Mechanisms for acceleration of halide anation reactions of platinum(IV) complexes. REOA versus ligand assistance and platinum(II) catalysis Without central ion exchange

Chloride anation of trans-Pt(CN)₄ClOH₂⁻ has been studied with and without Pt(CN)₄²⁻ present at 25.0°C by use of stopped-flow and conventional spectrophotometry and a 1.00 M perchlorate medium. The rate law in the absence of Pt(CN)₄²⁻ is Rate=(p₁ + p₂ [H⁺] ) [Cl⁻]² [complex]/(1 + q [Cl₋]) with p₁=(3.0 ± 0.1) × 10⁻⁵ M⁻²s⁻¹, p₂=(3.6 ± 0.1) × 10⁻⁵ M⁻³ s⁻¹ and q=(0.62 ± 0.02) M⁻¹. It is compatible with a chloride assistance via an intermediate of the type Cl-Cl-Pt(CN)₄···OH₂²⁻, in which the reactivity of the aqua ligand is enhanced due to a partial reduction of the platinum. This mechanism of halide assistance is in principle the same as the modified reductive elimination oxidative addition (REOA) mechanism proposed by Poë, in which the intermediate is not split into free halogen, platinum(II) and water, and in which electron transfer not necessarily involves complete reduction to platinum(II). To avoid confusion with complete reductive eliminations, reactions without split of the intermediates are here termed halide-assisted reactions. The pH-dependence indicates acid catalysis via a protonated intermediate ClClPt(CN)₄···OH₃⁻.The Pt(CN)₄²⁻accelerated path has the rate law Rate=

[Cl-] [Pt(CN)₄²⁻] [complex] where k=(39.9±0.5) M⁻² s⁻¹ and K_a=(4.0±0.2)10⁻² M is the protolysis constant of trans-Pt(CN)₄ClOH²⁻.Reaction between PtCl₅OH₂⁻ and chloride is accelerated by Pt(CN)₄²⁻ and gives PtCl₆²⁻ as the reaction product. The rate law is Rate=k [Cl⁻] [Pt(CN)₄²⁻] [PtCl₅OH₂⁻] with k=(5.6 ± 0.2)10⁻³ M⁻² s⁻¹ at 35.0°C and for a 1.50 M perchlorate acid medium. The reaction takes place without central ion exchange. Alternative mechanisms with two consecutive central ion exchanges can be excluded. The role of Pt(CN)₄²⁻ in this reaction is very similar to that of the assisting halide in the halide assisted anations. [p ]Reaction between trans-Pt(CN)₄ClOH₂⁻ and PtCl₄²⁻ gives Pt(CN)₄²⁻ and PtCl₅OH₂⁻ as products and has the rate law Rate=k[PtCl₄²⁻] [trans-Pt(CN)₄ClOH₂⁻] with k=(3.32 ± 0.02) M⁻¹ s⁻¹ at 25 °C for a 1.00 M perchloric acid medium. The formation of an aqua complex as the primary reaction product and the rate independent of [Cl⁻] shows that formation of a bridged intermediate of the type Pt(II)Cl₄ClPt(IV)(CN)₄OH₂³⁻ is formed in the initial reaction step, not five-coordinated PtCl₅³⁻.     

Oxidative dehydrogenation of an iron-tetraazacyclotetradecatriene complex

The oxidative dehydrogenation of the bis(N- methyl imidazole)(meso-5,5,7,12,12,14-hexamethyl- 1 4,8 11-tetraazacyclotetradeca-1,3,8-triene)iron(II) complex, forming the corresponding 1 3,8,10-tetraene product was investigated by cyclic voltammetry, spectroelectrochemistry and stopped-flow kinetics with [Fe(CN)₆]³⁻, at 25 °C, I = 0.50 M and pH 7–10. The results led to a mechanism consistent with a reversible one electron transfer process generating iron(III) species which lose a proton (pK_a = 9.77) and undergo induced electron transfer in the presence of the hexacyanoferrate(III) ion (k = 4.2 × 10⁵ M⁻¹ s⁻¹). The intermediate precursor complex (λ_max = 665 nm) formed at this step, converted to the tetraene product according to a first order kinetics, with k = 0.12 s⁻¹.     

