal), were determined from the kinetic measurements at various temperatures (10-30 degrees C) to understand the ET mechanisms. In the quenching reaction, the energy differences of Delta Delta H*(R- S) and T Delta Delta S*( R- S) at 25 degrees C were calculated to be -3.9+/-1.6 and -3.3+/-0.2 kJ mol(-1), respectively, whereas Delta Delta H*( R-S)=7.7+/-1.9 kJ mol(-1 )and T Delta Delta S*( R-S)=9.9+/-0.5 kJ mol(-1 )were found for the thermal ET reaction. Therefore, the thermal ET reaction to the ground state was proved to be dominated by the entropy term, and the large stereoselectivity may arise from the decrease in charge repulsion between donor and acceptor.     

Structure-dependent metallokinetics of antidiabetic vanadyl-picolinate complexes in rats: studies on solution structure,insulinomimetic activity,and metallokinetics

The insulinomimetic effect of vanadium is the most remarkable and important among its several biological actions. Vanadyl ion (+4 oxidation state of vanadium) and its complexes have been found to normalize the blood glucose levels of both type 1 and 2 diabetic animals. We have developed insulinomimetic vanadyl complexes having different coordination modes, emphasizing the possible usefulness of vanadyl-picolinate [VO(pa)(2)] and its related complexes with the VO(N(2)O(2)) coordination mode. In order to apply these complexes clinically in the future, the relationship between the chemical structure, insulinomimetic action, organ distribution of vanadium, and blood disposition of vanadyl species must be closely investigated. In the present investigation, we studied the blood disposition of the vanadyl-picolinate complexes in healthy rats, and tried to understand comprehensively the relationship between the structures, insulinomimetic activity, and metallokinetic parameters of the complexes, which had been recently prepared and specifically synthesized for the present study, by using an in vivo blood circulation monitoring -- electron spin resonance (BCM-ESR) method for analyzing ESR signals due to paramagnetic metal ions and complexes in the blood in real time. Metallokinetic parameters were estimated based on the blood clearance curves in terms of a two-compartment pharmacokinetic model, and vanadyl species were indicated to be distributed in peripheral tissues and gradually eliminated from the circulating blood, depending on their chemical structures. Vanadyl concentrations in the blood of rats given bis(5-iodopicolinato)oxovanadium(IV) [VO(5ipa)(2)] and bis(3-methylpicolinato)oxovanadium(IV) [VO(3mpa)(2)] with electron-withdrawing and donating groups, respectively, remained significantly higher and longer, due to their slower clearance rates from the blood, than in rats given other complexes, suggesting that the high exposure and long residence of vanadyl species bring about the high normoglyceric effect in diabetic animals. We then examined the relationship between insulinomimetic activity and metallokinetic parameters in the family of VO(pa)(2) for further development of insulinomimetic vanadyl complexes. IC(50), the 50% inhibitory concentration of the complexes on the free fatty acid release from isolated rat adipocytes treated with epinephrine, was found to be sufficiently correlated with metallokinetic parameters such as area under the concentration curve, mean residence time, total clearance, and distribution volume at steady-state. Furthermore, the in vivo antidiabetic activity of the complexes was enhanced with increasing exposure and residence of vanadyl species in the blood of animals. On the basis of these results, we concluded that in vitro insulinomimetic activity, metallokinetic character, and in vivo antidiabetic action of vanadyl-picolinate complexes are closely related to their chemical structures.     

Protein degradation by peroxide catalyzed by chromium (III): role of coordinated ligand

In order to understand the role of coordinated ligands in controlling the biotoxicity of chromium (III), interactions of three types of chromium (III) complexes viz. trans-diaquo [1,2 bis (salicyledeneamino) ethane chromium (III) perchlorate, [(Cr(salen)(H(2)O)(2)](ClO(4)); tris (ethylenediamine) chromium (III) chloride, [Cr(en)(3)]Cl(3), and monosodium ethylene diamine tetraacetato monoaquo chromiate (III), [Cr(EDTA)(H(2)O)]Na with BSA has been investigated. Spectroscopic and equilibrium dialysis studies show that the two cationic complexes Cr(salen)(H(2)O)(+)(2) and Cr(en)(3+)(3) bind to the protein with a protein-metal ratio of 1:8 and 1:4. The anionic complex Cr(EDTA)(H(2)O)(-) binds to the protein with a protein-metal ratio of 1:2. The binding constant K(b) as estimated from the fluorescence quenching studies has been found to be 7.6 +/- 0.4 x 10(3) M(-1), 3.1 +/- 0.2 x 10(2) M(-1), and 1.8 +/- 0.2 x 10(2) M(-1) for Cr(salen)(H(2)O)(+)(2), Cr(en)(3+)(3), and Cr(EDTA)(H(2)O)(-) respectively indicating that the thermodynamic stability of protein-chromium complex is Cr(salen)(H(2)O)(+)(2) > Cr(en)(3+)(3) approximately Cr(EDTA)(H(2)O)(-). The complexes Cr(salen)(H(2)O)(+)(2) and Cr(EDTA)(H(2)O)(-) in the presence of hydrogen peroxide have been found to bring about protein degradation, whereas Cr(en)(3+)(3) does not bring about any protein damage. This clearly shows that the nature of the chromium (III) complex plays a major role in the biotoxicity of chromium (III).     

The fate of cytoplasmic vanadium. Implications on (NA,K)-ATPase inhibition.

