Acidity constants of benzidine in aqueous solutions

The acidity constants of benzidine (Bz) in aqueous solutions determined potentiometrically at 25° were K_a1 = (1.11 ± 0.08) × 10⁻⁵, K_a2 = (1.45 ± 0.12) × 10⁻⁴. The apparent mixed constants in 0.1M sodium nitrate are K_a1 = (5.37 ± 0.28) × 10⁻⁶ and K_a2 = (1.14 ± 0.09) × 10⁻⁴. The ultraviolet spectra were recorded as a function of pH and analysed with these constants to obtain the absorption spectra of H₂Bz²⁺, HBz⁺ and Bz; the corresponding wavelengths of maximal absorption are 247, 273 and 278 nm, and molar absorptivities 1.63 × 10⁴, 1.76 × 10⁴ and 2.26 × 10⁴ 1.mole⁻¹.cm⁻¹.     

Characterization of antibodies against benzo[a]pyrene with thermodynamic and kinetic constants

Antibodies of a polyclonal antiserum against benzo[a]pyrene were characterized by determining thermodynamic and kinetic constants of the antigen–antibody reaction. Label-free binding assays with optical detection based on reflectometric interference spectroscopy were performed to determine these constants. Different evaluation methods for kinetic measurements were compared. Also, cross-reactivity against two other polycyclic aromatic hydrocarbons, chrysene and pyrene, was checked. The affinity constant between the antibodies and benzo[a]pyrene in homogeneous phase was determined to be K=(5.3±0.3)×10⁷ M⁻¹ which was in the middle of the usual range of antibody affinities. The association rate constant for the reaction at the surface was determined to be (3.8±0.9)×10⁵ M⁻¹ s⁻¹, the dissociation rate constant as (9.7±0.5)×10⁻³ s⁻¹. Different evaluation methods applied to the kinetic measurements led to the same results. This antiserum would be suitable for the selective determination of benzo[a]pyrene in concentrated samples.     

Flow injection analysis methods for determination of diffusion coefficients

Two flow injection analyses (FIA) methods for the determination of diffusion coefficients in a straight single tube FIA system were developed. Based on the analytical solution of the convection-diffusion equation, linear relationships of the logarithmic values of the dispersion coefficient (D) and the half-peak width (W_1/2) with the diffusion coefficient (D_m) were obtained. Experiments were designed to verify these methods. For example, for potassium hexacyanoferrate (III) a D_m value of 0.72 × 10⁵ cm² s⁻¹ was found versus a literature value of 0.76 × 10⁵ cm² s⁻¹ (error, 5%). For potassium hexacyanoferrate (II) a D_m value of 0.67 × 10⁵ cm² s⁻¹ was obtained versus a literature value of 0.63 × 10⁵ cm² s⁻¹ (error, 6%). The diffusion coefficients of some important biomedical compounds, such as dopamine, epinephrine, norepinephrine and ascorbic acid, were then determined. The values of 10⁵ D_m/cm² s⁻¹ are 0.60 ± 0.03, 0.44 ± 0.02, 0.60 ± 0.01 and 0.68 ± 0.06, respectively.     

TEMPO-initiated oxidation of 2-aminophenol to 2-aminophenoxazin-3-one

The oxidation reaction of 2-aminophenol (OAP) to 2-aminophenoxazin-3-one (APX) initiated by 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) has been investigated in methanol at ambient temperature. The oxidation of OAP was followed by electronic spectroscopy and the rate constants were determined according to the rate law −d[OAP]/dt=k_obs[OAP][TEMPO]. The rate constant, activation enthalpy and entropy at 298 K are as follows: k_obs (dm³ mol⁻¹ s⁻¹)=(1.49±0.02)×10⁻⁴, E_a=18±5 kJ mol⁻¹, ΔH^‡=15±4 kJ mol⁻¹, ΔS^‡=−82±17 J mol⁻¹ K⁻¹. The results of oxidation of OAP show that the formation of 2-aminophenoxyl radical is the key step in the activation process of the substrate.     

