Formation Of Reversible Complex Research Articles

The variable region-1 from tissue-type plasminogen activator confers specificity for plasminogen activator inhibitor-1 to thrombin by facilitating catalysis: release of a kinetic block by a heterologous protein surface loop

Substitution of the native variable region-1 (VR1/37-loop) of thrombin by the corresponding VR1 of tissue-type plasminogen activator (thrombin-VR1tPA) increases the rate of inhibition by plasminogen activator inhibitor type 1 (PAI-1) by three orders of magnitude, and is thus sufficient to confer PAI-1 specificity to a heterologous serine protease. A structural and kinetical approach to establish the function of the VR1 loop of t-PA in the context of the thrombin-VR1tPA variant is described. The crystal structure of thrombin-VR1tPA was resolved and showed a conserved overall α-thrombin structure, but a partially disordered VR1 loop as also reported for t-PA. The contribution of a prominent charge substitution close to the active site was studied using charge neutralization variants thrombin-E39Q(c39) and thrombin-VR1tPA-R304Q(c39), resulting in only fourfold changes in the PAI-1 inhibition rate. Surface plasmon resonance revealed that the affinity of initial reversible complex formation between PAI-1 and catalytically inactive Ser195→Ala variants of thrombin and thrombin-VR1tPA is only increased fivefold, i.e. KD is 652 and 128 nM for thrombin-S195A and thrombin-S195A-VR1tPA, respectively. We established that the partition ratio of the suicide substrate reaction between the proteases and PAI-1 was largely unaffected in any variant studied. Hirugen allosterically decreases the rate of thrombin inhibition by PAI-1 2.5-fold and of thrombin-VR1tPA 20-fold, by interfering with a unimolecular step in the reaction, not by decreasing initial complex formation or by altering the stoichiometry. Finally, kinetic modeling demonstrated that acylation is the rate-limiting step in thrombin inhibition by PAI-1 (k∼10−3 s−1) and this kinetic block is alleviated by the introduction of the tPA-VR1 into thrombin (k>1 s−1). We propose that the length, flexibility and different charge architecture of the VR1 loop of t-PA invoke an induced fit of the reactive center loop of PAI-1, thereby enhancing the rate of acylation in the Michaelis complex between thrombin-VR1t-PA and PAI-1 by more than two orders of magnitude.

Purification of xyloglucan endotransglycosylases (XETs): a generally applicable and simple method based on reversible formation of an enzyme‒substrate complex

Purification of xyloglucan endotransglycosylases (XETs): a generally applicable and simple method based on reversible formation of an enzyme‒substrate complex

A catalytic mechanism for caspase-1 and for bimodal inhibition of caspase-1 by activated aspartic ketones

We have evaluated 619 aspartic ketones with 9 different types of prime-side groups (acyloxymethyl, aryloxymethyl, arylthiomethyl, alkylthiomethyl, acylamino-oxymethyl, sulfonylaminomethyl, α-ketoamide, α-(1-phenyl-3-trifluoromethyl-pyrazol-5-yl)oxymethyl (PTP), and aliphatic ketones) as inhibitors of caspase-1. The inhibitory behaviors could be classified as reversible, inactivating, or bimodal (i.e. reversible inhibition followed by slow inactivation) based on the kinetically observed formation of reversible thiohemiketal complexes and conversion to an irreversible thioether adduct, and the mechanism of any given ketone was only poorly predictable on the basis of leaving group structure and chemistry. Among 201 bimodal inhibitors, the rate of conversion of the reversible thiohemiketal complex to the inactive thioether ( k i) was strictly first-order, consistent with direct conversion of the thiohemiketal to the thioether with no intermediate collapse to free ketone and thiolate. We have examined 22 crystallographic structures of caspase-1 complexed as a thiohemiketal with the inhibitors from 8 different ketone classes, and found the Cys285S–C–C α′-leaving group dihedral angle to be near either to 60° or to 180°. Only the 180° conformation was permissive for SN 2 displacement of the leaving group and, furthermore, positioned His237Nδ to stabilize developing charge on the leaving group. Among these structures and 19 additional complexes, all showed a strong interaction between His237Nδ and the ketone or thiohemiketal oxygen. We therefore propose a proteolytic mechanism for caspase-1 involving polarization of the scissile carbonyl by the His237 imidazolium group. During thiohemiketal/thioether conversion (but probably not during peptide hydrolysis), the leaving group is stabilized by the His237 imidazolium. ©

