Observed Equilibrium Constants Research Articles

Configurationally Nonselective Aquation of a Mononuclear Ru(II) Chloro Complex to Aquo Complex Isomers with Distinctive Aspects in Photoisomerization, Redox, and Catalytic Water Oxidation.

distal-[Ru(EtOtpy)(pynp)Cl]+ (d-EtO1Cl) (EtOtpy = 4'-ethoxy-2,2':6',2″-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine), and distal/proximal-[Ru(EtOtpy)(pynp)OH2]2+ (d/p-EtO1H2O) complexes were newly synthesized to investigate the synergistic influence of the geometric configuration coupled with substituent introduction of an ethoxy (EtO) group on the physicochemical properties and reactions of the Ru(II) complexes. Configurationally nonselective aquation of d-EtO1Cl was uniquely observed to form d/p-EtO1H2O isomers in water, in contrast to configurationally selective aquation of distal-[Ru(tpy)(pynp)Cl]+ (d-1Cl, tpy = 2,2':6',2″-terpyridine) without the EtO group [Yamazaki, H. . J. Am. Chem. Soc. 2011, 133, 8846-8849].The kinetic profiles of the aquation reactions of d-EtO1Cl were well analyzed using a sequential reversible reaction model assuming the reversible interconversion between d/p-EtO1H2O isomers via d-EtO1Cl. The observed equilibrium constant (Kiso) of isomerization between p/d-EtO1H2O was calculated from the kinetic analysis as Kiso = 0.45, which is consistent with the final concentration ratio (1:0.43) of p/d-EtO1H2O generated in the aquation reaction of d-EtO1Cl. The irreversible photoisomerization from d-EtO1H2O to p-EtO1H2O was observed in water with an internal quantum yield (Φ) of 0.44% at 520 nm. Electrochemical measurements showed that d-EtO1H2O undergoes a 2-step oxidation reaction of 1H+-coupled 1e- processes of RuII-OH2/RuIII-OH and RuIII-OH/RuIV═O at pH 1.3-9.7, whereas p-EtO1H2O undergoes a 1-step oxidation reaction of a 2H+-coupled 2e- process of RuII-OH2/RuIV═O in the pH range of 1.8-11.5. Any redox potential of d/p-EtO1H2O isomers was decreased by the electro-donating EtO substitution, compared with distal/proximal-[Ru(tpy)(pynp)OH2]2+ (d/p-1H2O). The turnover frequency (kO2 = 1.7 × 10-2 s-1) of d-EtO1H2O for water oxidation catalysis is higher than that (3.5 × 10-4 s-1) of p-EtO1H2O by a factor of 48.6. The kO2 value (1.7 × 10-2 s-1) for d-EtO1H2O is 4.5-fold higher than those of d-1H2O (3.8 × 10-3 s-1). The higher kO2 value of d-EtO1H2O compared with d-1H2O could be explained by the fast oxidation rate from RuIV═O to RuV═O involved in the rate-determining step due to the electron-donating EtO group.

Synthesis, Characterization, and Glutathionylation of Cobalamin Model Complexes [Co(N4PyCO2Me)Cl]Cl2 and [Co(Bn-CDPy3)Cl]Cl2

Synthetic Co(III) complexes containing N5 donor sets undergo glutathionylation to generate biomimetic species of glutathionylcobalamin (GSCbl), an important form of cobalamin (Cbl) found in nature. For this study, a new Co(III) complex was synthesized derived from the polypyridyl pentadentate N5 ligand N4PyCO(2)Me (1). The compound [Co(N4PyCO(2)Me)Cl]Cl(2) (3) was characterized by X-ray crystallography, UV-vis, IR, (1)H NMR, and (13)C NMR spectroscopies and mass spectrometry (HRMS). Reaction of 3 with glutathione (GSH) in H(2)O generates the biomimetic species [Co(N4PyCO(2)Me)(SG)](2+) (5), which was generated in situ and characterized by UV-vis and (1)H NMR spectroscopies and HRMS. (1)H NMR and UV-vis spectroscopic data are consistent with ligation of the cysteine thiolate of GSH to the Co(III) center of 5, as occurs in GSCbl. Kinetic analysis indicated that the substitution of chloride by GS(-) occurs by a second-order process [k(1) = (10.1 ± 0.7) × 10(-2) M(-1) s(-1)]. The observed equilibrium constant for formation of 5 (K(obs) = 870 ± 50 M(-1)) is about 3 orders of magnitude smaller than for GSCbl. Reaction of the Co(III) complex [Co(Bn-CDPy3)Cl]Cl(2) (4) with GSH generates glutathionylated species [Co(Bn-CDPy3)(GS)](2+) (6), analogous to 5. Glutathionylation of 4 occurs at a similar rate [k(2) = (8.4 ± 0.5) × 10(-2) M(-1) s(-1)], and the observed equilibrium constant (K(obs) = 740 ± 47 M(-1)) is slightly smaller than for 5. Glutathionylation showed a significant pH dependence, where rates increased with pH. Taken together, these results suggest that glutathionylation is a general reaction for Co(III) complexes related to Cbl.

