Reactions of ruthenium(II) para-substituted tetraphenylporphyrin complexes with carbon monoxide oxygen: A kinetic investigation

The reaction of Ru(XTPP)(DMF)₂, where XTPP is the dianion of para substituted tetraphenylporphyrins and X is MeO, Me, H, Cl, Br, I, F, with O₂ and CO were studied in DMF. The process was found to be first-order in metalloporphyrin, first-order in molecular oxygen and carbon monoxide, and second-order overall. Second-order rate constants for the CO reaction ranged from 0.170 to 0.665 M^?1 s^?1 at 25°C, those for the O₂ reaction from 0.132 to 0.840 M^?1 s^?1 at 25°C. Similar activation parameters (ΔH_CO^± = 87 ± 1 kJ mol^?1, ΔS_CO^± = 22 ± 4 JK^?1 mol^?1; ΔH_O2^± = 81 ± 1 kJ mol^?1, and ΔS_O2^± = 11 ± 5 JK^?1 mol^?1) were found within each series. Reactivities of X substituted metalloporphyrins were found to follow different Hammett σ functions. The CO reactions correlated with σ_? following normal behavior; the O₂ reactions correlated with σ₈° indicating O₂ is π-bonded in the transition states. A dissociative mechanism is postulated for the process.     

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.     

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⁻.     

Preparation,structure and properties of dicyano silver complexes of the M(en)3Ag2(CN)4 and M(en)2Ag2(CN)4 type

Complexes of the M(en)₃Ag₂(CN)₄ (M = Ni, Zn, Cd) and M(en)₂Ag₂(CN)₄ (M = Ni, Cu, Zn, Cd) type were prepared and identified by elemental analysis, infrared spectroscopy, measurement of magnetic susceptibility, and X-ray powder diffractometry. The crystal structures of Ni(en)₃Ag₂(CN)₄ (I) and Zn(en)₂Ag₂(CN)₄ (II) were determined by the method of monocrystal structure analysis. Complex I crystallizes in the space group C2/c, a = 1.2639(5), b = 1.3739(4), c = 1.2494(4) nm, β = 113.25(4)°, D_m = 1.86(1), D_c = 1.86 gcm⁻³ Z = 4, R = 0.0429. The crystal structure of I consists of complex cations [Ni(en)₃]²⁺ and complex anions [Ag(CN)₂]⁻. Complex II crystallizes in the space group I2/m, a = 0.9150(3), b = 1.3308(4), c = 0.6442(2) nm, β = 95.80(3)°, D_m = 2.14(1), D_c = 2.15 gcm⁻³, Z = 2, R = 0.0334. Its crystal structure consists of infinite, positively charged chains of the [-NCAgCNZn- (en)₂]_nⁿ⁺ type and isolated [Ag(CN)₂]⁻ anions. The atoms of Ag are positioned parallely to the z axis and the AgAg distance is equal to 0.3221(2) nm.     

Kinetics of imidazole addition to the axial site of the iron(II) complex of 2, 3, 9,10-tetraphenyl-1,4, 8,11-tetraaza-1,3, 8,10-cyclotetradecatetraene in dimethyl sulfoxide. Evidence for a dissociative-interchange mechanism

The axial adduct formation of the iron(II) complex of 2,3,9,10-tetraphenyl-l,4,8,11-tetraaza-1,3,8,10-cyclotetradecatetraene (L) with imidazole in dimethyl sulfoxide has been investigated spectrophotometrically at various temperatures and pressures. In the presence of a large excess of imidazole the reaction with the two phases has been observed. The first faster reaction is the formation of the monoimidazole complex of FeL²⁺, and the second slower reaction corresponds to the formation of the bisimidazole complex. Activation parameters are as follows: for the first step with k₁ (25.0°C) = (6.8 ±0.2)×10⁵ mol⁻¹ kg s⁻¹, ΔH³₁ = 47.5 ± 4.9 kJ mol⁻¹, ΔS³₁ = 26±16 J K⁻¹ mol⁻¹, and ΔV³₁ (30.0°C) = 27.2±1.5 cm³ mol⁻¹; for the second step with k₂ (25.0°C) = 26.8±0.8 mol⁻¹ kg s⁻¹, ΔH³₂ = 91.6± 0.8 kJ mol⁻¹, ΔS³₂ = 90±3 J K⁻¹ mol⁻¹, and ΔV³₂ (35.0°C) = 21.8±0.9 cm³ mol⁻¹. The large positive activation volumes strongly indicate a dissociative character of the activation process.     

