Kinetics of the oxidation of bromide ions by bis(2,2′-bipyridine)manganese(III) ions in aqueous perchlorate media

The kinetics of the oxidation of bromide ions by the bis(2,2′-bipyridine)manganese(III) ion have been investigated over a range of acid concentrations for several temperatures. Initial rates of reaction are measured and from their variation with [Mn^III] and [Br−] it is concluded that the observed order in [Mn(bipy)₂³⁺_aq] is one and that in [Br⁻] is intermediate between zero and one. It is shown kinetically that intermediate complexes between Mn^III and Br⁻ ions are involved and from the variation of the rate constant with acidity it is concluded that the decomposition of Mn(bipy)₂Br²⁺_aq is definitely involved as a rate controlling step and that the decomposition of the protonated complex Mn(bipy)(bipyH⁺)Br³⁺_aq is also probably involved as a rate controlling step. The kinetics and mechanism for this oxidation are compared with those found for other cations complexed with bipyridine, for aqua-cations and other complexes of Co^III.     

Reactivity of a cobalt(III)-peroxo complex in oxidative nucleophilic reactions

A mononuclear cobalt(III)-peroxo complex bearing a macrocyclic tetradentate N₄ ligand, [Co^III(TMC)(O₂)]⁺ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), was generated in the reaction of [Co^II(TMC)]²⁺ and H₂O₂ in the presence of triethylamine in CH₃CN. The reactivity of the cobalt(III)-peroxo complex was investigated in aldehyde deformylation with various aldehydes and compared with that of iron(III)- and manganese(III)-peroxo complexes, such as [Fe^III(TMC)(O₂)]⁺ and [Mn^III(TMC)(O₂)]⁺. In this reactivity comparison, the reactivities of metal-peroxo species were found to be in the order of [Mn^III(TMC)(O₂)]⁺ > [Co^III(TMC)(O₂)]⁺ > [Fe^III(TMC)(O₂)]⁺. A positive Hammett ρ value of 1.8, obtained in the reactions of [Co^III(TMC)(O₂)]⁺ and para-substituted benzaldehydes, demonstrates that the aldehyde deformylation by the cobalt(III)-peroxo species occurs via a nucleophilic reaction.     

The kinetics of the complex formation between iron(III)-ethylenediaminetetraacetate and hydrogen peroxide in aqueous solution

The kinetics of the formation of the purple complex [Fe^III(EDTA)O₂]³⁻, between Fe^III-EDTA and hydrogen peroxide was studied as a function of pH (8.22-11.44) and temperature (10-40 °C) in aqueous solutions using a stopped-flow method. The reaction was first-order with respect to both reactants. The observed second-order rate constants decrease with an increase in pH and appear to be related to deprotonation of Fe^III-EDTA ([Fe(EDTA)H₂O]⁻ ⇔ Fe(EDTA)OH]²⁻ + H⁺). The rate law for the formation of the complex was found to be d[Fe^IIIEDTAO₂]³⁻/dt=[(k₄[H⁺]/([H⁺] + K₁)][Fe^III-EDTA][H₂O₂], where k₄=8.15±0.05×10⁴ M⁻¹ s⁻¹ and pK₁=7.3. The steps involved in the formation of [Fe(EDTA)O₂]³⁻ are briefly discussed.     

Spectrophotometric kinetic study and analytical implications of the glucose oxidase-catalyzed reduction of [MIII(LL)2Cl2]+ complexes by d-glucose (M=Os and Ru, LL=2,2′-bipyridine and 1,10-phenanthroline type ligands)