Paramagnetic intermediates in the photolysis of 7-silanorbornadiene studied by means of spin chemistry method

The influence of an external magnetic field on the yield of photo-decomposition products of 7,7′-dimethyl-7-silanorbornadiene derivative has been detected in laser pulse photolysis experiments. The observations of magnetic field effects alterations in the presence of scavengers, O 2 and PPh 3, in combination with 1H-CIDNP data form the basis for the identification of the structure of paramagnetic intermediates involved in the process. It has been shown that magnetic field effects originate in biradical intermediates. These species result from both endocyclic SiC bond cleavage in the initial compound and the reaction of dimethylsilylene (in a singlet or a triplet excited state) with starting 7-silanorbornadiene. To explain the influence of O 2 upon the magnetic field effects, the reversible formation of oxygen complex with biradical species has been suggested.

Implication of His68 in the substrate site of Bacillus subtilis adenylosuccinate lyase by mutagenesis and affinity labeling with 2-[(4-bromo-2,3-dioxobutyl)thio]adenosine 5'-monophosphate.

Adenylosuccinate lyase of Bacillus subtilis is inactivated by 2-[(4-bromo-2,3-dioxobutyl)thio]adenosine 5'-monophosphate (2-BDB-TAMP) at pH 7.0. As the reagent concentration is increased, a maximum rate constant is approached, indicative of reversible enzyme-reagent complex formation (KR = 68 +/- 9 microM) prior to irreversible modification (kmax = 0.081 +/- 0.004 min-1). Complete inactivation occurs concomitant with about 1 mol of 2-BDB-[14C]TAMP incorporated/mol of enzyme subunit. Adenylosuccinate, or a combination of AMP and fumarate, decreases the inactivation rate and reduces incorporation of [14C] reagent, whereas either AMP or fumarate alone is much less effective. These observations suggest that 2-BDB-TAMP attacks the adenylosuccinate binding site. Proteolytic digestion of inactivated enzyme, followed by purification of the digest by HPLC, yields the radioactive peptide Ile62-Ala72, in which Arg67 and His68 are the most likely targets. Thus 2-BDB-TAMP reacts with adenylosuccinate lyase at a site distinct from the His141 attacked by 6-BDB-TAMP (Lee, Worby, Dixon, and Colman (1997) J. Biol. Chem. 272, 458-465). Site-directed mutagenesis was used to construct mutant enzymes with replacements for both Arg67 and His68, and either Arg67 or His68. The R67M mutant enzyme has almost the same specific activity as the wild-type enzyme under standard assay conditions, whereas the single mutant H68Q and double mutant R67M-H68Q enzymes exhibit specific activities that are decreased more than 100-fold. These results indicate that while Arg67 and His68 may both be in the region of the substrate site, only His68 is important for the catalytic activity of B. subtilis adenylosuccinate lyase. A role is proposed for His68 as a general acid-base catalyst.

Photodynamic Fluorescent Metal Ion Sensors with Parts per Billion Sensitivity

We report herein the synthesis and spectroscopic study of the first spiropyrandinolines that function as sensors for metal ions in the parts per billion range. These systems operate by either photochemically or chemically induced reversible formation of merocyanine metal ion complexes. The application of this novel photodynamic sensing material to sensor technology is discussed.

Equilibria, Kinetics and Mechanism for Complex Formation Between Hydrogen Sulfate/Sulfate and Palladium(II). Hydrolysis of Tetraaquapalladium(II).