A study on the binding of two water-soluble tetrapyridinoporphyrazinato copper(II) complexes to DNA

Interaction of N,N′,N″,N-tetramethyltetra-2,3-pyridinoporphyrazinatocopper(II), ([Cu(2,3-tmtppa)]⁴⁺) and N,N′,N″,N-tetramethyltetra-3,4-pyridinoporphyrazinatocopper(II), ([Cu(3,4-tmtppa)]⁴⁺) with calf thymus DNA was studied in 1 mM phosphate buffer and low ionic strength (5 mM NaCl) at various temperatures by UV-visible and circular dichroism (CD) spectroscopies and viscometric method. The binding constants were determined from the changes in the visible part of porphyrazine complexes spectra using SQUAD software. The values of K have been obtained (7.9±0.4)×10⁴ and (2.2±0.1)×10⁵ M⁻¹ for [Cu(2,3-tmtppa)]⁴⁺ and [Cu(3,4-tmtppa)]⁴⁺, respectively at 27 °C. The higher affinity of 3,4-isomer of Cu complex towards DNA with respect to the 2,3-isomer was attributed to favorable external positioning of the cationic charges in former, which enables superior interaction with the DNA duplex. The thermodynamic parameters (ΔG°, ΔH°, ΔS°) were calculated by van't Hoff equation. The enthalpy and entropy changes were determined, +34.2±3.6 kJ mol⁻¹ and +207.8±12.70 J mol⁻¹ K⁻¹ for [Cu(2,3-tmtppa)]⁴⁺ and +49.7±2.1 kJ mol⁻¹ and +267.8±7.9 J mol⁻¹ K⁻¹ for [Cu(3,4-tmtppa)]⁴⁺. The existence of extensive hypochromicity, large red shift and negative CD in the visible part of [Cu(3,4-tmtppa)]⁴⁺ spectra suggested an intercalation binding mode. Analysis of the moderate hypochromicity, red shift and bisignate CD in the Q-band absorption region of [Cu(2,3-tmtppa)]⁴⁺ spectra possibly led us to the coexistence of intercalation and outside binding modes. The influence of the ionic strength on the binding parameters and binding modes was investigated. The results show that increase in ionic strength causes the decrease in the binding constants. It was also concluded that increase in ionic strength affects the binding characteristics of positively charged complexes with DNA.

The increase in DNA viscosity in the presence of Cu–tmtppa complexes is attributed to the lengthening of DNA helix due to the intercalation. This result is consistent with conclusions obtained from the spectroscopic techniques.     

Persulphate quenching of the excited state of ruthenium(II) tris-bipyridyl dication: thermal reactions

The rate constant for the reaction between the sulphate radical (SO₄^√−) and the ruthenium (II) tris-bipyridyl dication (Ru(bipy)₃²⁺) is (3.3±0.2)×10⁹ mol⁻¹ dm³ s⁻¹ in 1 mol dm⁻³ H₂SO₄ and (4.9±0.5)×10⁹ mol⁻¹ dm³ s⁻¹ in 0.1 mol dm⁻³, pH 4.7 acetate buffer. The SO₄^√−radical produced by the electron transfer quenching of Ru(bipy)₃^2+* by S₂O₈²⁻ reacts rapidly with both acetate buffer and chloride ions. These side reactions result in a reduction in the overall quantum yield of Ru(bipy)₃³⁺ production and reduced reaction selectivity when Ru(bipy)₃^2+* is quenched by persulphate.     

Determination of absorptivity and formation constant of a chalcone association complex

A UV spectrometric method was developed to determine the molar absorptivity (_C) and formation constant (K_c) of the association complex of unsubstituted chalcone in cyclohexane, in the concentration range from 4.00·10⁻⁴ to 2.00·10⁻² mol dm⁻³. The thermodynamic and spectroscopic magnitudes such as K_c and _C contribute to the understanding of the physicochemical behavior of several ,β-unsaturated carbonylic compounds, of low solubility in water, as it is the case of numerous flavonoids of chemical and biological importance. The studied association complex, formed by two chalcone molecules, is characterized by the constants _C (300.8 nm)=4.98·10⁴ dm³ mol⁻¹ cm⁻¹ and K_c=5.58·10³. The method proposed is convenient for the study of solute–solute molecular associations particularly those due to dipole–dipole interactions.     