Dehydration from conserved stem regions is fundamental for ligand-dependent conformational transition of the adenine-specific riboswitch

Conformational dynamics and observed equilibrium constants for ligand binding of the adenine-specific riboswitch (add-A riboswitch) in the absence of Mg(2+) and presence of various concentrations of poly-ethylene glycol having an average molecular weight of 200 indicated that 54.2 water molecules were released from P2 and P3 stem regions of the add-A riboswitch during conformational transition upon the binding of 2-aminopurine, an analog of the natural ligand adenine.

Modified hydrolysis kinetics of the active lactone moiety of 10-hydroxycamptothecin by liposomal encapsulation

The key structural requirement for the antitumor activity of 10-hydroxycamptothecin (HCPT) is the intact lactone moiety which is always instability and suffered from pH-dependent hydrolysis. The aim of this study was to evaluate the protection effects of liposomal encapsulation on the labile lactone ring. Mono-modal dispersed quasi-spherical liposomes with mean diameter of 145 nm and high drug entrapment efficiency of 90% were obtained under optimal conditions. The in vitro hydrolysis kinetics behaviors of lactone were studied in varied pH buffers. Compared to that of free HCPT in solution formulation, both the hydrolysis half-life and observed equilibrium constant of liposomal HCPT were increased significantly along with the decreased apparent hydrolysis rate constant. The plasma pharmacokinetics was studied by assessing the lactone stability versus time profiles in vivo following intravenous administration of free and liposomal HCPT. The liposomal encapsulation led to a twofold increase in the AUC values and significant decrease in the plasma clearance of lactone (P < 0.05). There was a good correlation between in vitro and in vivo stability of HCPT-lactone. These results suggested a potential application of the novel liposome formulation for the stable delivery system of HCPT.

Salt Bridges in the Hyperthermophilic Protein Ssh10b Are Resilient to Temperature Increases

A double mutant cycle (DMC) approach was employed to estimate the effect of temperature on the contribution of two highly conserved salt bridges to protein stability in the hyperthermophilic protein Ssh10b. The coupling free energy were 2.4 +/- 0.4 kJ/mol at 298 K and 2.2 +/- 0.4 kJ/mol at 353 K for Glu-54/Arg-57, and 6.0 +/- 0.2 kJ/mol at 298 K and 5.9 +/- 0.6 kJ/mol at 353 K for Glu-36/Lys-68. The stability free energy of Ssh10b decrease greatly with increasing temperature, while the direct contribution of these two salt bridges to protein stability remain almost constant, providing evidence supporting the theoretical prediction that salt bridges are extremely resilient to temperature increases and thus are specially suited to improving protein stability at high temperatures. The reason for the difference in coupling free energy between salt bridges Glu-54/Arg-57 and Glu-36/Lys-68 is discussed. Comparing our results with published DMC data for the contribution of salt bridges to stability in other proteins, we found that the energy contribution of a salt bridge formed by two charged residues far apart in the primary sequence is higher than that of those formed between two very close ones. Implications of this finding are useful for engineering proteins with enhanced thermostability.