Reduction of [(FeTPP)2O]+SbF6− by iodide ion in dichloromethane

The oxidation in dichloromethane of the iodide ion of cetyltrimethyl ammonium iodide to iodine by the compound consisting of the hexafluoroantimonate anion and the dimeric iron tetraphenylporphyrin (bridged by oxygen) cation, [Fe(TPP)₂O]⁺SbF₆⁻, has been studied spectrophotometrically using the stopped-flow method. At 273 K, d[Fe(TPP)₂O]/dt is equal to 8 × 10⁴[[Fe(TPP)₂O]⁺]-[1⁻] mol dm⁻³ s⁻¹; from this and other measurements E_a is estimated to be 13 kJ mol⁻¹. These results will be compared with other relevant kinetic data, and a possible reaction mechanism will be considered.     

Reactions of Cr(III) tetraphenylporphyrin complex with superoxide ion: ESR evidence for the formation of a new superoxide adduct of Cr(IV) porphyrinate and its reactive character

The electron spin resonance (ESR) spectrum of the reaction product of superoxide ion, O₂⁻, with chloro-5,10,15,20-tetraphenylporphyrinatochromium(III) [Cr(III)(TPP)Cl] shows strong hyperfine interactions with the metal nucleus and the metal ligand, indicating the formation of a superoxide adduct, Cr(IV)(TPP)(Cl)(O₂⁻). The formation of this superoxide adduct was also confirmed by UV-Vis spectroscopy. The reactive character of this superoxide adduct was investigated by ESR spectrometry. It was found that Cr(IV)(TPP)(Cl)(O₂⁻) can oxidize t-butylamine and triphenylphosphine to give the corresponding radical species, respectively, but not pyridine, 4-cyanopyridine or imidazole. These results indicate that the reactive character of Cr(IV)(TPP)- (Cl)(O₂⁻) resembles that of the free superoxide ion.     

Measurement of intracellular chloride activity in mouse liver slices with microelectrodes

Steady-state membrane potential (V_m) and intracellular Cl⁻ activity (a_Clⁱ) were measured with double-barreled Cl⁻-selective microelectrodes in mouse liver slices. In bathing solutions (33.8° C) containing pyruvate, glutamate, fumarate, and glucose, V_m and a_Clⁱ were −27.6 ± 1.0 mV and 32.6 ± 1.5 mM, respectively. This apparent value of a_Clⁱ exceeded the level required for passive distribution of this ion (a_Cl^eq = 26.4 ± 1.3 mM) by 6.2 ± 1.0 mM. This difference was essentially unchanged in experiments where (i) Na⁺ was replaced by choline, (ii) HCO₃⁻ was removed, and (iii) Cl⁻ was replaced by gluconate. These data argue against the presence of Na⁺- or HCO₃⁻-coupled Cl⁻ transport mechanisms in the plasma membrane of mouse liver cells. This implies that a_Clⁱ is in fact at equilibrium and interference with the response of Cl⁻-selective microelectrodes by intracellular anions is responsible for the apparent difference between a_Clⁱ and a_Cl^eq. We found that Cl⁻-selective microelectrodes containing Corning 477315 ligand are sensitive to taurocholate, a representative bile salt. Their selectivity to taurocholate is about 60-times their selectivity towards Cl⁻. This suggests that interference of bile acids at concentrations normally present in hepatocytes with determinations of a_Clⁱ can account for the apparent difference a_Clⁱ − a_Cl^eq.     

Estimation of intracellular pH in muscle of fishes from different thermal environments

A technique based on homogenisation of rapidly frozen tissue was used to investigate the regulation of intracellular pH (pH_i) in freshwater and marine fish from diverse environmental temperatures. The following species were held at ambient temperatures of ca. 1°C (Notothenia coriiceps; Antarctica), 5°C (Pleuronectes platessa, Myoxocephalus scorpius; North Sea), and 26°C (Oreochromis niloticus; African lakes). The effects of seasonal acclimatisation to 4, 11 and 18°C were also examined in rainbow trout in the winter, autumn and summer, respectively. Extracellular (whole blood) pH (pH_e) did not follow the constant relative alkalinity relationship, where pH⁺=pOH⁻ for any particular temperature, over a range of 1–26°C (overall δpH_e/δT=0.009±0.002 U °C⁻¹; P<0.001), apparently being regulated by ionic fluxes and ventilation. Intracellular pH (pH_i) was also regulated independently of pN(=0.5 pK water) in all species of fish examined. The inverse relationship between pH_i and environmental temperature gave an overall δpH_i/δT of −0.010±0.001 U °C⁻¹ (for both white and red muscle) and −0.004±0.003 U °C⁻¹ (cardiac muscle). However, between 1 and 11°C δpH_i/δT was much higher (P<0.001), −0.022±0.003 U °C⁻¹ (white muscle) and −0.022±0.004 U °C⁻¹ (red muscle). The possible adaptive roles for these different acid–base responses to environmental temperature variation among tissues and species, and the potential difficulties of estimating pH_i, are discussed.     

