Association Constants (association + constant)

Distribution by Scientific Domains
Distribution within Chemistry


Selected Abstracts


Separation of amino alcohols using divalent dipeptides as counter ions in aqueous CE

ELECTROPHORESIS, Issue 10 2010
Jakob Haglöf
Abstract Divalent dipeptides have been introduced as counter ions in aqueous CZE. The dipeptides form ion pairs with amino alcohols in the BGE and facilitate the separation of amino alcohols. High concentrations of dipeptide caused reversed effective mobility for the analytes. The net charge of the dipeptide can be controlled using a buffer or a strong base, and regulates the interaction between the dipeptide and the amino alcohol. A stronger interaction and higher selectivity of amino alcohols was observed when the dipeptides were used as divalent counter ions, than in monovalent or uncharged form. Association constants for ion pairs between divalent dipeptides and amino alcohols can be used to enhance selectivity for amino alcohols in CZE. No chiral separation of amino alcohols was observed when using the dipeptides as ion-pairing chiral selectors in aqueous BGE, but addition of methanol to the BGE promoted enantioselectivity. [source]


Oligonucleotide Analogues with Integrated Bases and Backbone.

HELVETICA CHIMICA ACTA, Issue 12 2006

Abstract The self-complementary UA and AU dinucleotide analogues 41,45, 47, 48, and 51,60 were prepared by Sonogashira coupling of 6-iodouridines with C(5,) -ethynylated adenosines and of 8-iodoadenosines with C(5,) -ethynylated uridines. The dinucleotide analogues associate in CDCl3 solution. The C(6/I) -unsubstituted AU dimers 51 and 54 prefer an anti -oriented uracilyl group and form stretched linear duplexes. The UA propargyl alcohols 41 and 43,45 possess a persistent intramolecular O(5,/I)H,,,N(3/I) H-bond and, thus, a syn -oriented adeninyl and a gt - or tg -oriented ethynyl moiety; they form corrugated linear duplexes. All other dimers form cyclic duplexes characterized by syn -oriented nucleobases. The preferred orientation of the ethynyl moiety (the C(4,),C(5,) torsion angle) defines a conformation between gg and one where the ethynyl group eclipses O(4,/I). The UA dimers 42, 47, and 48 form Watson,Crick H-bonds, the AU dimers 56 and 58,60 H-bonds of the Watson,Crick -type, the AU dimers 53 and 55 reverse- Hoogsteen, and 57Hoogsteen H-bonds. The pairing mode depends on the substituent of C(5,/I) (H, OSiiPr3; OH) and on the H-bonds of HOC(5,/I) in the AU dimers. Association constants were derived from the concentration-dependent chemical shift for HN(3) of the uracilyl moiety; they vary from 45,104,M,1 for linear duplexes to 197,2307,M,1 for cyclic duplexes. The thermodynamic parameters were determined by van't Hoff analysis of the temperature-dependence of the (concentration-dependent) chemical shift for HN(3) of the uracilyl moiety. Neglecting stacking energies, one finds an average energy of 3.5,4.0,kcal/mol per intermolecular H-bond. Base stacking is evidenced by the temperature-dependent CD spectra. The crystal structure of 54 shows two antiparallel chains of dimers connected by Watson-Crick H-bonds. The chains are bridged by a strong H-bond between the propargylic OH and OC(4) and by weak reverse A,,,A Hoogsteen H-bonds. [source]


Effects of the Interaction Between , -Carboline-3-carboxylic acid N -Methylamide and Polynucleotides on Singlet Oxygen Quantum Yield and DNA Oxidative Damage

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2007
Ińigo X. García-Zubiri
The complexation of , -carboline-3-carboxylic acid N -methylamide (,CMAM) with the sodium salts of the nucleotides polyadenylic (Poly A), polycytidylic (Poly C), polyguanylic (Poly G), polythymidylic (Poly T) and polyuridylic (Poly U) acids, and with double stranded (dsDNA) and single stranded deoxyribonucleic acids (ssDNA) was studied at pH 4, 6 and 9. Predominant 1:1 complex formation is indicated from Job plots. Association constants were determined using the Benesi,Hildebrand equation. ,CMAM-sensitized singlet oxygen quantum yields were determined at pH 4, 6 and 9, and the effects on this of adding oligonucleotides, dsDNA and ssDNA were studied at the three pH values. With dsDNA, the effect on ,CMAM triplet state formation was also determined through triplet,triplet transient absorption spectra. To evaluate possible oxidative damage of DNA following singlet oxygen ,CMAM photosensitization, we used thiobarbituric acid-reactivity assays and electrophoretic separation of DNA assays. The results showed no oxidative damage at the level of DNA degradation or strand break. [source]