A nuclear magnetic resonance study of the kinetics of ligand exchange reactions in uranyl complexes. VII. Acetylacetonate exchange in bis(acetylacetonato)(N,N-dimethylformamide)dioxouranium(VI)

The exchange reaction of acac(acetylacetonate) in UO₂(acac)₂dmf (dmf=N,N-dimethylformamide) in o-dichlorobenzene has been studied by the NMR line-broadening method. The exchange rate depends on the concentration of the enol isomer of acetylacetone in its low region, and approaches the limiting value in its high region. It is proposed that the exchange reaction proceeds through the mechanism in which the dissociation of one end of the chelated acac is the rate-determining step. The kinetic parameters for this step are as follows: k (25 °C)=5.03 × 10⁻³ s⁻¹, ΔH3=91.6 ± 3.8 kJ mol⁻¹, and ΔS3 =17.2 ± 10.5 JK⁻¹ mol⁻¹. The exchange rate becomes slower by the addition of free DMF. This may be due to the competition of DMF with the enol isomer of acetylacetone in attacking a four-coordinated intermediate in the equatorial plane.     

Temperature and pressure dependent equilibrium studies of 1, 4, 8, 11-tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane nickel(ii) in coordinating solvents

In coordinating solvents, the complex 1, 4, 8, 11- tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane nickel(II) bisperchlorate exists as an equilibrium mixture involving four coordinate R,S,R,S-[Ni(tmc)]²⁺ and five coordinate R,S,R,S-[Ni(tmc)(solvent)]²⁺ species. Spectrophotometric measurements of this equilibrium in a number of solvents have been conducted over a range of temperatures and pressures. The stability order for the five coordinate complex in the solvents investigated is CH₃CN>DMF>DMSO>C₆H₅CN> H₂O>ClCH₂CN at 25 °C. Differences in stability are considered in terms of the measured thermodynamic parameters ΔH° and ΔS°. Both steric and electronic factors were found to influence solvent coordination with the macrocyclic complex.For the equilibrium in CH₃CN, C₆H₅CN, DMF and H₂O, reaction volumes, ΔV°, of −3.2±0.5, −4.2±0.5, −0.2±0.5 and −0.5±0.5 cm³ mol⁻¹ respectively have been determined. Each is significantly smaller than the corresponding solvent molar volume. The ΔV° for the equilibrium in CH₃CN is comparable with the previously determined activation volume for exchange of this solvent on R, S, R, S- [Ni(tmc)(CH₃CN)]²⁺. The equilibrium and measured volume parameters are discussed in relation to the mechanism for solvent exchange.     

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.     

Kinetics of reduction of ferrioxamine B by chromium(II), vanadium(II), and dithionite

Kinetic studies of the reduction of ferrioxamine B (Fe(Hdesf)⁺) by Cr(H₂O)₆²⁺, V(H₂O)₆²⁺, and dithionite have been performed. For Cr(H₂O)₆²⁺ and V(H₂O)₆²⁺, the rate is ?d[Fe(Hdesf)⁺]/dt = k[Fe(Hdesf)⁺][M²⁺]. For Cr(H₂O)₆²⁺, k = 1.19 × 10⁴ M^?1 sec^?1 at 25°C and μ = 0.4 M, and k is independent of pH from 2.6 to 3.5. For V(H₂O)₆²⁺, k = 6.30 × 10² M^?1 sec^?1 at 25°C, μ = 1.0 M, and pH = 2.2. The rate is nearly independent of pH from 2.2 to 4.0. For Cr(H₂O)₆²⁺ and V(H₂O)₆²⁺, the activation parameters are ΔH^≠ = 8.2 kcal mol^?1, ΔS^≠ ?12 eu and ΔH^≠ = 1.7 kcal mol^?1, ΔS^≠ = ?40 eu (at pH 2.2) respectively. Reduction by Cr(H₂O)₆²⁺ is inner-sphere, while reduction by V(H₂O)₆²⁺ is outer-sphere. Reduction by dithionite follows the rate law ?d[Fe(Hdesf)⁺]/dt =kK^{$\text{1}\text{2}$}[Fe(Hdesf)⁺][S₂O₄^2?]^{$\text{1}\text{2}$} where K is the equilibrium constant for dissociation of S₂O₄^2? into SO₂^? radicals. The value of k at 25°C and μ = 0.5 is 2.7 × 10³ M^?1 sec^?1 at pH 5.8, 3.5 × 10³ M^?1 sec^?1 at pH 6.8, and 4.6 × 10³ M^?1 sec^?1 at pH 7.8, and ΔH^≠ = 6.8 kcal mol^?1 and ΔS^≠ = ?19 eu at pH 7.8.     