On the conformational stability and dimerization of phosphotransferase enzyme I from Escherichia coli

The activity of enzyme I (EI), the first protein in the bacterial PEP:sugar phosphotransferase system, is regulated by a monomer–dimer equilibrium where a Mg²⁺-dependent autophosphorylation by PEP requires the homodimer. Using inactive EI(H189A), in which alanine is substituted for the active-site His189, substrate binding effects can be separated from those of phosphorylation. Whereas 1 mM PEP (with 2 mM Mg²⁺) strongly promotes dimerization of EI(H189A) at pH 7.5 and 20 °C, 5 mM pyruvate (with 2 mM Mg²⁺) has the opposite effect. A correlation between the coupling of N- and C-terminal domain unfolding, measured by differential scanning calorimetry, and the dimerization constant for EI, determined by sedimentation equilibrium, is observed. That is, when the coupling between N- and C-terminal domain unfolding produced by 0.2 or 1.0 mM PEP and 2 mM Mg²⁺ is inhibited by 5 mM pyruvate, the dimerization constant for EI(H189A) decreases from >10⁸ to <5 × 10⁵ or 3 × 10⁷ M⁻¹, respectively. With 2 mM Mg²⁺ at 15–25 °C and pH 7.5, PEP has been found to bind to one site/monomer of EI(H189A) with K_A′10⁶ M⁻¹ (ΔG′=−33.7±0.2 kJ mol⁻¹ and ΔH=+16.3 kJ mol⁻¹ at 20 °C with ΔC_p=−1.4 kJ K⁻¹ mol⁻¹). The binding of PEP to EI(H189A) is synergistic with that of Mg²⁺. Thus, physiological concentrations of PEP and Mg²⁺ increase, whereas pyruvate and Mg²⁺ decrease the amount of dimeric, active, dephospho-enzyme I.     

The reactions of a dinuclear ferric complex (oxo) di-iron(III) triethylenetetraamminehexaacetate, Fe2O(ttha), with oxidizing and reducing free radicals. A pulse radiolysis study

The rate constants at which oxidizing and reducing radicals react with the dinuclear iron(III) complex Fe₂O(ttha)²⁻ were measured in neutral aqueous solution. The rate constants for reduction of the complex by ·CO₂^.− CH₃^.CHOH and O₂^.− were found to be comparable with rate constants previously measured in mononuclear iron(III) polyaminocarboxylate systems. Fe₂O(ttha)²⁻ reacts slowly with O₂^.− (k₈ = (1.2 ± 0.2) × 10⁴ dm³ mol⁻¹ s⁻¹) and, hence, is a relatively poor catalyst for the dismutation of superoxide radical. The hydrated electron reduces the complex at a diffusion-controlled rate in a process which consumes one proton: e_aq⁻ + Fe₂O(ttha)²⁻ → Fe₂^III,IIO(ttha)³⁻ The reduction by carbon-centered radicals produces a (III,II) mixed-valence complex with an absorption spectrum different from that of the Fe₂(II,III) species produced from reduction by the hydrated electron. The oxidizing radicals ^.OH and ·CO₃⁻ appear to act as reductants of the complex via ligand oxidation rather than by oxidation of the Fe₂^IIIO core to Fe₂^III,IVO. In the former case ligand attack appears to occur mainly at the methylene carbon of a glycinate group. The decarboxylation product, CO₂, was detected by its aquation reaction in the presence of a pH sensitive dye, bromthymol blue.     