Thermodynamics and coordination characteristics of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 extraction complex

The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)₂·(H₃O⁺·χH₂O)₂·UO₂Cl₄²⁻ (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (K_ex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log K_ex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol⁻¹. The value of the entropy change, ΔS°, was calculated from ΔH° and K_ex and equals −690 J K⁻¹ mol⁻¹, whereas ΔG° = −6.45 kJ mol⁻¹. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K⁻¹ mol⁻¹, ΔH° = −101 kJ mol⁻¹ and ΔG° = −8.37 kJ mol⁻¹), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H₅O₂⁺ ions in it, in contrast to the H₃O⁺ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H₅O₂⁺ ions is held in the crown ether cavity by four hydrogen bonds. The H₅O₂⁺ ion has a central bond. The uranium atom forms UO₂Cl₄²⁻ as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above.     

Binuclear rhodium carbonyl halide complexes containing the 1,1-bis(diphenylphosphino)ethene ligand,and the crystal structure of [Rh2Cl2(μ-CO){Ph2PC(CH2)PPh2}2]

Treatment of [RhCl(CO)₂]₂ with 1,1-bis(diphenylphosphino)ethene (dppee) yields the cationic binuclear rhodium complex [Rh₂(μ-Cl)(CO)₂(μ-CO)(dppee)₂]⁺ which may be isolated as the [RhCl₂(CO)₂]⁻ salt at low temperature (230 K) but which readily forms the Cl⁻ salt on allowing the solution to warm to room temperature. On bubbling nitrogen through a solution of this cationic complex at room temperature, the monocarbonyl species, [Rh₂Cl₂(μ-CO)(dppee)₂] is obtained. The crystal structure of this complex has been determined. The crystals are orthorhombic, space group Pnca, a = 29.98(3), b = 23.70(2), c = 14.78(3) Å, Z = 8. Using 3019 unique reflections the structure was refined to R = 0.091. The rhodium-rhodium distance is 2.650(14) Å. Direct rhodium NMR data are reported for these complexes.     

Synthesis,characterization and antitumor activity of some metal complexes of 3- and 5-substituted salicylaldehyde o-hydroxybenzoylhydrazones

Complexes of Mn(II), Fe(III), Fe(II), Co(II), Ni(II), Cu(II), Zn(II) and Pt(II) with 3- and 5-substituted salicylaldehyde o-hydroxybenzoylhydrazones (XSBH, X = H, 3-NO₂, 3-CH₃O, 5-Cl, 5-Br, 5-CH₃ or 5-NO₂) have been prepared and characterized by elemental analysis, conductance measurements, magnetic susceptibilities (from room temperature down to liquid nitrogen temperature) and spectral studies. These studies indicate the following structures: monomeric, high-spin, distorted octahedral for Mn(XSBH)₂; monomeric, high-spin, five-coordinate for Fe(XSBH)SO₄·H₂O; dimeric, high-spin phenoxide bridged, five-coordinate for Fe(XSBH)Cl; dimeric, high-spin five-coordinate for Co(XSBH)Cl·2H₂O; dimeric low-spin, five-coordinate for Ni(XSBH)Cl·2H₂O; dimeric, four-coordinate for Zn(XSBH); and a square-planar structure for M(XSBH)Cl·H₂O (M = Cu(II) or Pt(II).Intermolecular antiferromagnetic exchange interactions are present in Fe(III) complexes, where the exchange parameter (J) is ca. −8.0 cm⁻¹ for these complexes. The Fe(III) complexes exhibit asymmetric quadrupole split doublets in their ⁵⁷Fe Mössbauer spectra. The asymmetry is found to be temperature dependent with relatively symmetrical doublets seen at low temperature. The polycrystalline ESR spectra of Cu(II) complexes are isotropic and indicate a d_x²−y² ground state in square-planar stereo-chemistry. All these metal complexes have been screened for their antitumor activity against the P 388 lymphocytic leukaemia test system in mice and enhanced antitumor activity relative to the free ligand was found but no significant activity at the dosages used.     