Metal complexes of cyclic diamines,dissociation kinetics of bis(1,5-diazacyclo-octane)nickel(II) in acidic and basic solution,and electrochemical studies

The preparation of the planar yellow [Ni([8]aneN₂)₂](ClO₄)₂ is described. The complex dissociates in basic solution, with rate = k_OH[NiL][OH^?] (L = 1,5-diazacyclo-octane). At 25 °C, k_OH = 4.5 x 10^?2 M^?1 s^?1 and the corresponding activation parameters are ΔH^≠ = 69.2 kJ mol^?1 and ΔS₂₉₈^≠ = ?38.6 J K^?1 mol^?1. Acid catalysed dissociation in quite slow even in strongly acidic solutions. The kinetic data in this case can be fitted to the expression K_obs = k_o + K_H[H⁺], where k_o relates to a solvolytic pathway and k_H to the acid catalysed pathway. At 60 °C, K_o = 2 x 10^?5 s^?1 and k_H is 2 x 10^?5 M^?1 s^?1. Possible mechanisms for these reactions are considered.The Ni(II)/Ni(III) redox couple for NiLⁿ⁺ is irreversible on Pt using MeCN as solvent.     

Charge-transfer photochemistry under high pressure: Reductive elimination from trans-diazidotetracyanoplatinate(IV)

The quantum yield for the reductive elimination of azide from trans-Pt(CN)₄(N₃)₂²⁻ to yield P-(CN)₄²⁻ and molecular nitrogen in aqueous solution was studied as a function of pressure up to 200 MPa. The resulting volume of activation of 8.1 ± 0.4 cm³ mol⁻¹ is interpreted as evidence for the formation of a caged radical species via simultaneous scission of both PtN₃ bonds in the CT excited state. Similar measurements in ethanol result in a volume of activation of 14.3 ± 0.9 cm³ mol⁻¹. An additional deactivation route for the CT excited state is suggested to account for this result.     

Dynamics of molecular organization in agarose sulphate

Nongelling solutions of structurally regular chain segments of agarose sulphate show disorder–order and order–disorder transitions (as monitored by the temperature dependence of optical rotation) that are closely similar to the conformational changes that accompany the sol–gel and gel–sol transitions of the unsegmented polymer. The transition midpoint temperature (T_m) for formation of the ordered structure on cooling is ～25 K lower than T_m for melting. Salt-induced conformational ordering, monitored by polarimetric stopped-flow, occurs on a millisecond time scale, and follows the dynamics expected for the process 2 coil ? helix. The equilibrium constant for helix growth (s) was calculated as a function of temperature from the calorimetric enthalpy change for helix formation (ΔH_cal = ?3.0 ± 0.3 kJ per mole of disaccharide pairs in the ordered state), measured by differential scanning calorimetry. The temperature dependence of the nucleation rate constant (k_nuc), calculated from the observed second-order rate constant (k_obs) by the relationship k_obs = k_nuc(1 ? 1/s) gave the following activation parameters for nucleation of the ordered structure of agarose sulphate (1 mg mL^?1; 0.5M Me₄NCl or KCl): ΔH* = 112 ± 5 kJ mol^?1; ΔS* = 262 ± 20 J mol^?1 K^?1; ΔG*₂₉₈ = 34 ± 6 kJ mol^?1; (k_nuc)₂₉₈ = (7.5 ± 0.5) × 10⁶ dm³ mol^?1 s^?1. The endpoint of the fast relaxation process corresponds to the metastable optical rotation values observed on cooling from the fully disordered form. Subsequent slow relaxation to the true equilibrium values (i.e., coincident with those observed on heating from the fully ordered state) was monitored by conventional optical rotation measurements over several weeks and follows second-order kinetics, with rate constants of (2.25 ± 0.07) × 10^?4 and (3.10 ± 0.10) × 10^?4 dm³ mol^?1 s^?1 at 293.7 and 296.2 K, respectively. This relaxation is attributed to the sequential aggregation processes helix + helix → dimer, helix + dimer → trimer, etc., with depletion of isolated helix driving the much faster coil–helix equilibrium to completion. Light-scattering measurements above and below the temperature range of the conformational transitions indicate an average aggregate size of 2–3 helices.     