Degradation of paclitaxel and related compounds in aqueous solutions I: Epimerization

Paclitaxel and other taxanes have complex structures including the presence of numerous hydrolytically sensitive ester groups and a chiral center that readily undergoes epimerization thus making their kinetics complex. The present study attempts to understand the mechanism of epimerization at the 7-position of paclitaxel, 7-epi-taxol, 10-deacetyltaxol, 7-epi-10-deacetyltaxol, baccatin III and 10-deacetylbaccatin III. Kinetics were studied as function of temperature, pH and buffer concentration and analyzed using a stability indicating assay and LC/MS to identify degradation products. Epimerization was base catalyzed with no evidence of acid catalysis noted. The observed equilibrium constant for epimerization, K, indicated a thermodynamically more favorable S-epimer and a small free energy change between the two epimers. For all of the compounds in this study, removal of the C10 acetyl group increases the epimerization rate in basic aqueous solutions. The observed base-catalyzed epimerization in near neutral to higher pH range suggests a possible rapid deprotonation/protonation of the C7 -OH, followed by a structural rearrangement through a retroaldol/aldol mechanism to form the epimer. Moreover, the rate-limiting step of structure rearrangement most likely occurs with the formation of an enolate intermediate.

Mechanistic Plasticity of DNA Topoisomerase IB: Phosphate Electrostatics Dictate the Need for a Catalytic Arginine

Four conserved amino acids of type IB topoisomerases (Arg130, Lys167, Arg223, and His265 in vaccinia topoisomerase) catalyze the attack by tyrosine on the scissile phosphodiester to form a DNA-(3'-phosphotyrosyl)-enzyme intermediate. The mechanism entails general acid catalysis (by Lys167 and Arg130) and transition-state stabilization (via contact of His265 with the pro-Sp oxygen). Here we query the function of Arg223, which accelerates transesterification by a factor of 10(5). The requirement for Arg223 is alleviated by a neutral Sp methylphosphonate (MeP) linkage at the cleavage site. Arg223 is not required for the 30,000-fold activation of the latent endonuclease activity of topoisomerase by the Sp MeP. The rate of autohydrolysis by the DNA-(3'-MeP)-topoisomerase intermediate approaches 10% of the rate of religation to a 5'-OH DNA strand. These findings underscore the importance of transition-state electrostatics in determining the composition of the active site and dictating the balance between strand transferase and hydrolase functions.

Studies on the formation of glutathionylcobalamin: any free intracellular aquacobalamin is likely to be rapidly and irreversibly converted to glutathionylcobalamin.

A decade ago Jacobsen and co-workers reported the first evidence for the presence of glutathionylcobalamin (GSCbl) in mammalian cells and suggested that it could in fact be a precursor to the formation of the two coenzyme forms of vitamin B(12), adenosylcobalamin and methylcobalamin (Pezacka et al. Biochem. Biophys. Res. Commun. 1990, 169, 443). It has also recently been proposed by McCaddon and co-workers that GSCbl may be useful for the treatment of Alzheimer's disease (McCaddon et al. Neurology 2002, 58, 1395). Aquacobalamin is one of the major forms of vitamin B(12) isolated from mammalian cells, and high concentrations of glutathione (1-10 mM) are also found in cells. We have now determined observed equilibrium constants, K(obs)(GSCbl), for the formation of GSCbl from aquacobalamin and glutathione in the pH range 4.50-6.00. K(obs)(GSCbl) increases with increasing pH, and this increase is attributed to increasing amounts of the thiolate forms (RS(-)) of glutathione. An estimate for the equilibrium constant for the formation of GSCbl from aquacobalamin and the thiolate forms of glutathione of approximately 5 x 10(9) M(-1) is obtained from the data. Hence, under biological conditions the formation of GSCbl from aquacobalamin and glutathione is essentially irreversible. The rate of the reaction between aquacobalamin/hydroxycobalamin and glutathione for 4.50 < pH < 11.0 has also been studied and the observed rate constant for the reaction was found to decrease with increasing pH. The data were fitted to a mechanism in which each of the 3 macroscopic forms of glutathione present in this pH region react with aquacobalamin, giving k(1) = 18.5 M(-1) s(-1), k(2) = 28 +/- 10 M(-1) s(-1), and k(3) = 163 +/- 8 M(-1) s(-1). The temperature dependence of the observed rate constant at pH 7.40 ( approximately k(1)) was also studied, and activation parameters were obtained typical of a dissociative process (DeltaH++ = 81.0 +/- 0.5 kJ mol(-1) and DeltaS++ = 48 +/- 2 J K(-1) mol(-1)). Formation of GSCbl from aquacobalamin is rapid; for example, at approximately 5 mM concentrations of glutathione and at 37 degrees C, the half-life for formation of GSCbl from aquacobalamin and glutathione is 2.8 s. On the basis of our equilibrium and rate-constant data we conclude that, upon entering cells, any free (protein-unbound) aquacobalamin could be rapidly and irreversibly converted to GSCbl. GSCbl may indeed play an important role in vitamin B(12)-dependent processes.