Equilibria and kinetics of the extraction of gallium with 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone

The equilibria and kinetics of the solvent extraction of gallium(III) from aqueous monochloroacetic acid [HA] media with a benzene solution of 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone [PMBP or HL] has been studied at 25 ± 0.1° and an ionic strength of 0.2. The species extracted was found to be GaL₃. The value of the acid dissociation constant of PMBP determined spectrophotometrically was 1.17 × 10⁻⁴. The values of the partition coefficient of PMBP and the extraction constant of its gallium complex between an aqueous and a benzene phase were found to be 3.72 × 10³ and 2.51 × 10⁴, respectively. The rate of extraction was first-order with respect to the concentrations of gallium(III) in the aqueous phase and PMBP in the organic phase, inversely first- and second-order with respect to the hydrogen-ion concentration and zero- and first-order with respect to the concentration of monochloroacetate ions. Two mechanisms operate for this extraction, depending on the pH of the aqueous phase, one where the formation of the first complex, GaL²⁺, between Ga³⁺ and L⁻ in the region of pH < 1.6 becomes the rate-determining step, and the other where the formation of the first complex between GaA²⁺ and L⁻ in the region of pH 2.0–2.3 is the rate-determining step. The rate constant for the first of these reactions was calculated to be 1.62 × 10⁴l.mole⁻¹.sec⁻¹, but that for the second could not be determined.     

New aspects of the reaction of silver(I) cations with the ethylenediaminetetraacetate ion

The highly neutralized ethylenediaminetetraacetate (EDTA) titrant (95–99% as Y⁴⁻ anion) precipitates with Ag⁺ cations to form the Ag₄Y species, in aqueous medium, which is well characterized from conductometric titration, thermal analysis and potentiometric titration of the silver content of the solid. The precipitate dissolves in excess Y⁴⁻ to form a complex, AgY³⁻. Equilibrium studies at 25°C and ionic strength 0.50 M (NaNO₃) have shown from solubility and potentiometric measurements that the formation constant (95% confidence level) β₁ = (1.93 ± 0.07) × 10⁵ M⁻¹ and the solubility products are K_S0 = [Ag +]⁴[Y⁴⁻] = (9.0 ± 0.4) × 10⁻¹⁸ M⁵ and K_S1 = [Ag +]³[AgY³⁻] = (1.74 ± 0.08) × 10⁻¹² M⁴. The presence of Na⁺, rather than ionic strength, markedly affects the equilibrium; the data at ionic strength 0.10 M are: β₁ = (1.19 ± 0.03) × 10⁶ M⁻¹, K_S0 = (1.6 ± 0.4) × 10⁻¹⁹ M⁵ and K_S1 = (1.9 ± 0.5) × 10⁻¹³ M⁴; at ionic strength tending to zero; β₁ = (1.82 ± 0.05) × 10⁷ M⁻¹, K_S0 = (2.6 ± 0.8) × 10⁻²² M⁵ and K_S1 = (5 ± 1) × 10⁻¹⁵ M⁴. The intrinsic solubility is 2.03 mM silver (I) in 0.50 M NaNO₃. Well-defined potentiometric titration curves can be taken in the range 1–2 mM with the Ag indicator electrode. Thermal analysis revealed from differential scanning calorimetry a sharp exothermic peak at 142°C; thermal gravimetry/differential thermal gravimetry has shown mass loss due to silver formation and a brown residue, a water-soluble polymeric acid (decomposition range 135–157°C), tending to pure silver at 600°C, consistent with the original Ag₄Y salt.     

Experimental and theoretical investigation of the reaction CF3 + NO + M → CF3NO + M from 0.5 to 12 Torr and from 253 to 373 K

The kinetics of the association reaction of CF₃ with NO was studied as a function of temperature near the low-pressure limit, using pulsed laser photolysis and time-resolved mass spectrometry. CF₃ radicals were generated by photolysis of CF₃I at 248 nm and the kinetics was determined by monitoring the time-resolved formation of CF₃NO. The bimolecular rate constants were measured from 0.5 to 12 Torr, using nitrogen as the buffer gas. The results are in very good agreement with recent data published by Vakhtin and Petrov, obtained at room temperature in a higher pressure range and, therefore, the two studies are quite complementary. A RRKM model was developed for fitting all the data, including those of Vakhtin and Petrov and for extrapolating the experimental results to the low- and high-pressure limits. The rate expressions obtained are the following: k₁(0) = (3.2 ± 0.8) × 10⁻²⁹ (T/298)^{−(3.4±0.6)} cm⁶ molecule⁻² s⁻¹ for nitrogen used as the bath gas and k₁(∞) = (2.0 ± 0.4) × 10⁻¹¹ (T/298)^(0±1) cm³ molecule⁻¹ s⁻¹. RRKM calculations also help to understand the differences in reactivity between CF₃ and other radicals, for the same association reaction with NO.     

