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.     

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.     

FeIII(SR)4]1- complexes can be synthesized by the direct reaction of thiolates with FeCl3

It is shown that the previously characterized [Fe(III)(SR)(4)](1-) (R=Et, i-Pr, Ph) complexes can be synthesized by the direct reaction of 4equiv. of LiSR with FeCl(3) in DMF solution. [Fe(III)(SR)(4)](1-) complexes are synthetic analogs for the [Fe(III)(S-Cys)(4)] center in rubredoxin proteins.     

Unprecedented forms of vanadium observed within the blood cells of Phallusia nigra using K-edge X-ray absorption spectroscopy

Fits to the vanadium K-edge X-ray absorption spectra (XAS) of five whole blood cell samples from the tunicate Phallusia nigra revealed unprecedented forms of intracellular vanadium. Endogenous vanadium was divided between the V(III) ion (74.2+/-5.1% of total V) and the vanadyl ion [V(IV)=O](2+) (25.2+/-5.4% of total V). The V(III) fraction included both [V(H(2)O)(6)](3+) (36.7+/-5.5%) modeled as VCl(3) in 1 M HCl, and three previously unprecedented chelated V(III) forms (37.5+/-4.6%). Two of these could be represented by the model ligand environments V(acetylacetonate)(3) (17.9+/-3.2%) and K(3)V(catecholate)(3) (13.1+/-4.7%), implying DOPA-like complexation. The third chelated form was represented by the 7-coordinate N(2)O(5) complex Na[V(edta)(H(2)O)] (8.0+/-1.8%). This coordination array, suggestive of a novel mononuclear V(III) protein site, contributed only to fits to samples 1, 2, 3 and 5, which were prepared in the presence of DTT. Endogenous V(IV) (25.2+/-5.4%) was principally modeled as VOCl(2) in 1 M HCl. EPR spectra (averages: A(parallel)=(1.842+/-0.006)x10(-2) cm(-1); A( perpendicular)=(0.718+/-0.007)x10(-2) cm(-1); g(parallel)=1.936+/-0.002; g( perpendicular)=1.990+/-0.001) confirmed the predominance of the aquated vanadyl ion. Blood cell sample five uniquely required the XAS spectrum of VOSO(4) in 0.1 M H(2)SO(4) solution (13.0%) and of [OV(V)(pivalate)(3)] (3.1%) to successfully fit the XAS pre-edge energy region. This endogenous V(V) signal is also unprecedented. These results are compared with those of analogous fits to the blood cells of Ascidia ceratodes and may support assignment of P. nigra to a different genus.     

Determination of ligand binding constants for the iron-molybdenum cofactor of nitrogenase: monomers, multimers, and cooperative behavior

Equilibrium titrations in N-methylformamide (NMF) of G-25 gel filtered (ox)-state FeMo cofactor [FeMoco(ox)] from Azotobacter vinelandii nitrogenase were carried out using sodium ethanethiolate and followed using UV/Vis absorption spectroscopy. For Fe-Moco(ox), a non-linear least squares (NLLSQ) fit to the data indicated a strong equilibrium thiolate-binding step with Keq = 1.3+/-0.2x10(6) M(-1). With 245 molar excess imidazole, cooperative binding of three ethanethiolates was observed. The best NLLSQ fit gave Keq=2.0+/-0.1x10(5) M(-2) and a Hill coefficient n=2.0+/-0.3. A Scatchard plot of these data was concave upward, indicating positive cooperativity. The fit to previously published data involving benzenethiol titration of the one-electron reduced (semi-reduced) cofactor, FeMoco(sr), as followed by EPR required a model that included both a sub-stoichiometric ratio of thiol to FeMoco(sr) and about five cooperative ligand binding sites. These constraints were met by modeling FeMoco(sr) as an aggregate, with fewer thiol binding sites than FeMoco(sr) units. The best fit model was that of FeMoco(sr) as a dodecamer with five cooperative benzenethiol binding sites, yielding a thiol binding constant of 3.32+/-0.09x10(4) M(-4.8) and a Hill coefficient n=4.8+/-0.6. The results of all the other published ligand titrations of FeMoco(sr) were similarly analyzed successfully in terms of equilibrium models that include both cooperative ligand binding and dimer-level aggregation. A possible structural model for FeMoco aggregation in NMF solution is proposed.     