Measurement of NO2, HNO3, NH3 and SO2 and related particulate matter at a rural site in Rampur, India

Abstract This paper presents the measurements of gaseous SO 2 , NO 2 , HNO 3 and NH 3 and particulate NH 4 + , NO 3 − and SO 4 2− at Rampur, a rural site of semi-arid region of India and annual mean concentrations are 3.7±2.2, 7.3±3.7, 0.7±0.6 and 6.7±4.2 and 1.0±0.4, 1.1±1.3 and 2.6±1.6 μg m −3 , respectively. Seasonal variation with higher concentration in winter is observed for gaseous SO 2 , NO 2 , and NH 3 and particulate NH 4 + . The concentration of HNO 3 and particulate NO 3 − and SO 4 2− are higher in summer. Summer to winter ratio of HNO 3 and particulate NO 3 − are 3.8 and 1.8, respectively, showing seasonal variation is more pronounced in case of HNO 3 than particulate NO 3 − . The ratio of gaseous NH 3 to particulate NH 4 + (NH 3 /NH 4 + ) is more than 1 probably due to basic nature of aerosol. Examination of equilibrium between gaseous NH 3 and HNO 3 and particulate NH 4 NO 3 shows observed equilibrium constant is lower than theoretical equilibrium constant in summer and vice versa in winter.

Hybridization of single-stranded DNA targets to immobilized complementary DNA probes: comparison of hairpin versus linear capture probes.

A microtiter-based assay system is described in which DNA hairpin probes with dangling ends and single-stranded, linear DNA probes were immobilized and compared based on their ability to capture single-strand target DNA. Hairpin probes consisted of a 16 bp duplex stem, linked by a T(2)-biotin.dT-T(2) loop. The third base was a biotinylated uracil (U(B)) necessary for coupling to avidin coated microtiter wells. The capture region of the hairpin was a 3' dangling end composed of either 16 or 32 bases. Fundamental parameters of the system, such as probe density and avidin adsorption capacity of the plates were characterized. The target DNA consisted of 65 bases whose 3' end was complementary to the dangling end of the hairpin or to the linear probe sequence. The assay system was employed to measure the time dependence and thermodynamic stability of target hybridization with hairpin and linear probes. Target molecules were labeled with either a 5'-FITC, or radiolabeled with [gamma-(33)P]ATP and captured by either linear or hairpin probes affixed to the solid support. Over the range of target concentrations from 10 to 640 pmol hybridization rates increased with increasing target concentration, but varied for the different probes examined. Hairpin probes displayed higher rates of hybridization and larger equilibrium amounts of captured targets than linear probes. At 25 and 45 degrees C, rates of hybridization were better than twice as great for the hairpin compared with the linear capture probes. Hairpin-target complexes were also more thermodynamically stable. Binding free energies were evaluated from the observed equilibrium constants for complex formation. Results showed the order of stability of the probes to be: hairpins with 32 base dangling ends > hairpin probes with l6 base dangling ends > 16 base linear probes > 32 base linear probes. The physical characteristics of hairpins could offer substantial advantages as nucleic acid capture moieties in solid support based hybridization systems.

Complexation of fullerenes by calix[n]arenes: molecular dynamic simulations and benesi-hildebrandequilibrium constants

MD simulations using a combination of AMI and AMBER are performed to elucidate the origin of complexation between fullerenes and calix[n]arenes (n = 4,6,8). Only calix[6]arenes and calix[8]arenes are suitable for complex formation. By introducing benzyl substitutents on the upper rim of the calixarenes the interaction energy between host and guest is even improved. Analysis of the data also reveals a strong stabilization by t-butyl or benzyl substitutents beside π-π interactions between the arene units of the calixarene and fullerenes. The calculated data perfectly correlate with the experimentally observed equilibrium constants.

Supramolecular organization of the photosynthetic apparatus of Rhodobacter sphaeroides.