Spectrophotometric equilibrium measurements indicate formation of the complexes [Pd(H2O)(3)HSO4](+) and [Pd(H2O)(3)SO4] in the reaction between [Pd(H2O)(4)](2+) and hydrogen sulfate/sulfate in the region: 0.10 less than or equal to [H+] less than or equal to 0.80 M. The stability constants are 0.7 +/- 0.2 and 19 +/- 6 M-1, respectively, at 25 degrees C and 1.00 M ionic strength. The protolysis constant for coordinated hydrogen sulfate, i.e. the equilibrium constant for the reaction [Pd(H2O)(3)HSO4](+) +H2O reversible arrow[Pd(H2O)(3)SO4] + H3O+, is 2.5 +/- 1.0 M. The stability constant for [Pd(H2O)(3)HSO4](+) and the protolysis constant for coordinated HSO4- are also derived from kinetic measurements as 0.6 +/- 0.2 M-1 and 2.3 +/- 1.3 M, respectively. The kinetics for the reversible complex formation reaction, studied by use of stopped-flow spectrophotometry, is first order with respect to palladium complex and total concentration of sulfate, [S(VI)], with an observed pseudo-first-order rate constant k(obsd) = k(f)[S(VI)] + k(r) for excess sulfate. Here k(f) and k(r) denote observed forward second-order and reverse first-order rate constants, respectively. The kinetic data are interpreted in terms of a reaction mechanism which involves parallel and reversible reactions between [Pd(H2O)(4)](2+) and HSO4- and SO42-, respectively, and between [Pd(H2O)(3)OH](+) and HSO4-. Forward and reverse rate constants for complex formation between [Pd(H2O)(4)](2+) and HSO4- are 119 +/- 6 M-1 s(-1) and 210 +/- 60 s(-1) at 25 degrees C, indicating that HSO4- has a similar nucleophilicity as other oxygen-donor ligands. The rate constants for the reactions of [Pd(H2O)(4)](2+) with SO42- and of [Pd(H2O)(3)OH](+) with HSO4- cannot be resolved because of a proton ambiguity. The mononuclear protolysis constant of [Pd(H2O)(4)](2+) is pK(h) = 3.0 +/- 0.1 at 25 degrees C and 1.00 M ionic strength as determined from rapid spectrophotometric equilibrium measurements. (Less)

Stoichiometric and Catalytic Reactions of the Polysiloxane‐Bound (Ether‐Phosphine)Rhodium(I) Complex [ClRh(PO)(P ∼ O)] in Interphases

AbstractThe reaction of four equivalents of the monomeric trimethoxysilyl(T)‐functionalized ether‐phosphine ligand CyP(CH2CH2OCH3)(CH2)3SiR3 (R = OMe [1 a(T0)], Me [1 b]) with [{μ‐ClRh(COE)2}2] yielded the monomeric pseudo 14 electron rhodium(I) complexes [ClRh(PO)(P∼O)] (2a(T0)2, 2 b). For the sol‐gel process the complex 2 a(T0)2 was protected by introduction of the volatile, reversibly binding ligand pyridine. Thus, the monomeric compound 2 a(T0)2 was co‐condensed with two and eight equivalents of the co‐condensation agent MeSi(OMe)2(CH2)6(MeO)2SiMe (D0‐C6‐D0) to give the polysiloxane‐bound congeners 2(Tn)2(Di‐C6‐Di)y (y = 2 and 8, respectively; i = 0–2; n = 0–3). The polysiloxane‐bound complex 2(Tn)2‐(Di‐C6‐Di)2 was treated with a variety of small molecules in the gas/solid and liquid/solid interphases. It was shown that a facile cleavage of the Rh‐O bond in the ether‐phosphine chelate occurred even in the solid state. The reaction of 2(Tn)2‐(Di‐C6‐Di)2 with carbon monoxide, carbon disulfide, and diphenylacetylene resulted in the irreversible coordination of the molecule to the metal. In the presence of pyridine, the polysiloxane‐bound complex 2(Tn)2‐(Di‐C6‐Di)2 oxidatively added hydrogen to give the octahedrally configurated complex [ClRhH2(Py)(P ∼ O)2] [6(Tn)2(Di‐C6‐Di)2]. Treatment of dry 2(Tn)2‐(Di‐C6‐Di)2 with ethene led to the reversible formation of the corresponding complex. Although the materials display low surface areas, at least 75% of the rhodium centers within the matrix are accessible to the rather bulky tolan molecules. The complexes 2(Tn)2(Di‐C6‐Di)y (y (y = 2, 8) show high activities and selectivities in the hydrogenation of tolan. The conversion was found to depend markedly on the amount of co‐condensate (D0‐C6‐D0) and on the polarity of the solvent. The polysiloxane‐bound complexes 2(Tn)2(Di‐C6‐Di)y are more active than their monomeric congener 2 a(T0)2.

Design of kallidin-releasing tissue kallikrein inhibitors based on the specificities of the enzyme's binding subsites.