Oxidation of copper(II)-coordinated diethylenetriamine by hexacyanoferrate(III) ion. A kinetic investigation

The stoichiometry and kinetics of the reaction between [Cu(dien)(OH)]⁺ and [Fe(CN)₆]³⁻ in aqueous alkaline medium are described. The rate equation − (d[Fe(III)]/dt = {k₁[OH⁻]²[[Cu(dien)(OH)]⁺] + k₂[OH⁻] × [[Cu(dien)(OH)]⁺]²}([Fe(III)]/[Fe(II)]) (Fe(III) = [Fe(CN)₆]³⁻; Fe(II) = [Fe(CN)₆]⁴⁻, the 4:4:1 OH⁻/Fe(III)/[Cu(dien)(OH)]⁺ stoichiometric ratio and the nature of the ultimate products identified in the reaction solution suggest the fast formation of a doubly deprotonated Cu(III)-diamido complex which slowly undergoes an internal redox process where the ligand is oxidised to the Schiff base H₂NCH₂CH₂NCHCHNH.The [[Cu(dien)(OH)]⁺]² term in the rate equation is explained with the formation of a transient μ-hydroxo mixed-valence Cu dimer. A two-electron internal reduction of the Cu(III) complex yielding a Cu(I) intermediate is suggested to account for the presence of monovalent copper in a precipitate which forms at relatively high reactant concentrations and in the absence of dioxygen.     

Imidazole,imidazolate, and hydroxide complexes of (protoporphyrin IX)iron(III) and its dimethyl ester as model systems for ferric hemoproteins: Electron paramagnetic resonance and electronic spectral study

The EPR and electronic spectral changes upon titration of systems consisting of (protoporphyrin IX)iron(III) chloride (Fe(PPIX)Cl) or its dimethyl ester (Fe-(PPIXDME)Cl) and imidazole derivatives with tetrabutylammonium hydroxide solution have been measured at 77 and 298 °K in various solvents. The EPR and electronic spectra of the melt of Fe(PPIXDME)Cl in imidazole derivatives have been also measured. The imidazole derivatives studied here were imidazole and 4-methyl-, 4-phenyl-, 2-methyl-, 2,4-dimethyl-, 1-methyl-, and 1-acetylimidazole. The spectral changes upon addition of hydroxide were markedly different between the systems containing NH imidazoles (BH), with a dissociable proton, and those containing NR imidazoles (BR), without it. In the former systems, five spectral species were successively formed at 77 °K and were assigned to following complexes: [Fe(P)(BH)₂]⁺, Fe(P)(BH)(B), [Fe(P)(B)₂]^?, Fe(P)(BH)(OH), and [Fe(P)(B)(OH)]^?, where P is PPIX or PPIXDME. In the latter systems, initial complex, [Fe(P)(BR)₂]⁺, was found to be changed to final complex, Fe(P)(BR)(OH), through an intermediate at 77 °K. At 298 °K, both systems were found to react with hydroxide to finally form Fe(P)(OH). The crystal field parameters were evaluated using the EPR g values in low-spin complexes studied here and in hemoproteins. The five regions corresponding to five low-spin complexes could be distinguished in crystal field diagrams.     

Kinetics and mechanism of the reaction between 1,2-diaminopropanetetra-acetatoferrate(III) and cyanide ion

The present study examines the kinetics and mechanism of the system [FePDTA(OH)]²⁻ + 5CN⁻ ⇌ [Fe(CN)₅OH]³⁻ + PDTA⁴⁻ at pH= 11.0±0.02, I= 0.25 M and temperature = 25 ± 0.1 °C. The reaction has been studied spectrophotometrically at 395 nm (λ_max of [Fe(CN)₅OH]³⁻). The data show that the reaction has three distinguishable stages; the first stage is formation of [Fe(CN)₅OH]³⁻, the second is conversion of [Fe(CN)₅OH)]³⁻ to [Fe(CN)₆]³⁻ and last is reduction of [Fe(CN)₆]³⁻ to [Fe(CN)₆]³⁻ by the released ligand, viz., PDTA. The first reaction shows variable order dependence on cyanide concentration, one at high cyanide concentration and two at low cyanide concentration. The second reaction exhibits first order dependence on the concentration of [Fe(CN)₅OH]³⁻ as well as cyanide. The reverse reaction between [Fe(CN)₅OH]³⁻ and PDTA is first order in [Fe(CN)₅OH]³⁻ and PDTA, and inverse first order in cyanide. On the basis of forward and reverse rate studies, a five-step mechanism has been proposed for the first reaction.     