 Glucose oxidase-catalyzed reduction of cis[M^III (LL)₂Cl₂]⁺ (M=Os and Ru) complexes to cis[M^II (LL)₂Cl₂] (LL=2,2′-bipyridine and 1,10-phenanthroline type ligands) by d-glucose is a first-order process in the complex and the enzyme in aqueous buffered solution. The reaction follows MichaelisMenten kinetics in d-glucose and the rate is independent of d-glucose concentration above 0.03 M. The reactivity decreases in the series [Ru(bpy)₂Cl₂]⁺ > [Os(phen)₂Cl₂]⁺ > [Os(4,4′-Me₂bpy)₂Cl₂]⁺ > [Os(4,7Me₂phen)₂Cl₂]⁺. The measured second-order rate constant for the oxidation of reduced glucose oxidase by [Os(phen)₂Cl₂]⁺ in air equals 1.2×10⁵ M^–1 s^–1 at pH 6.7, [d-glucose] 0.05 M, and 25  °C, which is ca. 20% less than that when the reaction solutions are purged with argon. In the case of [Ru(bpy)₂Cl₂]⁺ the rate constant equals 1.8×10⁵ M^–1 s^–1 under similar conditions in air, showing higher reactivity of Ru complexes compared with Os ones. The reduction is pH-dependent with a maximum around 7. Added for solubilization of poorly soluble metal complexes, surfactants decrease the rates of the enzymatic reaction. The retardation effect increases in the series: cetyltrimethylammonium bromide < Triton X-100 < sodium dodecyl sulfate, i.e. on going from positively charged to neutral and then to negatively charged surfactants. The behavior of the Os^III and Ru^III complexes toward reduced glucose oxidase contrasts to that of recently studied ferricenium cations. As opposed to the latter, the former do not show kinetically meaningful binding with the enzyme, and the Michaelis kinetics typical of the ferricenium case is not realized for the Os^III, and Ru^III species. The systems Os^III- or Ru^III-glucose oxidase are convenient for routine "one pot" spectrophotometric monitoring of the d-glucose content in samples, since the metal reduction to M^II is accompanied by a strong increase in absorbance in the visible spectral region. Received: 1 July 1998 / Accepted: 13 January 1999     

Reduction of pentaamminenitrocobalt(III) by hexaaquachromium(II) ion. Reinvestigation of the mechanism

Products of the reduction of [CoNO₂(NH₃)₅]²⁺ by Cr²⁺ were separated and identified under the conditions of [Cr²⁺]₀/[Co(IlI)]₀⩽3 and 0.02 M ⩽[H⁺] ⩽ 0.75 M. The product distribution was dependent on both [Cr²⁺]_o and [H⁺]. The following mechanism is proposed: [CoNO₂(NH₃)₅]²⁺ + Cr²⁺→Co²⁺ + [CrONO(H₂O)₅]²⁺ (i) [CrONO(H₂O)₅]²⁺ + H⁺→[Cr(H₂O)₆]³⁺ + HNO₂ (ii) [CrONO(H₂O)₅]²⁺ + Cr²⁺→Cr(IV) + [CrNO(H₂O)₅]²⁺ (iii) Cr(IV) + Cr²⁺→[(H₂O)₄Cr(OH)₂Cr(H₂O)₄]⁴⁺ (iv) HNO₂ + 2Cr²⁺→[Cr(H₂O)₆]³⁺ + [CrNO(H₂O)₅]²⁺ (v)     

Na+ and H+ dependent Mn2+ binding to phosphatidylserine vesicles as a test of the Gouy-Chapman-Stern theory

Summary Mn²⁺ binding to phosphatidylserine (PS) vesicles was measured by EPR as a function of [Na⁺] and pH. At nearly physiological monovalent salt concentration the apparent Mn²⁺ affinity (K _a) increased monitonically over the pH range 5.7–8.35, withK _a roughly [H⁺]^–1 above pH 7.3. It was found, moreover, thatK _a fell off more rapidly with added NaCl at pH 6.1 than at pH 7.87. Qualitatively, these results are consistent with two types of Mn²⁺-PS binding: (i) simple adsorption and (ii) adsorption with the release of an amino proton from PS. The existence of Mn²⁺-induced H⁺ displacement from PS was verified through titration measurements, employing a pH electrode.When H⁺ displacement is taken into account, the variation inK _a with [Na⁺] observed at pH 6.1 is found to be in reasonably good agreement with that expected from the Gouy-Chapman-Stern theory of ionic binding to charged surfaces.     