The fate of vanadate (+5 oxidation state of vanadium) taken up by the red cell was studied using EPR spectroscopy. The appearance of an EPR signal indicated that most of the cytoplasmic vanadate is reduced to the +4 oxidation state with axial symmetry characteristic of vanadyl ions. The signal at 23 degrees C was characteristic of an immobilized system indicating that the vanadyl ions in the cytoplasm are associated with a large molecule. [48V]Vanadium eluted with hemoglobin when the lysate from Na3[48V[O4-treated red cells was passed through a Sephadex G-100 column and rabbit anti-human hemoglobin serum caused a hemoglobin-specific precipitation of 48V when added to the red cell lysate. Both results indicate that hemoglobin is the protein which binds cytoplasmic vanadyl ions. However, neither sodium vanadate nor vanadyl sulfate bound to purified hemoglobin in vitro. Finally, transient kinetics of vanadyl sulfate interaction with the sodium-and potassium-stimulated adenosine triphosphatase showed that the +4 oxidation state of vanadium is less effective than the +5 oxidation state in inhibiting this enzyme. These results indicate that oxidation-reduction reactions in the cytoplasm are capable of relieving vanadate inhibition of cation transport.     

Consistent simulation of X- and Q-band EPR spectra of an unsymmetric dinuclear Mn2(II,III) complex

Simulation of X- and Q-band electron paramagnetic resonance (EPR) spectra of an unsymmetric dinuclear [Mn(2)(II,III)L(mu-OAc)(2)]ClO(4) complex (1), (L is the dianion of 2-{[N,N-bis(2-pyridylmethyl)amino]methyl}-6-{[N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N-(2-pyridylmethyl)amino]methyl}-4-methylphenol) was performed using one consistent set of simulation parameters. Rhombic g-tensors and hyperfine tensors were necessary to obtain satisfactory simulation of the EPR spectra. The anisotropy of the effective hyperfine tensors of each individual (55)Mn ion was further analyzed in terms of intrinsic hyperfine tensors. Detailed analysis shows that the hyperfine anisotropy of the Mn(III) ion is a result of the Jahn-Teller effect and thus an inherent character. In contrast, the anomalous hyperfine anisotropy of the Mn(II) ion is attributed as being transferred from the Mn(III) ion through the spin exchange interaction. The anisotropy parameter for the Mn(II) is deduced as D(II)=-1.26+/-0.2cm(-1). This is the first reported D(II) value for a Mn(II) ion in a weakly exchange coupled mixed-valence Mn(2)(II,III) complex with a bis-mu-acetato-bridge. The [see text] electronic configuration of the Mn(III) ion in 1 is revealed by the negative sign of its intrinsic hyperfine tensor anisotropy, Deltaa(III)=a(z)-a(x,y)=-46cm(-1). Lower spectral resolution of the Q-band EPR spectrum as compared to the X-band EPR spectrum is associated to large line width broadening of the x- and y-components in contrast to the z-component. The origins of the unequal distribution of line width between the z- and x-, y-components are discussed.     

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.     

Studies on metal carboxylates. VIII. Reactions of acetylacetonates of the first row transition elements with pyridine-2,6-dicarboxylic acid

The acetylacetonates VO(acac)₂, M(acac)₃, where M = V, Mn or Fe and [M′(acac)₂]_n, where M′ = Co, Ni or Cu, have been reacted with pyridine-2,6-dicarboxylic acid (dipicH₂) in acetone to afford the complexes VO(dipic)·2H₂O, M(acac)(dipic)·xH₂O [M = V, Mn or Fe and x = 1 or 0] and M₂(dipic) (dipicH)₂·yH₂O [M = Co, Ni or Cu and y = 2 or 0]. The cobalt(II) and nickel(II) complexes are converted to polymeric [M(dipic)]_n in ethanol and all three complexes formulated as M₂(dipic)(dipicH)₂ react with 2,2′2″-terpyridyl to yield M(dipic)(terpy)·3H₂O. The vanadium(III) complex V(acac)(dipic) is oxidized to VO(dipic)·4H₂O in aqueous solution via the vanadium(III) intermediate V(OH)(dipic)·2H₂O. Tentative structural conclusions are drawn for certain of these new complexes based upon room temperature spectral and magnetic measurements. The characterization of these complexes has included selected studies of their X-ray photoelectron spectra.     

The first enantioselective synthesis of the amavadin ligand and its complexation to vanadium

The ligand of the naturally occurring vanadium compound amavadin found in Amanita muscaria, (2S, 2'S)-N-hydroxyimino-2,2'-dipropionic acid (1), was synthesized stereoselectively in two steps with 43% overall yield. After complexation of this ligand to vanadyl acetate, amavadin was isolated in quantitative yield. Due to the chirality at vanadium amavadin consists of a mixture of delta and lambda diastereoisomers. Directly after its synthesis, the delta to lambda ratio of amavadin is 2.27 and it decreases to 0.80 after equilibrium has been reached. During this epimerization the optical rotation for V[(2S,2'S)-N-hydroxyimino-(2,2')-dipropionate]2 (=amavadin) changes from [alpha](D)25 = +36 degrees to +114.0 degrees (c = 0.5, H2O). For V[(2R,2'R)-N-hydroxyimino-(2,2')-dipropionate] the optical rotation changes from [alpha](D)25 = -36 degrees to -113.2 degrees (c = 0.5, H2O).