Turning up the heat on subzero fish: thermal dependence of sustained swimming in an Antarctic notothenioid

We determined the maximum sustained swimming speed (U_crit), and resting and maximum ventilation rates of the Antarctic fish Pagothenia borchgrevinki at five temperatures between −1°C and 8°C. We also determined resting metabolic rate (VO₂) at −1°C, 2°C, and 4°C. U_crit of P. borchgrevinki was highest at −1°C (2.7±0.1 BL s⁻¹) and rapidly decreased with temperature, representing a thermal performance breadth of only 5°C. This narrow thermal performance supports our prediction that specialisation to the subzero Antarctic marine environment is associated with a physiological trade-off in performance at high temperatures. Resting oxygen consumption and ventilation rate increased by more than 200% across the temperature range, which most likely contribute to the decrease in aerobic swimming capabilities at higher temperatures.     

Kinetics and mechanism of the oxidation of L-ascorbic acid by trisoxalatocobaltate(III) in basic aqueous solution

The kinetics and mechanism of the oxidation of L- ascorbic acid by trisoxalatocobaltate(III) were studied as a function of pH, ascorbate concentration, ionic strength and temperature in a weakly basic aqueous solution. The pH dependence of the process can be ascribed to the oxidation of the doubly deprotonated ascorbate ion for which k = 20 M⁻¹ s⁻¹ at 25 °C, ΔH^# = 34 ± 2 kJ mol⁻¹ and ΔS^# = −108 ± 7 J K⁻¹ mol⁻¹. The results are discussed in reference to literature data for this reaction in weakly acidic medium and for the oxidation by a series of other oxidants.     

Strategies of freeze avoidance in larvae of the goldenrod gall moth,Epiblema scudderiana: Laboratory investigations of temperature cues in the regulation of cold hardiness

Laboratory manipulations of ambient temperature were used to investigate the role of temperature in triggering or modulating cold-hardiness adaptations, supercooling-point depression and cryoprotectant accumulation, in larvae of the goldenrod gall moth, Epiblema scudderiana (Clemens), a freeze-intolerant species. Low temperature strongly facilitated cryoprotectant synthesis; larvae subjected to a 1°C per day decrease in temperature showed a major increase in the rate of glycerol synthesis when temperature fell below 5°C with highest rates of synthesis, greater than 90 μmol g⁻¹ d⁻¹, at temperatures between 0 and −10°C. Conversely, abrupt rewarming of larvae from −18 to 23°C in mid-November stimulated a rapid loss of glycerol (from a starting level of 1763 ± 278 μmol/g wet weight) with a half time of only 1.5 days. Supercooling-point depression was not keyed to ambient temperature but appeared to be an endogenous event occurring over the same time interval in laboratory animals held at warm or cold temperatures, as well as in outdoor animals. Rewarming of cold-adapted larvae in November resulted in only a small rise in supercooling point (and did not break diapause) but rewarming in February resulted in a 19°C increase in supercooling point in 4 days, followed rapidly by pupation.     

Studies on the reactions of PtCl2en,cis-Pt(NH3)2Cl2 and their aqua species with adenosine,deoxyadenosine and adenine using ion-pair HPLC

The reactions of PtCl₂en or cis-Pt(NH₃)₂Cl₂ and their aqua species with adenine and adenosine were studied by means of ion-pair HPLC. From the chromatograms, it was found that the first binding site of Pt(II) was the N(7) site of adenine under both acidic and neutral conditions. The rates of Pt(II) binding at the (N7) site of adenosine and deoxyadenosine were measured. The rate constants, k₁, were obtained for the reactions of PtCl₂en or cis-Pt(NH₃)₂Cl₂ with adenosine and deoxyadenosine at pH 3 and 7 over the temperature range 9–25 °C. The k₁ values were 6.8–7.7 × 10⁻⁴ dm³ mol⁻¹ s⁻¹ at 25 °C. For the aqua species, the rate of [cis-Pt(NH₃)₂ClH₂O]⁺ with adenosine N(7) was measured. The rate constants, k₂ which were found to be smaller than those of hydrolysis, k_h, were calculated at pH 3 over the temperature range 25–40 °C. The k₂ value obtained at 25 °C was 1.1 × 10⁻² dm³ mol⁻¹ s⁻¹, 15 time larger than k¹. The activation parameters were also calculated.     