Mechanistic studies on oxidation of hydrogen peroxide and hydrazine by a metal-bound superoxide

In aqueous acetate buffer, hydrogen peroxide and hydrazine reduce the bridging superoxide in [(en)(dien)Co^III(O₂)Co^III(en)(dien)](ClO₄)₅ (1) to the corresponding hydroperoxo complex [(en)(dien)Co^III(μ-O₂H)Co^III(en)(dien)]⁵⁺ (2). In the presence of excess [H₂O₂] and [N₂H₅⁺] over [1], both the reactions obeyed first-order kinetics and exhibited inverse proton dependence. Protonation of 1 at equilibrium generates [(en)(dien)Co^III(μ-O₂H)Co^III(en)(dien)]⁶⁺ (1H), the conjugate acid from 1, which appears to be a kinetic dead-end and that accounts for the observed inverse proton dependence on rate. Reaction rates significantly decrease with increasing proportion of D₂O replacing H₂O in the solvent and an H-atom transfer (HAT) from the reducing species to the bridging superoxide in 1 seems reasonable at the rate step.     

The dynamics of mitochondrial Ca fluxes

We have investigated the kinetics of mitochondrial Ca²⁺ influx and efflux and their dependence on cytosolic [Ca²⁺] and [Na⁺] using low-Ca²⁺-affinity aequorin. The rate of Ca²⁺ release from mitochondria increased linearly with mitochondrial [Ca²⁺] ([Ca²⁺]_M). Na⁺-dependent Ca²⁺ release was predominant al low [Ca²⁺]_M but saturated at [Ca²⁺]_M around 400 μM, while Na⁺-independent Ca²⁺ release was very slow at [Ca²⁺]_M below 200 μM, and then increased at higher [Ca²⁺]_M, perhaps through the opening of a new pathway. Half-maximal activation of Na⁺-dependent Ca²⁺ release occurred at 5-10 mM [Na⁺], within the physiological range of cytosolic [Na⁺]. Ca²⁺ entry rates were comparable in size to Ca²⁺ exit rates at cytosolic [Ca²⁺] ([Ca²⁺]_c) below 7 μM, but the rate of uptake was dramatically accelerated at higher [Ca²⁺]_c. As a consequence, the presence of [Na⁺] considerably reduced the rate of [Ca²⁺]_M increase at [Ca²⁺]_c below 7 μM, but its effect was hardly appreciable at 10 μM [Ca²⁺]_c. Exit rates were more dependent on the temperature than uptake rates, thus making the [Ca²⁺]_M transients to be much more prolonged at lower temperature. Our kinetic data suggest that mitochondria have little high affinity Ca²⁺ buffering, and comparison of our results with data on total mitochondrial Ca²⁺ fluxes indicate that the mitochondrial Ca²⁺ bound/Ca²⁺ free ratio is around 10- to 100-fold for most of the observed [Ca²⁺]_M range and suggest that massive phosphate precipitation can only occur when [Ca²⁺]_M reaches the millimolar range.     

Sodium sensing induces different changes in free cytosolic calcium concentration and pH in salt-tolerant and -sensitive rice (Oryza sativa) cultivars

Perception of salt stress in plant cells induces a change in the free cytosolic Ca²⁺, [Ca²⁺]_cyt, which transfers downstream reactions toward salt tolerance. Changes in cytosolic H⁺ concentration, [H⁺]_cyt, are closely linked to the [Ca²⁺]_cyt dynamics under various stress signals. In this study, salt‐induced changes in [Ca²⁺]_cyt, and [H⁺]_cyt and vacuolar [H⁺] concentrations were monitored in single protoplasts of rice (Oryza sativa L. indica cvs. Pokkali and BRRI Dhan29) by fluorescence microscopy. Changes in cytosolic [Ca²⁺] and [H⁺] were detected by use of the fluorescent dyes acetoxy methyl ester of calcium‐binding benzofuran and acetoxy methyl ester of 2′, 7′‐bis‐(2‐carboxyethyl)‐5‐(and‐6) carboxyfluorescein, respectively, and for vacuolar pH, fluorescent 6‐carboxyfluorescein and confocal microscopy were used. Addition of NaCl induced a higher increase in [Ca²⁺]_cyt in the salt‐tolerant cv. Pokkali than in the salt‐sensitive cv. BRRI Dhan29. From inhibitor studies, we conclude that the internal stores appear to be the major source for [Ca²⁺]_cyt increase in Pokkali, although the apoplast is more important in BRRI Dhan29. The [Ca²⁺]_cyt measurements in rice also suggest that Na⁺ should be sensed inside the cytosol, before any increase in [Ca²⁺]_cyt occurs. Moreover, our results with individual mesophyll protoplasts suggest that ionic stress causes an increase in [Ca²⁺]_cyt and that osmotic stress sharply decreases [Ca²⁺]_cyt in rice. The [pH]_cyt was differently shifted in the two rice cultivars in response to salt stress and may be coupled to different activities of the H⁺‐ATPases. The changes in vacuolar pH were correlated with the expressional analysis of rice vacuolar H⁺‐ATPase in these two rice cultivars.     