The kinetics of ligand substitution in bis(N-alkylsalicylaldiminato)nickel(II) Complexes: a study on the reactivity of square-planar versus octahedral coordination

The bis(N-alkylsalicylaldiminato)nickel(II) complexes Ni(R-sal)₂ with R = CH(CH₂OH)CH(OH)Ph (I), R = CH(CH₃)CH(OH)Ph (II) and R = CH₂CH₂Ph (III; Ph = phenyl) were prepared and characterized. In the solid state I and II are paramagnetic (μ = 3.2 and 3.3 BM at 20 °C, respectively), whereas III is diamagnetic. It follows from the UV-Vis spectra that in acetone solution I is six-coordinate octahedral and III is four-coordinate planar, the spectrum of II showing characteristics of both modes of coordination. Vis spectrophotometry and stopped-flow spectrophotometry were applied to study the kinetics of ligand substitution in I–III by H₂salen (= N,N′-disalicylidene-ethylenediamine) in the solvent acetone at different temperatures. The kinetics follow a second-order rate law, rate = k[H₂-salen] [complex]. At 20 °C the sequence of rate constants is k(III):k(II):k(I) = 11 850:40.6:1. The activation parameters are ΔH^≠(I) = 112, ΔH^≠(II) = 40.7, ΔH^≠(III) = 35.7 kJ mol⁻¹ and ΔS^≠(I) = 92, ΔS^≠(II) = −103, ΔS^≠(III) = −89 J K⁻¹ mol⁻¹. The enormous difference in rate between complexes I, II and III, which is less pronounced in methanol, is attributed to the existence of a fast equilibrium planar ⇌ octahedral, which is established in the case of I and II by intramolecular octahedral coordination through the hydroxyl groups present in the organic group R. An A-mechanism is suggested to control the substitution in the sense that the entering ligand attacks the four-coordinate planar complex, the octahedral complex being kinetically inert.     

Isokinetic Relationship in the Oxidation of Cuprous Stellacyanin by Cobalt(III) Complexes

Rate parameters have been obtained for the oxidation of cuprous stellacyanin by cobalt(III) ions of the form cis(N)-[CoN₂O₄]^?, including cis(N)-[Co(NTA)(gly)]^?, cis(N)-[Co(IDA)₂]^?, [Co(en)(ox)₂]^?(μ 0.5 M(phosphate), pH 7.0), and Co(EDTA)^?(μ 0.1 M(NaCl), pH 7.2, 0.001 M phosphate). An excellent isokinetic correlation between the activation parameters ΔH^≠ and ΔS^≠ exists for the reactions of aminopolycarboxylatocobalt(III) ions with reduced stellacyanin (β = 300 ± 12 K; correlation coefficient = 0.995). It is concluded that enthalpy-entropy compensation in these reactions may be understood in terms of differing orientations preferred by the various oxidants in forming precursor complexes with the reduced blue protein. While ΔH^≠ and ΔS^≠ values for electron transfer from stellacyanin to cis(N)-[CoN₂O₄]^? ions vary over ranges of 10.7 kcal/mol and 34 cal/mol-deg, respectively, room temperature rate constants are relatively constant (3.6–34.5 M^?1 sec^?1), as expected from Marcus theory for outer sphere electron transfer.     

The crystallochemistry of tetracyanocomplexes. The crystal and molecular structures of tris(1,2-diaminoethane)zinc(II) tetracyanoniccolate(II) monohydrate and tris(1,2-diaminoethane)zinc(II) tetracyanoniccolate(II)

C₁₀H₂₆N₁₀ONiZn, tris(1,2-diaminoethane) zinc(II) tetrakis(cyano)niccolate(II) monohydrate (I), orthorhombic, Pbca, a = 1.1680(4), b = 1.5844(3), c = 1.9981(6) nm, Z = 8 d(meas) = 1.54, d(calc) = 1.53 g cm^?3. C₁₀H₂₄N₁₀NiZn, tris(1,2-diaminoethane) zinc(II) terakis(cyano)niccolate(II), (II), monoclinic, P2₁/n, a = 0.7957(2), b = 1.5170(5), c = 1.4932(4) nm, β = 96.41(2)°, Z = 4, d(meas) = 1.49, d(calc) = 1.51 g cm^?3. Both the structures (I) and (II) have been solved by the heavy atom method and refined by full-matrix least-squares to R(I) = 0.086 for 1890 independent reflections and R(II) = 0.058 for 1689 independent reflections, respectively. In the case of (II) the superlattice structure problem was solved. The crystal structure of (I) consists of [Zn(en)₃]²⁺ cations, [Ni(CN)₄]^2? anions and water molecules. Two of the cyano groups in trans positions are bonded to water molecules by hydrogen bonds, the distances CN?O being 0.289 and 0.291 nm, respectively. The crystal structure of (II) is constituted by [Zn(en)₃]²⁺ cations and [Ni(CN)₄]^2? anions.     