The tissue kallikrein inhibitors reported in the present work were derived by selectively replacing residues in Nalpha-substituted arginine- or phenylalanine-pNA (where pNA is p-nitroanilide), and in peptide substrates for these enzymes. Phenylacetyl-Arg-pNA was found to be an efficient inhibitor of human tissue kallikrein (Ki 0.4 microM) and was neither a substrate nor an inhibitor of plasma kallikrein. The peptide inhibitors having phenylalanine as the P1 residue behaved as specific inhibitors for kallidin-releasing tissue kallikreins, while plasma kallikrein showed high affinity for inhibitors containing (p-nitro)phenylalanine at the same position. The Ki value of the most potent inhibitor developed, Abz-Phe-Arg-Arg-Pro-Arg-EDDnp [where Abz is o-aminobenzoyl and EDDnp is N-(2,4-dinitrophenyl)-ethylenediamine], was 0.08 microM for human tissue kallikrein. Progress curve analyses of the inhibition of human tissue kallikrein by benzoyl-Arg-pNA and phenylacetyl-Phe-Ser-Arg-EDDnp indicated a single-step mechanism for reversible formation of the enzyme-inhibitor complex.

Identification of His141 in the Active Site of Bacillus subtilis Adenylosuccinate Lyase by Affinity Labeling with 6-(4-Bromo2,3-dioxobutyl)thioadenosine 5′-Monophosphate

Adenylosuccinate lyase of Bacillus subtilis is inactivated by 25-400 microM 6-(4-bromo-2,3-dioxobutyl)thioadenosine 5'-monophosphate (6-BDB-TAMP) at pH 7.0 and 25 degrees C. The initial inactivation rate constant exhibits nonlinear dependence on the concentration of 6-BDB-TAMP, implying there is reversible formation of enzyme-reagent complex (K(I) = 30 +/- 4 microM) prior to irreversible modification (kmax = 0.139 +/- 0.005 min(-1)). The tetrameric enzyme incorporates about 1 mol of 6-BDB-[32P]TAMP per mol of enzyme subunit concomitant with complete inactivation. Protection against inactivation and incorporation of [32P]reagent is provided by adenylosuccinate or a combination of AMP and fumarate, whereas either AMP or fumarate alone is much less effective. These observations suggest that 6-BDB-TAMP targets the adenylosuccinate-binding site. Hydrolyzed 6-BDB-TAMP is a competitive inhibitor with respect to adenylosuccinate in the catalytic reaction and also decreases the rate of inactivation by 6-BDB-TAMP. These results account for the decrease in the inactivation rate as the reaction of 6-BDB-TAMP with the enzyme proceeds. Purification by chromatography on dihydroxyboryl-agarose and high performance liquid chromatography of the tryptic digest of inactivated enzyme yields a single radioactive peptide, Thr140-Phe150, as determined by gas-phase sequencing. Modified His141 is the reaction product of 6-BDB-TAMP and adenylosuccinate lyase. We conclude that 6-BDB-TAMP functions as a reactive adenylosuccinate analog in modifying His141 in the substrate-binding site of adenylosuccinate lyase, where it may serve as a general base accepting a proton from the succinyl group during catalysis.

Modification of ketoprofen bead structure produced by the spherical crystallization technique with a two-solvent system

Poorly-compressible crystals of ketoprofen were agglomerated by the spherical crystallization technique with a two-solvent system (acetone/demineralized water). In order to study a possible modification of particle texture, spherical crystals were formulated with low concentrations of additives. The results showed that the procedure was possible with ethylcellulose, cross-linked PVP and cross-linked CMC, all at a concentration of 1%. However, no spherical beads could be obtained with the water-soluble compounds tested (PVP and Eudragit L100-55®), Eudragit RS100® and colloidal silica. These formulation trials have indicated two main factors with their possible influences: the polymer solubility and viscosity in the solvent/non-solvent system leading to two kinds of nucleation and changes in mass transfer and drug/polymer interactions leading to the formation of reversible complexes or irreversible complexes and possibility of drug release modifications. Moreover, formulations with the methacrylic acid derivatives were found to be incompatible with the operating conditions, in terms of temperatures changes, stirring or residence time. Furthermore, an optimization of the formulation with ethylcellulose yielded a controlled release form with 1% of the polymer, whereas the addition of very low concentrations increased the drug release.

Signal-transducing protein phosphorylation cascades mediated by Ras/Rho proteins in the mammalian cell: the potential for multiplex signalling.