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.     

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.     

Novel cytotoxic chelators that bind iron(II) selectively over zinc(II) under aqueous aerobic conditions

To achieve cellular iron deprivation by chelation, it is important to develop chelators with selective metal-binding properties. Selectivity for iron has long been the province of certain oxygen-donor chelators such as desferrioxamine, which target Fe(III) and exploit the strength of a relatively ionic Fe(III)-O interaction. We have been studying novel chelators that possess mechanisms to selectively chelate +2 biometals, particularly tachpyr [N,N',N"-tris(2-pyridylmethyl)-1,3,5-cis,cis-triaminocyclohexane] and derivatives from N,N',N"-trialkylation and pyridine ring alkylation. Metal-exchange and metal-binding competition reactions have been conducted at pH 7.4, 37 degrees C and time periods until no further change was observed (generally 24-48 h). Under anaerobic conditions, tachpyr is strongly selective for iron, binding 95+/-5% Fe(II) versus 5+/-5% Zn(II) in the forms [Fe(tachpyr)](2+) and [Zn(tachpyr)](2+) respectively. Under aerobic conditions, tachpyr complexes Fe(II) more effectively than Fe(III), forming iminopyridyl complexes [Fe(tachpyr-ox-n)](2+) (n=2, 4) by O(2)-induced and iron-mediated oxidative dehydrogenation. Complexes [Fe(tachpyr-ox-n)](2+) are also strongly bound forms of iron that are unaffected by an excess of Zn(II) (75 mol zinc:1 mol iron complex). The preference of tachpyr for iron over zinc under aerobic conditions appears to be hindered by oxidation of Fe(II) to Fe(III), such that the proportions bound are 44+/-10% Fe(II) versus 56+/-10% Zn(II), in the respective forms [Fe(tachpyr-ox-n)](2+) and [Zn(tachpyr)](2+). However, upon addition of the reducing agent Na(2)S(2)O(4) that converts Fe(III) to Fe(II), the binding proportions shift to 76+/-10% Fe(II) versus 24+/-10% Zn(II), demonstrating a clear preference of tachpyr for Fe(II) over Zn(II). Iron(II) is in the low-spin state in [Fe(tachpyr)](2+) and [Fe(tachpyr-ox-n)](2+) (n=2, 4), which is a likely cause of the observed selectivity. N-methylation of tachpyr [giving (N-methyl)(3)tachpyr] results in the loss of selectivity for Fe(II), which is attributed to the steric effect of the methyl groups and a resulting high-spin state of Fe(II) in [Fe(N-methyl)(3)tachpyr)](2+). The relationship of chelator selectivity to cytotoxicity in the tach family will be discussed.     

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.     

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 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.     

Photochemistry of the [Fe4(mu3-S)3(NO)7]- complex in the presence of S-nucleophiles: a spectroscopic study.

Biological systems usually contain cysteine, glutathione or other sulfur-containing biomolecules. These S-nucleophiles were found to affect drastically the [Fe(4)(mu(3)-S)(3)(NO)(7)](-) photolysis pathway generating products completely different from that of the neat cluster, which produces Fe(II) and NO and S(2-). The effect is interpreted in terms of formation of a pseudo-cubane adduct, [Fe(4)(mu(3)-S)(3)(mu(3)-SR)(NO)(7)](2-), whose existence in equilibrium with the parent complex has no detectable influence on the spectral properties, whereas shifts the redox potential and induces photoconversion leading to the Fe(III) species and N(2)O. Characteristic bond lengths, bond angles and atomic Mulliken charges were calculated using semi-empirical quantum chemical methods for the RBS anion and a series of pseudo-cubane complexes with S-donor or N-donor ligands. The results justify the hypothesis of the adduct formation and show that only in case of S-ligands the higher contribution of the Fe(III)-NO(-) components in adduct than in RBS is observed, which on excitation can undergo heterolytic cleavage yielding Fe(III) and NO(-), converted rapidly into N(2)O. These results are crucial in understanding the physiological activity of RBS. Fe(III) formation can be detected only when the S-ligand enables formation of a stable Fe(III) compound; the effect was recorded in the presence of sulfide, thioglycolate, 2-mercaptopropionate, mercaptosuccinate, penicillamine, 2,3-dimercaptosuccinate, 2,3-dimercaptopropanol, and thiocyanate. For all these S-ligands the Fe(III) photoproducts were identified and characterised. In the case of other thiolates, their excess results in fast reduction of Fe(III) to Fe(II), whereas N(2)O can be still detected. Quantum yields of Fe(III) formation in the presence of the S-ligands are considerably higher than that of the Fe(II) photoproduction from neat [Fe(4)(mu(3)-S)(3)(NO)(7)](-).     