Cyanide thermodynamics 2. Stability constants of copper(I) cyanide complexes in aqueous acetonitrile mixtures

The stability (formation) constants of the binary Cu(I)-CN⁻ complexes have been measured in five aqueous mixtures containing from 10% to 70% v/v acetonitrile (MeCN) by glass electrode potentiometry at 25 °C and an ionic strength of l M (NaClO₄). The constants show monotonic increases with MeCN concentration, the changes being greatest for the higher order complexes, consistent with the unfavourable solvation of CN ⁻ in these mixtures. The sparing solubility of CuCN(s) prevented determination of the stability constant for CuCN ⁰ (soln.) at low MeCN concentrations.     

Protonation constant of monoaza-12-crown-4 ether and stability constants with selected metal ions in aqueous solution in the presence of an excess of sodium ion: a potentiometric and differential pulse polarographic study at fixed ligand to metal ratio and varied pH

The ligand monoaza-12-crown-4 ether (A12C4) was studied in aqueous solution at 298 K and an ionic strength of 0.5 mol dm⁻³ in the presence of an excess of sodium ion (0.5 mol dm⁻³ NaNO₃). The protonation constant of A12C4, determined by glass electrode potentiometry (GEP) in the same background electrolyte, was found to be log K=9.36±0.03. Polarographic experimental and calculated complex formation curves (ECFC and CCFC) for labile metal–ligand systems, studied at a fixed total ligand (L_T) to total metal (M_T) concentration ratio and varied pH, were used for the modelling of the metal species formed and the refinement of their stability constants. The metal–ligand model and formation constants are optimised by solving mass-balance equations written for the assumed model and by fitting the CCFC to the ECFC. The CCFC can be generated for any metal–ligand model, including polynuclear metal species, for any L_T:M_T ratio, and for more than one ligand competing in the complex formation reaction. Three lead complexes with the ligand A12C4, viz. PbL²⁺, PbL(OH)⁺ and PbL(OH)₂, were found and their overall stability constants from differential pulse polarography (DPP), as log β, were estimated to be 3.75±0.03, 9.30±0.05 and 12.70±0.05, respectively. Two copper complexes CuL²⁺ and CuL(OH)₂ are reported and their stability constants (from DPP) were estimated to be 6.00±0.05 and 21.77±0.1, respectively. Two cadmium complexes CdL²⁺ and CdL(OH)⁺ are reported. The stability constant for CdL²⁺ was estimated from DPP and GEP as 2.80±0.05 and 2.68±0.03 (the latter value was obtained from a few potentiometric experimental points), respectively, and the stability constant for CdL(OH)⁺ from DPP was estimated to be 7.88±0.05. GEP could not be used for the stability constants determination of other metal complexes studied because of precipitation occurring prior the completion of a complex formation reaction.     

Dimesitylborane monomer-dimer equilibrium in solution, and the solid-state structure of the dimer by single crystal neutron and X-ray diffraction

Dimesitylborane dimer has been shown to exist in equilibrium with dimesitylborane monomer in solution. This equilibrium has been investigated by variable concentration and variable temperature multinuclear NMR spectroscopy and values for the dissociation constant, enthalpy and entropy of dissociation were found to be K_diss=(3.2±0.4)×10⁻³ M, ΔH=70 kJ mol⁻¹, and ΔS=212 J K⁻¹mol⁻¹, respectively. Ab initio methods have been used to investigate the gas-phase structures and energies of both monomer and dimer, and calculated ¹¹B-NMR shifts are also presented. The solid-state structure of dimesitylborane dimer has been investigated by single crystal X-ray diffraction at 100 K and the position of the bridging hydrogen atoms (B---H=1.340(2), 1.342(2) Å, H---B---H=92.46(14)°) has been determined accurately, for the first time, by single crystal neutron diffraction at 20 K.     