The features of three distinct protein phosphorylation cascades in mammalian cells are becoming clear. These signalling pathways link receptor-mediated events at the cell surface or intracellular perturbations such as DNA damage to changes in cytoskeletal structure, vesicle transport and altered transcription factor activity. The best known pathway, the Ras-->Raf-->MEK-->ERK cascade [where ERK is extracellular-signal-regulated kinase and MEK is mitogen-activated protein (MAP) kinase/ERK kinase], is typically stimulated strongly by mitogens and growth factors. The other two pathways, stimulated primarily by assorted cytokines, hormones and various forms of stress, predominantly utilize p21 proteins of the Rho family (Rho, Rac and CDC42), although Ras can also participate. Diagnostic of each pathway is the MAP kinase component, which is phosphorylated by a unique dual-specificity kinase on both tyrosine and threonine in one of three motifs (Thr-Glu-Tyr, Thr-Phe-Tyr or Thr-Gly-Tyr), depending upon the pathway. In addition to activating one or more protein phosphorylation cascades, the initiating stimulus may also mobilize a variety of other signalling molecules (e.g. protein kinase C isoforms, phospholipid kinases, G-protein alpha and beta gamma subunits, phospholipases, intracellular Ca2+). These various signals impact to a greater or lesser extent on multiple downstream effectors. Important concepts are that signal transmission often entails the targeted relocation of specific proteins in the cell, and the reversible formation of protein complexes by means of regulated protein phosphorylation. The signalling circuits may be completed by the phosphorylation of upstream effectors by downstream kinases, resulting in a modulation of the signal. Signalling is terminated and the components returned to the ground state largely by dephosphorylation. There is an indeterminant amount of cross-talk among the pathways, and many of the proteins in the pathways belong to families of closely related proteins. The potential for more than one signal to be conveyed down a pathway simultaneously (multiplex signalling) is discussed. The net effect of a given stimulus on the cell is the result of a complex intracellular integration of the intensity and duration of activation of the individual pathways. The specific outcome depends on the particular signalling molecules expressed by the target cells and on the dynamic balance among the pathways.

The Kinetic Factors That Determine the Affinity and Selectivity for Slow Binding Inhibition of Human Prostaglandin H Synthase 1 and 2 by Indomethacin and Flurbiprofen

We present here for the first time a method for determining the rate constants associated with slow binding inhibition of prostaglandin H synthase (PGHS). The rate constants were determined by a method using initial steady-state conditions, which minimize the impact of catalytic autoinactivation of the enzyme. The currently available methods for determining the kinetic constants associated with slow binding enzyme inhibition do not distinguish between rate decreases due to enzyme inhibition or due to autoinactivation of the enzyme. A mathematical model was derived assuming a rapid reversible formation of an initial enzyme-inhibitor complex (EI) followed by a slow reversible formation of a second enzyme-inhibitor complex (EI*). The two enzyme inhibitor complexes are assumed to be in slow equilibrium. This method was used to evaluate the kinetic parameters associated with the binding and selectivity of the nonsteroidal antinflammatory drugs (NSAIDs), flurbiprofen and indomethacin. The KI values associated with the formation of the first reversible complex (EI) for flurbiprofen with PGHS1 and PGHS2 were 0.53 +/- 0. 06 and 0.61 +/- 0.08 microM, respectively; the rate constants for the forward isomerization, k2, into the second reversible complex (EI*) were 0.97 +/- 0.99 and 0.11 +/- 0.01 s-1, respectively, and rates of the reverse isomerization from EI*, k-2, were 0.031 +/- 0.004 and 0.0082 +/- 0.0008 s-1, respectively. Indomethacin was estimated to form the EI complex with the same affinity for both PGHS1 and PGHS2, 10.0 +/- 2.8 microM and 11.2 +/- 2.0 microM, respectively, and dissociate from EI* at approximately the same rate 0.0011 +/- 0.0002 s-1 and 0.0031 +/- 0.0003 s-1, respectively. However, the rate of isomerization into EI* from EI was much greater for PGHS1 than PGHS2, 0.33 +/- 0.08 s-1 as compared with 0.034 +/- 0.004 s-1. These results show that the overall affinity for the inhibition of PGHS1 versus PGHS2 was 30-fold greater for indomethacin (KI* = 0.032 +/- 0.005 and 1.02 +/- 0.08 microM, respectively) and 3-fold greater for flurbiprofen (KI* = 0.017 +/- 0.002 and 0.045 +/- 0.004 microM, respectively). The results also show that for both PGHS1 and PGHS2, flurbiprofen was bound tighter to the initial EI complex than indomethacin; however, the rate of dissociation from EI* was slower for indomethacin than flurbiprofen. The rate of the forward isomerization to EI* is primarily responsible for the selectivity of both NSAIDs for PGHS1. This analysis shows the quantitative importance of the different kinetic parameters upon the overall binding affinity of these NSAIDs and should greatly assist in our understanding of the structural interactions that promote enzyme-inhibitor binding.