Kinetics of the reaction of pentacyanonitrosylferrate(II) with aliphatic amines

The reactions of pentacyanonitrosylferrate(II) with ethyl-, n-propyl-, n-butyl-, cyclohexyl- and benzylamines were studied in dilute aqueous solution at 8.6–9.6 pH and 15–35 °C. Nitrosation, diazotation and deamination processes take place in the reactions resulting in alcohols and N₂ gas as final products. On the basis of spectrophotometric and gasvolumetric experiments the rate law was determined as follows.v = k[RNH₂][Fe(CN)₅NO^2?] The dependence on pH was interpreted by the protonation equilibria of the amines. From the function of the logarithm of rate constants vs. reciprocal temperature, relatively small activation enthalpies (15–70 kJ mol^?1) and extremely high negative activation entropies [(?80) ? (?240) J K^?1 mol^?1] were found. The mechanism was interpreted by the analogy with nitrous acid diazotation.A parallel trend was observed between the rate constants at 25 °C and the basicity constants of the amines.     

A calorimetric study of the binding of 2'-deoxyuridine-5'-phosphate and 5-fluoro-2'-deoxyuridine-5'-phosphate to thymidylate synthetase.

The thermodynamic parameters, ΔH′, ΔG′, and ΔS′, and the stoichiometry for the binding of the substrate 2′-deoxyuridine-5′-phosphate (dUMP) and the inhibitor 5-fluoro-2′-deoxyuridine-5′-phosphate (FdUMP) to Lactobacillus casei thymidylate synthetase (TSase) have been investigated using both direct calorimetric methods and gel filtration methods. The data obtained show that two ligand binding sites are available but that the binding of the second mole of dUMP is extremely weak. Binding of the first mole of dUMP can best be illustrated by dUMP + TSase + H⁺?(dUMP-TSase-H⁺). [1] The enthalpy, ΔH₁′, for reaction [1] was measured directly on a flow modification of a Beckman Model 190B microcalorimeter. Experiments in two different buffers (I = 0.10 m) show that ΔH₁′ = ?28 kJ mol^?1 and that 0.87 mol of protons enters into the reaction. Analysis of thermal titrations for reaction [1] indicates a free energy change of ΔG₁′ = ?30 kJ mol^?1 (K₁ = 1.7 × 10⁵ m^?1). From these parameters, ΔS₁′ was calculated to be +5 J mol^?1 degree^?1, showing that the reaction is almost totally driven by enthalpy changes. Gel filtration experiments show that at very high substrate concentrations, binding to a second site can be observed. Gel filtration experiments performed at low ionic strength (I = 0.05 m) reveal a stronger binding, with ΔG₁′ = ?35 kJ mol^?1 (K₁ = 1.2 × 10⁶ m^?1), suggesting that the forces driving the interaction are, in part, electrostatic. Addition of 2-mercaptoethanol (0.10 m) had the effect of slightly increasing the dUMP binding constant. Binding of FdUMP to TSase is best illustrated by 2FdUMP + TSase + n_HH⁺?FdUMP₂ ? TSase ? (H⁺)_nH. [2] The enthalpy for this reaction, ΔH₂, was also measured calorimetrically and found to be ?30 kJ mol^?1 with n_H = 1.24 at pH 7.4 Assuming two FdUMP binding sites per dimer as established by Galivan et al. [Biochemistry15, 356–362 (1976)] our calorimetric results indicate different binding energies for each site. Based on the binding data, a thermodynamic model is presented which serves to rationalize much of the confusing physical and chemical data characterizing thymidylate synthetase.     