Native tubular membranes were purified from the purple non-sulfur bacterium Rhodobacter sphaeroides. These tubular structures contain all the membrane components of the photosynthetic apparatus, in the relative ratio of one cytochrome bc1 complex to two reaction centers, and approximately 24 bacteriochlorophyll molecules per reaction center. Electron micrographs of negative-stained membranes diffract up to 25 A and allow the calculation of a projection map at 20 A. The unit cell (a = 198 A, b = 120 A and gamma = 103 degrees) contains an elongated S-shaped supercomplex presenting a pseudo-2-fold symmetry. Comparison with density maps of isolated reaction center and light-harvesting complexes allowed interpretation of the projection map. Each supercomplex is composed of light-harvesting 1 complexes that take the form of two C-shaped structures of approximately 112 A in external diameter, facing each other on the open side and enclosing the two reaction centers. The remaining positive density is tentatively attributed to one cytochrome bc1 complex. These features shed new light on the association of the reaction center and the light-harvesting complexes. In particular, the organization of the light-harvesting complexes in C-shaped structures ensures an efficient exchange of ubihydroquinone/ubiquinone between the reaction center and the cytochrome bc1 complex.

Synthetic zinc tetrapyrroles complexing with pyridine as a single axial ligand

Zinc chlorins were prepared from chlorophyll- a. Visible spectra in benzene showed that synthetic zinc chlorins complexed with pyridine as an axial ligand to form the monopyridine adducts. The equilibrium constants for the complexation were dependent upon the chlorin structure: substitution of electron-withdrawing groups at the peripheral position enhanced the coordinated ability of the central zinc. 1H NMR spectra in benzene- d 6 also indicated that single pyridine coordinated to the central zinc. Comparison of the equilibrium constant in a zinc chlorin with those of the corresponding zinc bacteriochlorin (7,8-dihydrochlorin) and porphyrin (17,18-dedihydrochlorin) led to an increase in the saturation and flexibility of the tetrapyrrole π-plane ligands making the central zinc more axial-ligated. All the zinc tetrapyrroles in benzene complexed with pyridine to form 5-coordinated (1:1) complexes, not 6-coordinated bis-adducts. The observed equilibrium constants were consistent with the energy changes of the complexation calculated from molecular modeling.

High-Level ab Initio Molecular Orbital Calculations of Imine Formation

Ab initio molecular orbital calculations at the G2(MP2,SVP) level are reported for the reaction between methylamine and formaldehyde to form the Schiff-base, N-methylmethanimine, with loss of water. The gas-phase barrier to carbinolamine formation through intramolecular proton transfer is found to be dramatically reduced when explicit water molecules are employed to facilitate the proton transfer. The lowest barrier lies 14.7 kJ mol-1 above the complex between methylamine, formaldehyde, and two water molecules and passes through a zwitterionic intermediate. The structural features of the transition state for this process closely resemble those predicted experimentally. Although intramolecular proton transfer through water molecules to form the imine from the carbinolamine requires significantly less energy than without the water, the barrier remains quite large. With two waters the barrier is calculated to be 111.9 kJ mol-1. A considerably smaller barrier is predicted for the protonated carbinolamine, 67.9 kJ mol-1, and while there is no experimental evidence for internal proton transfer, the calculated structure shows characteristics of the experimentally predicted transition state. Calculated aqueous-phase dissociation energies are too small in comparison with the experimentally observed equilibrium constants.

Iron(III) Thiocyanate Revisited: A Physical Chemistry Equilibrium Lab Incorporating Ionic Strength Effects

A physical chemistry laboratory exercise is presented in which the thermodynamic equilibrium constant for Fe3+ + SCN- Fe(SCN)2+ in 0.5 M acid is calculated from the experimentally observed equilibrium constant and activity coefficients generated by the Davies extension of the Debye-Huckel theory.

Reaction of glutathione with the electrophilic metabolites of 1,1-dichloroethylene