The first 1,2-dibora-[2]ferrocenophane and its dynamic behaviour in solution

1,2-Bis(dimethylamino)-1,2-dibora-[2]ferrocenophane (1) was prepared by the reaction of 1,1′-dilithioferrocene with 1,2-dichlorobis(dimethylamino)diborane(4). In addition to hindered rotation about the B-N bond (ΔG^‡ > 80 kJ mol⁻¹), another dynamic process was revealed by ¹H and ¹³C NMR in solution at low temperature, and interpreted as motion of the cyclopentadienyl rings between staggered and eclipsed conformations (ΔG_{(233 K)}^‡ = 44 ± 1 kJ mol⁻¹).     

Pulse radiolysis studies of 4,7-phenanthroline(II)

The one-electron reduction of 4,7-phenanthroline (P) in aqueous solutions at neutral pH has been further studied by pulse radiolysis. The spectral and kinetic properties of the transient formed due to the reaction of 4,7-phenanthroline with hydrated electron were investigated. The transient absorption spectrum obtained 5μs after the pulse exhibits a broad band with a λ_max at 420 nm. The λ_max is 10 nm blue shift compared with the absorption spectrum obtained at pH 2.9 where the reactant was the protonated form. The bimolecular'rate constant of the reaction of 4,7-phenanthroline with hydrated electron was 0etermined to be (2.2±0.1)×10¹⁰ dm³ mol⁻¹ s⁻¹. It was found that the decay of the transient was mainly following a first-order kinetics. The first-order decay rate constant was determined to be (1.25±0.1)×10⁴s⁻¹.     

Lifetime measurements of electronically excited C3(Πu) radicals in different vibrational states

The radiative lifetimes of nine vibrational levels of the C₃(¹Π_u) radical were obtained from decay time studies of the C₃(¹Π_u → ¹Σ⁺_g) fluorescence induced by a tunable dye laser. The lifetimes of the different vibronic levels were found to be constant within the experimental error limits, namely, τ_o = (200 ± 10) ns. The collisional deactivation of the C₃(¹Π_u) states by helium gives rate constants between 2.5 and 4 in 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ units.     

Water diffusion in polyethylene modified by ion irradiation

Water diffusion in polyethylene (PE) modified by irradiation with 40keV Ar⁺ ions to fluences of 1 × l0¹² to 1 × l0¹⁵cm⁻² was studied. The diffusion experiments were performed in a temperature interval from 25 to 100 °C and for diffusion times of up to 3 h. The water incorporated was detected by infra-red spectroscopy, microgravimetry and by measuring contact angles. Maximum water uptake observed for the ion fluence of 1 × 10¹³cm⁻² may be connected with an increase in free volume fraction and oxidative processes in the PE surface layer modified by ion irradiation. For higher ion fluences, the available free volume is reduced owing to carbonization and crosslinking of the polymer and, accordingly, the water content decreases. The water incorporated in PE is mostly in the form of isolated molecules. At elevated temperature rediffusion of water is observed.     

Kinetic study of gas-phase Lu(D3/2) with O2, N2O and CO2

The second-order rate constants of gas-phase Lu(²D_3/2) with O₂, N₂O and CO₂ from 348 to 573 K are reported. In all cases, the reactions are relatively fast with small barriers. The disappearance rates are independent of total pressure indicating bimolecular abstraction processes. The bimolecular rate constants (in molecule⁻¹ cm³ s⁻¹) are described in Arrhenius form by k(O₂)=(2.3±0.4)×10⁻¹⁰exp(−3.1±0.7 kJmol⁻¹/RT), k(N₂O)=(2.2±0.4)×10⁻¹⁰exp(−7.1±0.8 kJmol⁻¹/RT), k(CO₂)=(2.0±0.6)×10⁻¹⁰exp(−7.6±1.3 kJmol⁻¹/RT), where the uncertainties are ±2σ.