New method for the resolution of the enantiomers of 5,6-dihydroxy-2-methyl-aminotetralin by selective derivatization and HPLC analysis: application to biological fluids.

A new chiral derivatization procedure for the HPLC resolution of chiral catecholamines and structurally related compounds is described. The homochiral reagent, (+)-(R)-1-phenylethyl isocyanate (RPEIC), was added to separate and quantitate the enantiomers of rac-5,6-dihydroxy-2-methyl-aminotetralin, the main metabolite of rac-5, 6-diisobutyryl-2-methyl-aminotetralin, a potent dopamine agonist, by reversed-phase HPLC analysis. To avoid catecholamine degradation in the basic reaction medium and to obtain the selective and quantitative derivatization of the amino group of the compound, the reversible complex formation between diphenylborinic acid (DPBA) and the catechol group, in alkaline medium, was performed before homochiral isocyanate addition. The RPEIC derivatization was completed in 30 min and then the DPBA complex was dissociated by adding dilute acid. The structure of intermediates and urea derivatives was confirmed by mass spectometry. The use of an electrochemical detector, operating in redox mode, allowed HPLC quantitation of enantiomers at the nanogram level in plasma and urine. The derivatization procedure is also suitable for other catecholamine-related compounds.

Inhibition of human steroid 5alpha reductases type I and II by 6-aza-steroids: structural determinants of one-step vs two-step mechanism.

We have discovered that 17beta-[N,N-(diethyl)carbamoyl]-6-azaandrost-4-en-3-one is a time-dependent inhibitor of type II 5alpha-reductase, as is the drug finasteride. Unlike finasteride, the 6-aza-steroid is not a time-dependent inhibitor of type I 5 alpha-reductase. Finasteride inhibition of type II enzyme proceeds in a two-step mechanism. At pH 6 and 37 degrees C, an initial finasteride-reductase complex is formed with a K(i)(app) of 11.9 +/- 4.1 nM. In a second step, an irreversible complex is formed with a rate constant of inactivation of 0.09 +/- 0.01 s(-1). In contrast, the 6-aza-steroid is a reversible inhibitor. From the results of a simplified mathematical analysis, based on the rapid equilibrium approximation, the inhibitor and the enzyme form an initial complex with a K(i) of 6.8 +/- 0.2 nM. The reversible formation of a final complex, with an overall K(i) of 0.07 +/- 0.02 nM, is characterized by a first-order isomerization rate constant 0.0035 +/- 0.0001 s(-1) for the forward step and 0.00025 +/- 0.00006 s(-1) for the backward step. All rate constants for the two-step mechanism were obtained by using a general numerical integration method. The best fit values for the association and dissociation rate constants were 5.0 microM(-1) s(-1) and 0.033 +/- 0.008 s(-1), respectively, and the isomerization rate constants were 0.0035 +/- 0.007 s(-1) and 0.000076 +/- 0.000019 s(-1). These values correspond to an initial K(i) of 6.5 nM and an overall dissociation constant of 0.14 nM. The data presented here show that both finasteride and the 6-aza-steroid analogs are potent against type II 5alpha-reductase, although their mechanisms of inhibition are different.

Quantum-chemical Structure-Activity Relationships in carbamate insecticides

A non-empirical Quantitative Structure-Activity Relationship (QSAR) method is employed to analyze the reversible complex formation during the reaction of phenyl-N-methylcarbamates with the enzyme acetylcholinesterase. No common equation for theortho, meta andpara-substituted molecules could be obtained. A good description of the reversible complex formation is achieved by separating the molecules according to the position of the aromatic substituent. The introduction of a substituent orientation parameter helps account for the percentage of molecules attaining the proper orientation to interact with their partner. This parameter is useful in describing physical effects depending on the rotational partition function. A model for the carbamate-acetylcholinesterase reversible complex is proposed.