Nanosized Ball Joints Constructed from C60 and Tribenzotriquinacene Sockets: Synthesis, Component Self-Assembly and Structural Investigations

CHEMISTRY - A EUROPEAN JOURNAL, Issue 35 2007
Björn Bredenkötter Dr.
Abstract The formation of supramolecular host,guest complexes of fullerene (C60) and two novel tribenzotriquinacene based hosts (5,a and 5,b) was investigated in solution and in the solid state. Stability constants for 1:1 and 2:1 complexes were obtained from spectroscopic (UV/Vis, 1H,NMR) titration experiments. Association constants of K1=(2908±360),L,mol,1 and K2=(2076±300),L,mol,1 for C60/5,a, and K1=(5608±220),L,mol,1 and K2=(673±160),L,mol,1 for C60/5,b were obtained. Single crystal X-ray structural analysis of compound C60,5,b,3,toluene revealed that a molecule of C60 was located at short van der Waals contact distances in the open pre-organised cavity of the rigid host. The supramolecular complex created resembles an engineered nanosized ball joint and represents the first member for a future nanomechanics construction kit. [source]


Capillary electrophoretic study of the binding of zinc(II) ion to bacitracin A1 in water-2,2,2-trifluoroethanol

ELECTROPHORESIS, Issue 10 2003
Massimo Castagnola
Abstract Binding of Zn2+ to bacitracin A1 was studied by capillary electrophoresis in water/2,2,2-trifluoroethanol (70/30 v/v) at different apparent pH values in order to estimate the association constant of metal, the acidic dissociation constants and the Stokes radii of both free and bounded peptide in apolar environment. The Stokes radii of the free peptide species were compared with those in aqueous solution, as obtained in a recent study performed by our group, indicating that apolar environment stabilizes bacitracin A1 in a conformational structure with the lateral chain of apolar amino acids exposed on the external surface. This conformation of the macrocyclic dodecapeptide is ready to interact with Zn2+ ion, as pointed out by the strong increase of the association constant measured in water/2,2,2-trifluoroethanol with respect to the value obtained in aqueous solution. In addition, whereas Zn2+ ion binding in aqueous solution provides a sensible reduction of peptide Stokes radius, no sensible variations following to ion binding were observed in hydro-organic solution. The present results suggest that the apolar environment, rather than the metal ion binding, could be responsible for the conformational transition that brings bacitracin A1 towards its biologically active structure.* [source]


Influence of response factors on determining equilibrium association constants of non-covalent complexes by electrospray ionization mass spectrometry

JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 5 2003
Valérie Gabelica
Abstract A method for determining the equilibrium association constant of a complexation reaction A + B , AB by electrospray ionization mass spectrometry is described. The method consists in measuring the relative intensities of the peaks corresponding to A and to AB in equimolar A,B solutions at different concentrations C0. The results are fitted by a non-linear least-squares procedure, with the two variable parameters being the equilibrium association constant Ka and a factor R, defined by I(AB)/I(A) = R × [AB]/[A]. The factor R is the ratio between the response factors of AB and A, and corrects for the relative electrospray responses of the complex and the free substrate A, mass discrimination of instrumental origin and/or moderate in-source dissociation. The method is illustrated with the following two systems: complexes between a double-stranded 12-base pair oligonucleotide and minor groove binders, and cyclodextrin complexes with ,,,-dicarboxylic acids. For the oligonucleotide complexes, it is found that the response of the complex is not dramatically different to the response of the free oligonucleotide duplex, as the double helix conformation is disturbed by the drug only to a minor extent. In the case of cyclodextrin complexes, these complexes were found to have a much higher response than free cyclodextrin. This may be due to the fact that cyclodextrin is neutral in solution, whereas the complex is charged, but it can also stem from the fact that a significant proportion of the complex is in a non-inclusion geometry. The present method requires the exact determination of the concentrations of the reactants and is applicable to 1 : 1 complexes. Copyright © 2003 John Wiley & Sons, Ltd. [source]


Alteration of the intravenous pharmacokinetics of a synthetic ozonide antimalarial in the presence of a modified cyclodextrin