Effects of dehydration and low temperatures on the oxidation of high-potential cytochrome c by photosynthetic reaction centers in Ectothiorhodospira shaposhnikovii

Effects of temperature and dehydration on the efficiency of electron transfer from membrane-bound high-potential cytochromes c_h to the reaction-center bacteriochlorophyll (P-890) in Ectothiorhodospira shaposhnikovii have been studied. A kinetic analysis of the cytochrome oxidation suggests that there are at least two conformational states of the c_h-P-890 complex, of which only one allows photoinduced electron transfer from cytochrome to P-890⁺. Lowering the temperature of dehydration leads to a change in the proportion of the populations in the two conformations. The observed 2-fold deceleration of cytochrome oxidation can be related only to the diminution of the amount of photoactive cytochromes per reaction center. The rate constant for the transfer of an electron from cytochrome c_h to bacteriochlorophyll is 2.8 · 10⁵ s⁻¹ and is independent of temperature and dehydration (as estimated within the accuracy of the experiments). The effects produced by low temperature and dehydration are completely reversible. The thermodynamic parameters of the transition of the cytochrome from the nontransfer to electron-transfer conformation were estimated. For room temperature (+ 20°C) in chromatophore preparations, ΔG = −5.4 kJ · M⁻¹, ΔH = 60 kJ · M⁻¹, ΔS = 0.22 kJ · M⁻¹ · K⁻¹. For Triton X-100 subchromatophore preparations, the absolute values of the above parameters are significantly lower: ΔG = −2.8 kJ · M⁻¹, ΔH = 18 kJ · M⁻¹, and ΔS = 0.075 kJ · M⁻¹ · K⁻¹. To a larger extent, the above parameters are diminished for chromatophore preparations in an 80% glycerol solution: ΔG = −1.7 kJ · M⁻¹, ΔH = 6 kJ · M⁻¹, ΔS = 0.025 kJ · M⁻¹ · K⁻¹. The data suggest the hydrophobic character of the forces that maintain the P-890-c_h complex in the electron-transfer conformation. The results obtained suggest that electron tunneling within the complex cannot occur until a specific conformational configuration of the complex is formed. The efficiency of cytochrome c_h oxidation is determined by the temperature, the degree of dehydration and the environmental conditions, whereas the transfer of an electron itself in the electron-transfer configuration is essentially independent of temperature and hydration.     

Cryopreservation of Babesia caballi cultures

Babesia caballi cultures were cryopreserved with a solution of 10% (w/v) polyvinylpyrrolidone 40 as cryoprotectant. Samples were cooled at rates of 1, 10, 30 and 100°C min⁻¹ using a programmable freezer. Additionally, a styrofoam box designed to cool samples at an approximate rate of 10°C min⁻¹ when placed in a −80°C freezer was used. Samples were stored in liquid nitrogen, thawed rapidly and inoculated into cultures. Although, a high loss of infectivity was observed after cryopreservation, cultures could be initiated reliably from cryo-stabilates frozen at a rate of 10 and 30°C min⁻¹ or frozen with the styrofoam box.     

Kinetics and mechanism of malonate replacement in bis(malonato)oxovanadate(IV) by oxalate

The kinetics of malonate replacement in bis- (malonato)oxovanadate(IV), [VO(mal)₂H₂O]²⁻(hereafter water molecule will be omitted), by oxalate has been studied by the stopped-flow method. The reaction was found to consist of two consecutive steps (k₁ and k₂: first-order rate constants) passing through a mixed ligand complex, [VO(mal)(ox)]²⁻. The rates for each step depended linearly on the concentrations of free oxalate species, Hox⁻ and ox²⁻. The second-order rate constants for the replacement by ox²⁻ were much larger in the k₁ step than in the k₂ step and the activation parameters were determined as follows: ΔH^≠= 43.5 ± 5.6 kJ mol⁻¹, ΔS±-53 ± 19 J K⁻¹ mol⁻¹ and ΔH≠= 43.6 ± 0.5 kJ mol⁻¹, δS≠ = -62 ± 2 J K^−l mol⁻¹ for the k₁ and k₂ steps, respectively. The volume of activation was determined to be -0.65 ± 0.75 cm³ mol⁻¹ at 20.2 °C by the high-pressure stopped-flow method for the apparent rate constants.