1,1-Dichloroethylene (DCE) requires cytochrome P450-catalyzed bioactivation to electrophilic metabolites (1,1-dichloroethylene oxide, 2-chloroacetyl chloride and 2,2-dichloroacetaldehyde) to exert its cytotoxic effects. In this investigation, we examined the reactions of these metabolites with glutathione by spectroscopic and Chromatographic techniques. In view of the extreme reactivity of 2-chloroacetyl chloride, primary reactions are likely to include alkylation of cytochrome P450, conjugation with GSH to give S-(2-chloroacetyl)-glutathione, or hydrolysis to give 2-chloroacetic acid. Our results showed conjugation of GSH with 1,1-dichloroethylene oxide, through formation of the mono- and di-glutathione adducts, 2- S-glutathionyl acetate and 2-( S-glutathionyl) acetyl glutathione, respectively. The observed equilibrium constant between the hydrate of 2,2-dichloroacetaldehyde and S-(2,2-dichloro-1-hydroxy)ethylglutathione was estimated from 1H-NMR experiments to be 14 ± 2 M −1. Thus, 2,2-dichloroacetaldehyde is unlikely to make a significant contribution to GSH depletion as GSH concentrations above normal physiological levels would be necessary to form significant amounts of S-(2,2-dichloro-1-hydroxy)ethylglutathione. We also compared the formation of the glutathione conjugates in rat and mouse liver microsomes using 14C-DCE. The results demonstrated a species difference; the total metabolite production was 6-fold higher in microsomes from mice, compared with samples from rat. Production of DCE metabolites in hepatic microsomes from acetone-pretreated mice was 3-fold higher than those from untreated mice suggesting a role for P450 2E1 in DCE bioactivation. These results indicate that the epoxide is the major metabolite of DCE that is responsible for GSH depletion, suggesting that it may be involved in the hepatotoxicity evoked by DCE. Furthermore, this metabolite is formed to a greater extent in mouse than in rat liver microsomes and this difference may underlie the enhanced susceptibility found in the former species.

Mass-spectrometric study of the production and recombination of BaOH+ ions and free electrons from barium added to flames of H2+ O2+ Ar

Barium has been added in trace amounts to a variety of atmospheric-pressure flames of H2+ O2+ Ar. These are in effect tubular reactors without walls and cover the temperature range 1850–2460 K. The ions present were studied by continuously sampling a flame into a quadrupole mass spectrometer. Barium formed no negative ions and the free electron was always the dominant negatively charged species. By far the major positive ion was BaOH+, although Ba+ and BaOH+·H2O were also detected. Chemi-ionisation via BaO + H ⇄ BaOH++ e–(14) and Ba + OH ⇄ BaOH++ e–(15) produces and removes ions and free electrons. This mechanism is accompanied by the rapid equilibrium BaOH++ H = Ba++ H2O (12) as well as a modest hydration of BaOH+. Chemi-ionisation often initially produces more ions than required for thermodynamic equilibrium. Nevertheless, measurements of ionic concentrations for equilibrium were made and indicated that the ionisation potential of BaOH is 4.62 ± 0.30 eV. It is found also that tiny additions of barium to the hottest flame are totally ionised; this fact enables ion currents, measured with this technique, to be converted into ionic concentrations. The overall recombination coefficient for the sum of the reverse steps of reactions (14) and (15) was found to be (1.4 ± 0.3)× 10–7 ml ions–1 s–1 at 2200 K and varied with temperature as T–0.5 ± 1.0. The rates of ion production were also measured in these flames. Reactions (14) and (15) appear indistinguishable, but detailed balancing holds, i.e. the observed equilibrium constant of e.g. reaction (14) is found to equal the ratio of the observed effective forward and reverse rate constants for (14). Thus it is concluded that reactions (14) and (15) are the major ones producing and removing ions. The addition of potassium or a hydrocarbon, such as CH4, to a flame usually only interacts with the above ionisation scheme for Ba in so far as more free electrons are thereby produced. Thus any effects of proton transfer in e.g. H3O++ BaO → BaOH++ H2O are normally negligible, even though the rate constant is close to 10–9 ml molecule–1 s–1. Likewise, no contribution from BaOH++ K → K++ BaOH is observed. That reaction (12) is rapid and equilibrated enables determinations of [Ba+]/[BaOH+] to be used as measurements of the concentration of free hydrogen atoms in a flame. It also proved possible to assess the perturbations of an ion spectrum due to a sample of the flame adjusting its composition on being cooled when entering the first vacuum chamber of the mass spectrometer; a sample is cooled by ca. 500–600 K as a result of losing heat to the sampling probe.