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.     

Redox and ligand exchange reactions of potential gold(I) and gold(III)-cyanide metabolites under biomimetic conditions

Biomimetic pathways for the oxidation of [Au(CN)(2)](-), a gold metabolite, and further cyanation of the gold(III) products to form Au(CN)(4)(-) were investigated using 13C NMR and UV-Visible spectroscopic methods. Hypochlorite ion, an oxidant released during the oxidative burst of immune cells, was employed. The reaction generates mixed dicyanoaurate(III) complexes, trans-[Au(CN)(2)X(2)](-), where X(-) represents equilibrating hydroxide and chloride ligands, and establishes the chemical feasibility of dicyanoaurate oxidation by OCl(-) to gold(III) species. This oxidation reaction suggests a new procedure for synthesis of H[Au(CN)(2)Cl(2)]. Reaction of trans-[Au(CN)(2)X(2)](-) (X(-)=Cl(-) and Br(-)) or [AuCl(4)](-) with HCN in aqueous solution at pH 7.4 leads directly to [Au(CN)(4)](-) without detection of the anticipated [Au(CN)(x)X(4-x)](-)intermediates, which is attributed to the cis- and trans-accelerating effects of the cyanides. The reduction of [Au(CN)(4)](-) by glutathione and other thiols is a complex, pH-dependent process that proceeds through two intermediates and ultimately generates [Au(CN)(2)](-). These studies provide further insight into the possible mechanisms of an immunogenically generated gold(I)/gold(III) redox cycle in vivo.     

The dynamics of 57Fe nuclei in Fe(II)-DNA and [Fe(II)(1-methyl-2-mercaptoimidazole)2]-DNA condensates

Alcoholic solutions of FeCl(2) and Fe(II)(Hmmi)(2)Cl(2) (Hmmi=1-methyl-2-mercaptoimidazole) induce calf thymus DNA condensation from aqueous solutions buffered at pH 7.4. A 1:1 Fe(II)-(DNA monomer) stoichiometry is assumed. The (57)Fe M?ssbauer hyperfine parameters suggest an octahedral coordination environment, severely distorted, in both Fe(II)-(DNA monomer) and [Fe(II)(Hmmi)(2)]-(DNA monomer) condensates. The dynamic properties of iron nuclei in freeze-dried samples were investigated by means of variable temperature (57)Fe M?ssbauer spectroscopy. Mean square displacements, (T), were calculated, such as the effective vibrating mass and the M?ssbauer lattice temperature of the solids. increases linearly with the temperature in the whole temperature range explored; the absolute values are typical for lattice or solid-state vibrations. Very similar values for the effective vibrating masses were extracted, suggesting comparable covalency of the bonding interaction between the metal atom and its ligands, while the M?ssbauer lattice temperatures show a softening of the lattice for [Fe(II)(Hmmi)(2)]-(DNA monomer) with respect to Fe(II)-(DNA monomer) condensate.     

Perturbing the DNA sequence selectivity of metallointercalator-peptide conjugates by single amino acid modification.