Searching for switchable molecular conductors: Salts of [M(dcbdt)2] (M = Ni, Au) anions with [Fe(sal2-trien)] and [Fe(phen)3]

Salts of [Fe^III(sal₂-trien)]⁺and [Fe^II(phen)₃]²⁺ cations and M[(dcbdt)₂]⁻ anions with M = Ni and Au (dcbdt = dicyanobenzenedithiolate) with formula [Fe(sal₂-trien)] [M(dcbdt)₂] and [Fe(phen)₃] [M(dcbdt)₂]₂ were obtained and characterized by single X-ray diffraction and magnetic measurements. None of these salts shows a clear spin crossover behaviour and their magnetic properties are due essentially to the cations in a high spin S = 5/2 and low spin states for the Fe^III and Fe^II salts respectively. The magnetic Ni sublattices in both compounds appear to have a negligible direct contribution to the magnetization but enhance the AF interactions in the cation sublattice.     

Equilibrium constants under physiological conditions for the reactions of the nonphosphorylated pathway of L-serine biosynthesis

The observed equilibrium constants (K_obs) for the reactions of d-2-phosphoglycerate phosphatase, d-2-Phosphoglycerate^3? + H₂O → d-glycerate^? + HPO₄^2?; d-glycerate dehydrogenase (EC 1.1.1.29), d-Glycerate^? + NAD⁺ → NADH + hydroxypyruvate^? + H⁺; and l-serine:pyruvate aminotransferase (EC 2.6.1.51), Hydroxypyruvate^? + l-H · alanine^± → pyruvate^? + l-H · serine^±; have been determined, directly and indirectly, at 38 °C and under conditions of physiological ionic strength (0.25 m) and physiological ranges of pH and magnesium concentrations. From these observed constants and the acid dissociation and metal-binding constants of the substrates, an ionic equilibrium constant (K) also has been calculated for each reaction. The value of K for the d-2-phosphoglycerate phosphatase reaction is 4.00 × 10³m [ΔG⁰ = ?21.4 kJ/mol (?5.12 kcal/mol)]([H₂0] = 1). Values of K_obs for this reaction at 38 °C, [K⁺] = 0.2 m, I = 0.25 M, and pH 7.0 include 3.39 × 10³m (free [Mg²⁺] = 0), 3.23 × 10³m (free [Mg²⁺] = 10^?3m), and 2.32 × 10³m (free [Mg²⁺] = 10^?2m). The value of K for the d-glycerate dehydrogenase reaction has been determined to be 4.36 ± 0.13 × 10^?13m (38 °C, I = 0.25 M) [ΔG⁰ = 73.6 kJ/mol (17.6 kcal/mol)]. This constant is relatively insensitive to free magnesium concentrations but is affected by changes in temperature [ΔH⁰ = 46.9 kJ/mol (11.2 kcal/mol)]. The value of K for the serine:pyruvate aminotransferase reaction is 5.41 ± 0.11 [ΔG⁰ = ?4.37 kJ/mol (?1.04 kcal/mol)] at 38 °C (I = 0.25 M) and shows a small temperature effect [ΔH⁰ = 16.3 kJ/ mol (3.9 kcal/mol)]. The constant showed no significant effect of ionic strength (0.06–1.0 m) and a response to the hydrogen ion concentration only above pH 8.5. The value of K_obs is 5.50 ± 0.11 at pH 7.0 (38 °C, [K⁺] = 0.2 m, [Mg²⁺] = 0, I = 0.25 M). The results have also allowed the value of K for the d-glycerate kinase reaction (EC 2.7.1.31), d-Glycerate^? + ATP^4? → d-2-phosphoglycerate^3? + ADP^3? + H⁺, to be calculated to be 32.5 m (38 °C, I = 0.25 M). Values for K_obs for this reaction under these conditions and at pH 7.0 include 236 (free [Mg²⁺] = 0) and 50.8 (free [Mg²⁺] = 10^?3m).     