Aldehyde production by alcohol oxidation with Gluconobacter oxydans

The microbial oxidation of various primary alcohols to the corresponding aldehydes has been investigated. A focused screening performed amongst some acetic acid bacteria showed that a newly isolated strain of Gluconobacter oxydans oxodizes various short-chain aliphatic alcohols to the corresponding aldehydes with negligible acid production. 3-Methyl-1-butanol (isoamyl alcohol) proved to be the better substrate with high yields (more than 90%) without by-product formation. This biotransformation also occurs with continuous or semicontinuous addition of substrate since the volatile product is removed from the medium under vigorous aeration conditions. Product recovery is attained either by the use of cold traps or by reversible complex formation.

Interaction of slightly crosslinked gels of poly(diallyldimethylammonium bromide) with sodium dodecyl sulfate and cetylpyridinium bromide

AbstractThe interaction of slightly crosslinked gels of poly(diallyldimethylammonium bromide) (PDADMAB) with both the anionic surfactant sodium dodecyl sulfate (SDS) and the cationic surfactant cetylpyridinium bromide (CPB; system. name: N‐hexadecylpyridinium bromide) in aqueous medium has been investigated. The absorption of the anionic surfactant by the oppositely charged gel leads to contraction or collapse of the network. The absorption of the cationic surfactant by the gel‐SDS complexes results in a reswelling of the network. By means of IR and UV spectroscopy it has been shown that in the presence of the anionic detergent the cationic network absorbs the cationic surfactant. It has been shown that both reversible and irreversible complex formation in the gel phase is possible depending on the initial ratio between the components. Small‐angle‐X‐ray scattering experiments demonstrated that the complexes may have a regular microstructure.

Complex formation between nickel(II) and 2-(2-aminoethyl) benzimidazole: A kinetic and equilibrium study

The reversible complex formation between 2-(2-aminoethyl) benzimidazole (AEB) and nickel(II) was studied by stopped flow spectrophotometry at I = 0.30 mol dm−3. Both the neutral and monoprotonated form of AEB reacted to give the NiAEB2+ chelate. At 25 °C, the rates and activation parameters for the reactions NiII + AEB \(\xrightarrow{{k_f^L }}\) NiAEB2+ and NiII + AEBH+\(\xrightarrow{{k_f^{HL} }}\) NiAEB2+ + H+ are kfL(dm−3 mol−1 s−1) = (2.17 ± 0.24) × 103, ΔH≠ (kJ mol−1) = 40.0 ± 0.8, ΔS≠ (JK−1 mol−1) = − 47 ± 3 and k inff pHL (dm3 mol−1 s−1) = 33 ± 10, ΔH≠(kJ mol−1) = 42.0 ±2.7, ΔS≠(JK−1 mol−1) = − 72 ± 9. The dissociation of NiAEB2+ was acid catalysed and kobs for this process increased linearly with [H+] in the 0.01–0.15 mol dm−3 (10–30 °C) range with kH(dm3 mol−1s−1) (25 °C) = 329 ± 6, ΔH≠(kJ mol−1) = 40 ± 2 and ΔS≠ (JK−1 mol−1) = − 61 ± 8. The results also indicated that the formation of NiAEB2+ involves a chelation-controlled, rate-limiting process. Analysis of the ΔS ° data for the acid ionisation of AEBH inf2 p2+ and the formation of NiAEB2+ showed that the bulky AEBH+ ion has a solvent structure breaking effect as compared to AEB [saqS ° (AEBH+) − saq ° (AEB) = 69 JK−1 mol−1], while AEBH inf2 p2+ is a solvent ordering ion relative to NiAEB2+ [saq° (NiAEB2+) − ovSaq ° (AEBH inf2 p2+ ) = 11 JK−1 mol−1].

Kinetics and mechanism of the complex formation between oxalatopentaamminecobalt(III) and aluminium(III) and gallium(III); a comparative study

Summary The reversible complex formation between oxalatopentaammine cobalt(III), aluminium(III) and gallium(III) was investigated by the stopped flow technique at 30 + 0.1 ~ and I = 1.0 tool din- 3. The reactivity sequence: Gam > A1 m is observed, however, the major path for gallium(III) was (NH3)sCoC20,H 2 + + GaOH 2 + ~(NH3)5CoC204Ga 4 + + H20. The formation and dissociation rate constants of the binuclear species have been compared with the analogous data for iron(III) and nickel(II) reported earlier. The results reflect the fact that the half-bonded exalato moiety of (NH3)5CoC20 ~ acts as a chelating agent for the metal ions.