Metallointercalator-peptide conjugates that provide small molecular mimics to explore peptide-nucleic acid recognition have been prepared. Specifically, a family of peptide conjugates of [Rh(phi)(2)(phen')](3+) [where phi = 9,10-phenanthrenequinone diimine and phen' = 5-(amidoglutaryl)-1,10-phenanthroline] has been synthesized and their DNA-binding characteristics examined. Single amino acid modifications were made from the parent metallointercalator-peptide conjugate [Rh(phi)(2)(phen')](3+)-AANVAIAAWERAA-CONH(2), which targets 5'-CCA-3' site-specifically. Moving the glutamate at position 10 in the sequence of the appended peptide to position 6 {[Rh(phi)(2)(phen')](3+)-AANVAEAAWARAA-CONH(2)} changed the sequence preference of the metallointercalator-peptide conjugate to 5'-ACA-3'. Subsequent mutation of the glutamate at position 6 to arginine {[Rh(phi)(2)(phen')](3+)-AANVARAAWARAA-CONH(2)} caused more complex changes in DNA recognition. Thermodynamic dissociation constants were determined for these metallointercalator-peptide conjugates by photoactivated DNA cleavage assays with the rhodium intercalators. At 55 degrees C in the presence of 5 mM MnCl(2), [Rh(phi)(2)(phen')](3+)-AANVAIAAWERAA-CONH(2) binds to a 5'-CCA-3' site with K(d) = 5.7 x 10(-)(8) M, whereas [Rh(phi)(2)(phen')](3+)-AANVAEAAWARAA-CONH(2) binds to its target 5'-ACA-3' site with K(d) = 9.9 x 10(-8) M. The dissociation constant for [Rh(phi)(2)(phen')](3+) with random-sequence DNA is 7.0 x 10(-7) M. Structural models have been developed and refined to account for the observed sequence specificities. As with much larger DNA-binding proteins, with these metal-peptide conjugate mimics, single amino acid changes can lead to single or multiple base changes in the DNA site targeted.     

Cobalt complexes of terpyridine ligand: crystal structure and photocleavage of DNA

Two new cobalt complexes, [Co(pytpy)(2)](ClO(4))(2), 1, and [Co(pytpy)(2)](ClO(4))(3), 2 where pytpy=pyridine terpyridine, have been synthesized and characterized. Single-crystal X-ray structure of both the complexes has been resolved. The structure shows the complexes to be a monomeric cobalt(II) and cobalt(III) species with two pytpy ligands coordinated to the metal ion to give a six coordinate complex. Both cobalt(II) and cobalt(III) complexes crystallize in meridional configuration. The interaction of these complexes with calf thymus DNA has been explored by using absorption, emission spectral, electrochemical studies and viscosity measurements. From the experimental results the DNA binding constants of 1 and 2 are found to be (1.97+/-0.15)x10(4)M(-1) and (2.7+/-0.20)x10(4)M(-1) respectively. The ratio of DNA binding constants of 1 and 2 have been estimated to be 0.82 from electrochemical studies, which is in close agreement with the value of 0.73 obtained from spectral studies. The observed changes in viscosity of DNA in the presence of increasing amount of complexes 1 and 2 suggest intercalating binding of these complexes to DNA. Results of DNA cleaving experiments reveal that complex 2 efficiently cleaves DNA under photolytic conditions while complex 1 does not cleave DNA under similar conditions.     

Reaction of a model siderophore with Ni(II), Co(II) and Zn(II) in aqueous solution: kinetics and spectroscopy

A spectroscopic study was performed showing that the [Fe(III)(L(2-))(2)](1-) (L(2-)=dopacatecholate) complex reacts with Ni(II), Co(II) and Zn(II) in an aqueous solution containing S(2)O(3)(2-) resulting in the soluble [M(L(1-))(3)](1-) (L(1-)=dopasemiquinone; M=Ni(II), Co(II) or Zn(II) complex species. The Raman and IR spectra of the [CTA][M(L(1-))(3)] complexes, CTA=hexadecyltrimethylammonium cation, in the solid state were obtained. The kinetic constants for the metal substitution reactions were determined at four different temperatures, providing values for DeltaH(not equal), DeltaS(not equal) and DeltaG(not equal). The reactions were slow (k=10(-11) Ms(-1)) and endothermic. The system investigated can be considered as a simplified model to explain some aspects of siderophore chemistry.     