Hydrogen peroxide constricts rat arteries by activating Na+-permeable and Ca2+-permeable cation channels

Oxidative stress is associated with many cardiovascular diseases, such as hypertension and arteriosclerosis. Oxidative stress reportedly activates the L-type voltage-gated calcium channel (VDCC_L) and elevates [Ca²⁺]_i in many cells. However, how oxidative stress activates VDCC_L under clinical setting and the consequence for arteries are unclear. Here, we examined the hypothesis that hydrogen peroxide (H₂O₂) regulates membrane potential (Em) by altering Na⁺ influx through cation channels, which consequently activates VDCC_L to induce vasoconstriction in rat mesenteric arteries. To measure the tone of the endothelium-denuded arteries, a conventional isometric organ chamber was used. Membrane currents and Em were recorded by the patch-clamp technique. [Ca²⁺]_i and [Na⁺]_i were measured with microfluorometry using Fura2-AM and SBFI-AM, respectively. We found that H₂O₂ (10 and 100 µM) increased arterial contraction, and nifedipine blocked the effects of H₂O₂ on isometric contraction. H₂O₂ increased [Ca²⁺]_i as well as [Na⁺]_i, and depolarised Em. Gd³⁺ (1 µM) blocked all these H₂O₂-induced effects including Em depolarisation and increases in [Ca²⁺]_i and [Na⁺]_i. Although both nifedipine (30?nM) and low Na⁺ bath solution completely prevented the H₂O₂-induced increase in [Na⁺], they only partly inhibited the H₂O₂-induced effects on [Ca²⁺]_i and Em. Taken together, the results suggested that H₂O₂ constricts rat arteries by causing Em depolarisation and VDCC_L activation through activating Gd³⁺-and nifedipine-sensitive, Na⁺-permeable channels as well as Gd³⁺-sensitive Ca²⁺-permeable cation channels. We suggest that unidentified Na⁺-permeable cation channels as well as Ca²⁺-permeable cation channels may function as important mediators for oxidative stress-induced vascular dysfunction.     

Solution equilibria leading to the formation of metal-metal bonds in partially oxidized bisoxalatoplatinate(II) systems

The reaction between [Pt^II(Ox)₂]²⁻ and an appropriate oxidant resulted in the formation of the dimeric unbridged platinum complex [{Pt^III(Ox)₂ }₂]²⁻ where (Ox) is oxalate. This complex was moderately stable under ambient conditions and was studied via a variety of NMR and spectrophotometric techniques. Reaction of the [{Pt^III(Ox)₂}₂]²⁻ complex with [Pt^II(Ox)₂]²⁻ in the presence of H⁺ lead to the formation of a series of longer platinum oligomers with non-integral oxidation states, culminating in the formation of partially oxidized platinum polymers of general formula [{Pt(Ox)₂}_n]ⁿ⁻. The concentration of H⁺ was an important factor leading to higher oligomers and the approximate number of protons associated with each oligomer was determined. The analogous [{Pt^III(Mal)₂}₂]²⁻ complex, where (Mal) is the malonate anion, was also synthesized and studied but was shown to be significantly less stable.     