Equilibrium studies of α-diimine displacement in cationic allylpalladium(II) complexes by monodentate N-donors and the mechanism of allyl amination by triethylamine and pyridine

In the cationic complexes [Pd(η3-allyl)(L–L)]ClO4[L–L = 1,2-bis(imino)ethanes or 2-(iminomethyl)-pyridines] the chelated α-diimine was rapidly and reversibly displaced by secondary amines (N-methylaniline, morpholine or piperidine), triethylamine and 4-substituted pyridines. The observed equilibrium constants Ke, increased with increasing basicity and decreasing steric requirements of the entering N-donor. They strongly depend on the α-diimine and decrease in the order RNCHCHNR RNC(Me)C(Me)NR ≈ NC5H4(CHNR)-2 (R = C6HaOMe-4). The cationic complex [Pd(η3-C3H5){NC5H4(CHNC6H4OMe-4)-2}]+ underwent a slow allyl amination by triethylamine or pyridine (L′) in the presence of fumaronitrile (fn), yielding [Pd(η2-fn){NC5H4(CHNC6H4OMe-4)-2}] and Et3[graphic omitted]CH2CHCH2 or C5H5[graphic omitted]CH2CHCH2. Kinetic studies showed that the pseudo-first-order rate constants for amination (kobs) are given by kobs=k2[L′], suggesting a direct bimolecular attack of L′ on the η3-allyl ligand. Amination hardly proceeds in the presence of the less-activated olefin dimethyl fumarate (dmf). The π-accepting properties of the olefinic ligands play an important role also in the reaction of Et3[graphic omitted]CH2CHCH2 or C5H5[graphic omitted]CH2CHCH2 with [Pd(η2-olefin){NC5H4(CHNC5H4OMe-4)-2}](olefin = fn or dmf), i.e. the reverse of the amination reaction.

Inclusion of quaternary ammonium compounds by calixarenes

In weakly polar solvents, strong association occurs between calixarene anions and tetraalkylammonium cations, with the magnitude of the observed equilibrium constants depending upon the charge on the anion, the solvent, the ring size of the calixarene and the nature of the alkyl group of the cation. Large upfield shifts of the methyl resonances of the [(CH3)4N]+ cation in solutions of [(CH3)4N]2[p-t-butylcalix[6]arene-2H] indicate cation inclusion in a structure which is possibly identical with that found for the solid ‘salt’ by X-ray crystallography. This shows one of the cations to be included within a partial cone structure of a ‘hinged 3-up, 3-down’ conformation of the calixarene. The functionalised tetramethylammonium ions, choline and acetyl choline, are also strongly included by various calixarene anions but attempts to detect significant modification of the reactivity of acetyl choline resulting from inclusion have not been successful.

Placing hydroxide in the thermodynamic trans influence order of the cobalt corrinoids: equilibrium constants for the reaction of some ligands with aquahydroxocobinamide

Diaquacobinamide (DAC) was prepared by reducing aquacyanocobinamide (Factor B) with zinc and oxidising in aqueous HCl. The macroscopic acid dissociation constants p K Co1 and p K Co2 for coordinated H 2O in DAC were found spectrophotometrically to be 5.91 ± 0.04 and 10.30 ±0.24 at 25 °C; from their temperature dependence it was found that Δ H=45±4 and 27±2 kJ mol −1, and Δ S=40±14 and −109±8 J K −1 mol −1, for p K Co1 and p K Co2, respectively. Equilibrium constants for the reaction of the cobinamide with cyanide were determined in the pH range 8–12 at 25 °C, μ=1.0 M (NaClO 4). Two CN − ligands bind to aquahydroxocobinamide and pH dependence of the observed equilibrium constants can be accounted for by assuming that both dihydroxocobinamide and hydroxocyanocobinamide are inert, while equilibrium constants for reaction of the labile species aquahydroxocobinamide and aquacyanocobinamide with HCN are insignificantly small compared to those for reaction with CN − anion. A fit of the data shows that the macroscopic equilibrium constant, log β 2, for reaction of 2CN − with the two diastereomers of aquahydroxocobinamide to produce dicyanocobinamide is 19.0±0.1. Hence the macroscopic log K for binding of CN − to the two diastereomers of aquahydroxocobinamide is 11 since log K for binding of CN − to aquacyanocobinamide is 8. Equilibrium constants for binding of azide, pyridine, N- methylimidazole and 3-amino-l-propanol to aquahydroxocobinamide were determined at 25 °C, μ=1.0 M (NaClO 4), pH 12. In these cases only a single ligand is bound; the macroscopic log K values are N-methylimidazole, 6.06±0.04; 3-amino-l-propanol, 4.66±0.04; pyridine, 4.19±0.01; and azide, 3.45±0.04. A comparison of the present results with others available shows that hydroxide is above water and 5, 6-dimethylbenzimidazole, but below cyanide, sulfite and alkyl ligands in the trans influence order of the cobalt corrinoids.