Association Constants for Metal Hexacyanide Binding to Cytochrome c

The binding of[Co(CN)₆]^3?, and that of[Fe(CN)₆]^3? and [Ru(CN)₆]^4? using a competitive method, to horse cytochrome c has been studied by ⁵⁹ Co NMR spectroscopy. At I = 0.07 M, without added salt and in ²H₂O at ph* 7.3 (measured in ²H₂O) and 25°C, there are at least two binding sites on ferricytochrome c and ferrocytochrome c for [Co(CN)₆]^3?. Association constants were determined to be 2.0 ± 0.6 × 10³M^?1 and 1.5 ± 0.5 × 10²M^?1 respectively. with no effect of the oxidation state of the cytochrome. At higher ionic strength (I = 0.12 M adjusted with KCl the binding markedly decreased, and, although it was not possible to determine the precise binding stoichiometry and magnitude of association constants, it is clear that the association constants are ≤ 1.5 × 10^tM^?1 The binding of [Ru(CN)₆]^4? at I = 0.07, without added salt and in ²H₂O at pH 1.3 and 23°C, was not precisely defined, but its binding strength relative to that of [Fe(CN)₆]^3? was determined. Extrapolating this to I = 0.12 (KCl) suggests that under these conditions the association constant for [Ru(CN)₆]^4? binding to ferricytochrome c is ≤ 3 × 10²M^?1.     

Kinetic studies on the reaction of ethylenediaminetetraacetatochromium(III) complex with hydrogen peroxide

The rate of reaction of [Cr(III)Y]_aq (Y is EDTA anion) with hydrogen peroxide was studied in aqueous nitrate media [μ = 0.10 M (KNO₃)] at various temperatures. The general rate equation, Rate = $\text{k}{}_1\text{+}\text{k}{}_2\text{K}{}_1\text{[H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{?1}}\text{1 +}\text{K}{}_1\text{[H}{}^{\mathrm{+}}\text{]}{}^{\mathrm{?1}}$ [Cr(III)Y]_aq[H₂O₂] holds over the pH range 5–9. The decomposition reaction of H₂O₂ is believed to proceed via two pathways where both the aquo and hydroxo-quinquedentate EDTA complexes are acting as the catalyst centres. Substitution-controlled mechanisms are suggested and the values of the second-order rate constants k₁ and k₂ were found to be 1.75 × 10^?2 M^?1 s^?1 and 0.174 M^?1 s^?1 at 303 K respectively, where k₂ is the rate constant for the aquo species and k₂ is that for the hydroxo complex. The respective activation enthalpies (ΔH^*₁ = 58.9 and ΔH^*₂ = 66.5 KJ mol^?1) and activation entropies (ΔS^*₁ = ?85 and ΔS^*₂ = ?40 J mol^?1 deg^?1) were calculated from a least-squares fit to the Eyring plot. The ionisation constant pK₁, was inferred from the kinetic data at 303 K to be 7.22. Beyond pH 9, the reaction is markedly retarded and ceases completely at pH ? 11. This inhibition was attributed in part to the continuous loss of the catalyst as a result of the simultaneous oxidation of Cr(III) to Cr(VI).     

Sulphate transport in the cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. leopoliensis) PCC 7942

Synechococcus R-2 (PCC 1942) actively accumulates sulphate in the light and dark. Intracellular sulphate was 1.35 ± 0.23 mol m^?3 (light) and 0.894 ± 0.152 mol m^?3 (dark) under control conditions (BG-11 media: pH_o, 7.5; [SO₄^2?]_o, 0.304 mol m^?3). The sulphate transporter is different from that found in higher plants: it appears to be an ATP-driven pump transporting one SO₄^2?/ATP [ΔμSO₄^2?_i,o=+ 27.7 ± 0.24 kJ mol^?1 (light) and + 24 ± 0.34 kj mol^?1 (dark)]. The rate of metabolism of SO₄^2?at pH_o, 7.5 was 150 ± 28 pmol m^?2 s^?1 (n = 185) in the light but only 12.8 ± 3.6 pmol m^?2 s^?1 (n = 61) in the dark. Light-driven sulphate uptake is partially inhibited by DCMU and chloramphenicol. Sulphate uptake is not linked to potassium, proton, sodium or chloride transport. The alga has a constitutive over-capacity for sulphate uptake [light (n= 105): K_m= 0.3 ± 0.1 mmol m^?3, V_max, = 1.8 ± 0.6 nmol m^?2 s^?1; dark (n= 56): K_m= 1.4 ± 0.4 mmol m^?3, V_max= 41 ± 22 pmol m^?2 s^?1]. Sulphite (SO₃^2?) was a competitive inhibitor of sulphate uptake. Selenate (SeO₄^2?) was an uncompetitive inhibitor.