Out-of-plane dimeric Mn quadridentate Schiff-base complexes: Synthesis, structure and magnetic properties

Four Mn^III quadridentate Schiff-base compounds have been prepared and structurally characterized: [Mn(salpn)(CH₃OH)₂]BPh₄ (1), [Mn₂(salpn)₂(N₃)₂] (2), [Mn₂(salpn)₂(NCS)₂] (3), [Mn₂(salpn)₂(H₂O)₂](H₂O)(ClO₄)₂ (4) (salpn = N,N′-(1,2-propylene)-bis-(salicylideneiminate)). Among them, 1 is a discrete Mn^III monomeric complex with a square-bipyramidal geometry. Complexes 2, 3 and 4 form the similar phenolate-bridged out-of-plane dimers. Magnetic susceptibility studies reveal that 2, 3 and 4 all exhibit ferromagnetic intra-dimer coupling between Mn^III ions.     

Spectral and kinetic investigation on oxidation and reduction of water soluble porphyrin-manganese(III) complex

In this work the oxidation and reduction reactions of Mn^III-Coproporphyrin-I (Mn^III-CPI) have been studied and four forms of manganese-CPI complexes have been characterized. This complex was observed to be highly reactive (at basic pH) towards Mn(II), hypochlorite, hydrogen peroxide and oxone, forming [Mn^IV(O)CPI(OH)]⁻ that was unstable and, after a short time, formed again [Mn^IIICPI(OH)₂]⁻. With an excess of NaClO, a further oxidation of the complex [Mn^IV(O)CPI(OH)]⁻, provoked a significant spectral change for the [Mn^V(O)CPI(OH)] formation that showed, in the time, a partial polymerization. [Mn^IIICPI(OH)₂]⁻ was reduced by sodium dithionite to form the very unstable complex of [Mn^IICPI(OH)]⁻ that successively degraded with Mn(II) release.     

Reaction of trans-dichlorobis(ethylenediamine)nickel(III) in aqueous hydrochloric acid

Both EPR and electronic absorbance spectroscopy have been used to follow the disappearance of [Ni^III(en)₂Cl₂]⁺ in aqueous HCI solutions. The rate of Ni(III) reduction is influenced by both the H⁺ and Cl^? concentrations, although the rate is not linear with respect to the concentration of either species. A mechanism is proposed in which the first step in the reaction is the proton-induced chelate ring opening which is followed by the reduction of Ni(III) by chloride ions. In the presence of H₂SO₄ the coordinated Cl^? ions are rapidly replaced by HSO_4^? ions and the resulting complex is much more stable, even in a 6 N acid solution.     

Syntheses of linear-type S-bridged trinuclear complexes composed of heavy d metal ions and 2-aminoethanethiolate: comparative characterization by X-ray diffraction, spectroscopy and electrochemistry

The S-bridged trinuclear complexes composed of heavy d⁶ metal ions, [Rh^III{M(aet)₃}₂]³⁺ (M=Ir^III(1), Rh^III(2); aet = 2-aminoethanethiolate), have been prepared by the reactions of fac(S)-[M(aet)₃] with RhCl₃ · 3H₂O. The complexes were separated into meso (1a, 2a) and rac (1b, 2b) isomers by SP-Sephadex C-25 column chromatography. 1b and 2b were optically resolved by the column chromatographic method and characterized by CD spectroscopy. Crystal structures of 1a, 1b and 2a were determined by X-ray diffraction, and it was found that they consist of linear-type trinuclear structures. The central Rh(III) ion in the present complexes has d⁶ electronic configuration with the non-degenerated A-type cubic field term, and showed long Rh?M distances, acute S-M-S angles and obtuse Rh-S-M angles. These are in contrast with the complexes having the degenerated T-type cubic field term such as [M^′{M(aet)₃}₂]ⁿ⁺ (M^′=V^III, Mo^IV and Re^III, M=Ir^III, Rh^III, n=3 or 4). All the isomers have been comparatively characterized and discussed in solid state and the solution for spectrochemical and electrochemical properties.     