Cytotoxicity studies of chromium(III) complexes on human dermal fibroblasts

The cytotoxicity of certain Cr(III) complexes, such as [Cr(salen)(H(2)O)(2)](+), [Cr(edta)(H(2)O)](-), [Cr(en)(3)](3+), [Cr(ox)(3)](3-), [Cr(pic)(3)], and CrCl(3), which differ in ionic character and ligand environment in human dermal skin fibroblasts, has been studied. After 72 h of exposure to 100 microM doses of chromium(III) complexes, the order in which the complexes had an inhibitory effect on cell viability was [Cr(en)(3)](3+) > [Cr(salen)(H(2)O)(2)](+) > [Cr(ox)(3)](3-) > [Cr(edta)(H(2)O)](-) > [Cr(pic)(3)] > CrCl(3). Based on viability studies it was confirmed that [Cr(en)(3)](3+), a triply charged cation, inhibits cell proliferation, and therefore, it was chosen to carry out further investigations. [Cr(en)(3)](3+), at a dose of 50 microM, was found to bring about surface morphological changes, evidenced by cellular blebbing and spike formation accompanied by nuclear damage. TEM analysis revealed substantial intracellular damage to fibroblasts in terms of the formation of apoptotic bodies and chromatin condensation, thus reflecting cell death. FACS analysis further revealed DNA damage by formation of a sub-G(1) peak with 84.2% DNA as aneuploid DNA and arrest of the G(2) / M phase of the cell cycle. Cellular DNA damage was confirmed by agarose gel electrophoresis with the characteristic appearance of a DNA streak in DNA isolated from [Cr(en)(3)](3+)-treated fibroblasts. The proposed mechanism suggests the plausible role of Cr(V), formed as a result of oxidation of Cr(III) by cellular oxidative enzymes, in the cytotoxic response. Consequently, any Cr(III) complex that is absorbed by cells and can be oxidized to Cr(V) must be considered a potential carcinogen. This has potential implications for the increased use of Cr(III) complexes as dietary supplements and highlights the need to consider the cytotoxicity and genotoxicity of a variety of Cr(III) complexes and to understand the potential hazards of Cr(III) complexes encountered in research laboratories.     

The role of photoinduced electron transfer processes in photodegradation of the [Fe4(μ3-S)3(NO)7] cluster

Spectroscopic and electrochemical study of the [Fe(4)(mu(3)-S)(3)(NO)(7)](-) photochemical reaction and thermodynamic calculations of relevant systems demonstrate the redox character of this process. The photoinduced electron transfer between substrate clusters in excited and ground state (probably via exciplex formation) results in dismutation yielding unstable [Fe(4)(mu(3)-S)(3)(NO)(7)](2-) and [Fe(4)(mu(3)-S)(3)(NO)(7)](0). Back electron transfer between the primary products is responsible for fast reversibility of the photochemical reaction in deoxygenated solutions. In the presence of an electron acceptor (such as O(2), MV(2+) or NO) an oxidative quenching of the (*)[Fe(4)(mu(3)-S)(3)(NO)(7)](-) is anticipated, although NO seems to participate as well in the reductive quenching. The electron acceptors can also regenerate the substrate from its reduced form ([Fe(4)(mu(3)-S)(3)(NO)(7)](2-)), whereas the other primary product ([Fe(4)(mu(3)-S)(3)(NO)(7)](0)) decomposes to the final products. The suggested mechanism fits well to all experimental observations and shows the thermodynamically favored pathways and explains formation of all major (Fe(2+), S(2-), NO) and minor products (N(2)O, Fe(3+)). The photodissociation of nitrosyl ligands suggested earlier as the primary photochemical step cannot be, however, definitely excluded and may constitute a parallel pathway of [Fe(4)(mu(3)-S)(3)(NO)(7)](-) photolysis.