Properties of Ca2+ uptake and release by Golgi membrane vesicles from rat intestine

A previous study of energy-independent in vitro Ca²⁺ uptake by rat intestinal epithelial membrane vesicles demonstrated that uptake by Golgi membrane vesicles was greater than that by microvillus or lateral-basal membrane vesicles, was markedly decreased in vitamin D-deficient rats, and responded specifically to 1,25-(OH)₂D₃ repletion (R. A. Freedman, M. M. Weiser, and K. J. Isselbacher, 1977, Proc. Nat. Acad. Sci. USA74, 3612–3616; J. A. MacLaughlin, M. M. Weiser, and R. A. Freedman, 1980, Gastroenterology78, 325–332). In the present study, properties of Ca²⁺ uptake and release by intestinal Golgi membrane vesicles have been investigated. The initial rate of uptake was found to be saturable, suggesting carrier-mediated uptake. Uptake was markedly inhibited by Mg²⁺ and Sr²⁺, but not by Na⁺ or K⁺. Lowering the external [H⁺] or raising the internal [H⁺] resulted in enhancement of the initial rate of uptake; the intial rate was found to correlate with the internal-to-external [H⁺] gradient. The initial rate of uptake could be enhanced by preloading the vesicles with MgCl₂ or SrCl₂ but not CaCl₂, NaCl, or KCl. Vesicles preloaded with K₂SO₄ failed to show enhanced uptake in the presence of valinomycin, suggesting that enhancement in uptake by vesicles preloaded with MgCl₂ was not due to transmembrane potentials. The internal volume of the Golgi membrane vesicles was determined and found to be 9 μl/mg protein; this volume could accomodate less than 1% of the Ca²⁺ uptake maintained at equilibrium. Therefore, the remainder of the Ca²⁺ taken up was presumably bound to the Golgi membranes. A dissociation constant of 3.8 × 10^?6m was found for this binding. The bound Ca²⁺ could be rapidly released by external Mg²⁺ or Sr²⁺, but not Ca²⁺, Na⁺, or K⁺. Release of bound Ca²⁺ could also be induced by raising the [H⁺] of the external medium. Failure of external Ca²⁺ to release bound Ca²⁺ suggested that the release induced by external Mg²⁺, Sr²⁺, or H⁺ was not due to competitive displacement of Ca²⁺ from its binding sites. These results indicated that Ca²⁺ uptake by intestinal Golgi membrane vesicles consists of carrier-mediated transport followed by binding of Ca²⁺ to the vesicle. The effects of H⁺, Mg²⁺, and Sr²⁺ on Ca²⁺ uptake and release suggest the existence of cation countertransport in the Golgi membrane vesicles.     

A new heterobinuclear FeIIICuII complex with a single terminal FeIII–O(phenolate) bond. Relevance to purple acid phosphatases and nucleases

A novel heterobinuclear mixed valence complex [Fe^IIICu^II(BPBPMP)(OAc)₂]ClO₄, 1, with the unsymmetrical N₅O₂ donor ligand 2-bis[{(2-pyridylmethyl)aminomethyl}-6-{(2-hydroxybenzyl)(2-pyridylmethyl)}aminomethyl]-4-methylphenol (H₂BPBPMP) has been synthesized and characterized. A combination of data from mass spectrometry, potentiometric titrations, X-ray absorption and electron paramagnetic resonance spectroscopy, as well as kinetics measurements indicates that in ethanol/water solutions an [Fe^III–()OH–Cu^IIOH₂]⁺ species is generated which is the likely catalyst for 2,4-bis(dinitrophenyl)phosphate and DNA hydrolysis. Insofar as the data are consistent with the presence of an Fe^III-bound hydroxide acting as a nucleophile during catalysis, 1 presents a suitable mimic for the hydrolytic enzyme purple acid phosphatase. Notably, 1 is significantly more reactive than its isostructural homologues with different metal composition (Fe^IIIM^II, where M^II is Zn^II, Mn^II, Ni^II, or Fe^II). Of particular interest is the observation that cleavage of double-stranded plasmid DNA occurs even at very low concentrations of 1 (2.5 M), under physiological conditions (optimum pH of 7.0), with a rate enhancement of 2.7×10⁷ over the uncatalyzed reaction. Thus, 1 is one of the most effective model complexes to date, mimicking the function of nucleases.Electronic Supplementary Material Supplementary material is available for this article at .