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 2 2006
Susan A. Charman
Abstract The pharmacokinetic profile and renal clearance of a novel synthetic ozonide antimalarial (1) was found to be significantly altered when intravenously administered to rats as a cyclodextrin-based formulation (0.1 M Captisol®, a sulfobutylether ,-cyclodextrin derivative (SBE7 -,-CD)) compared to a cyclodextrin-free isotonic buffered glucose formulation. There was an 8.5-fold decrease in the steady-state blood volume of distribution, a 6.6-fold decrease in the mean residence time and a greater than 200-fold increase in renal clearance of 1 when administered in the cyclodextrin formulation. Analysis of the whole blood and plasma concentration profiles revealed an essentially constant blood to plasma ratio when 1 was administered in the cyclodextrin-free formulation, whereas this ratio changed as a function of time when administered in the presence of the cyclodextrin derivative. It is postulated that the observed differences were due to a very strong complexation interaction between 1 and the cyclodextrin, resulting in a slow dissociation of the complex in vivo, and altered distribution and excretion profiles. Preliminary studies using isothermal titration calorimetry (ITC) indicated that the association constant for the 1/Captisol® complex was approximately two orders of magnitude higher than reported for typical drug/cyclodextrin complexes. © 2005 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 95:256,267, 2006 [source]


A pulsed field gradient NMR diffusion investigation of enkephalin peptide-sodium dodecyl sulfate micelle association

MAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2006
Brandon A. Begotka
Abstract Pulsed field gradient NMR (PFG-NMR) diffusion experiments were used to investigate the binding of leucine and methionine enkephalin peptides to anionic sodium dodecyl sulfate (SDS) micelles. The study was undertaken to investigate the mechanism of interaction between enkephalin peptides and SDS micelles and to determine if NMR-derived association constants, Keq, can predict the elution order in electrokinetic chromatography (EKC). In EKC, peptides are separated on the basis of their interactions with micelles. The Leu-enkephalin peptide,micelle association constant increased from 130 ± 8 to 1459 ± 57 and 1744 ± 64 M,1, respectively, when an Arg or Lys was added to the C -terminus. The association constant of Leu-enkephalinamide was approximately equal to that of Leu-enkephalin-Arg. Substitution of Phe4 with a Trp or Gly2 with an Ala in the Leu-enkephalin peptides also increased the micelle binding affinity. These results confirm that the interaction of Leu-enkephalin peptides with SDS micelles is largely electrostatic and that the non-polar amino acid side chains interact with the hydrophobic micelle core. The peptide,micelle association constants for the cationic Met-enkephalin peptides were also greater than their zwitterionic counterparts. For example, the Met-enkephalin Keq value was 162 ± 9 M,1, while the association constants for Met-enkephalin-Arg, Met-enkephalin-Lys, and Met-enkephalinamide were, respectively, 674 ± 31, 426 ± 23, and 453 ± 27 M,1. In both Met-enkephalin and Met-enkephalinamide, replacing Gly2 with an Ala did not significantly increase the association constant. These results confirm that with the Met-enkephalin peptides, there was little or no interaction of the amino acid side chains with the micelle core. For both the Leu-enkephalin and Met-enkephalin peptides, the association constants were consistent with EKC results, in that the peptides with smaller Keq values were found to elute before those with larger association constants. Copyright © 2006 John Wiley & Sons, Ltd. [source]


Change in dimerization mode by removal of a single unsatisfied polar residue located at the interface

PROTEIN SCIENCE, Issue 9 2000
P. R. Pokkuluri
Abstract The importance of unsatisfied hydrogen bonding potential on protein-protein interaction was studied. Two alternate modes of dimerization (conventional and flipped form) of an immunoglobulin light chain variable domain (VL) were previously identified. In the flipped form, interface residue Gln89 would have an unsatisfied hydrogen bonding potential. Removal of this Gln should render the flipped dimer as the more favorable quaternary form. High resolution crystallographic studies of the Q89A and Q89L mutants show, as we predicted, that these proteins indeed form flipped dimers with very similar interfaces. A small cavity is present in the Q89A mutant that is reflected in the ,100 times lower association constant than found for the Q89L mutant. The association constant of Q89A and Q89L proteins (4 × 106 M,1 and > 108 M,1) are 10- and 1,000-fold higher than that of the wild-type protein that forms conventional dimers clearly showing the energetic reasons for the flipped dimer formation. [source]


Electrochemical Quartz Crystal Impedance and Fluorescence Quenching Studies on the Binding of Carbon Nanotubes (CNTs)-Adsorbed and Solution Rutin with Hemoglobin

BIOTECHNOLOGY PROGRESS, Issue 2 2007
Yuhua Su
Electrochemical quartz crystal impedance (QCI) technique was utilized to monitor in situ the adsorption of rutin (RT) onto a carbon nanotubes (CNTs)-modified gold electrode and to study the binding process of solution hemoglobin (Hb) to RT immobilized on the electrode. Time courses of the QCI parameters including crystal resonant frequency were simultaneously obtained during the RT adsorption and Hb-RT binding. In contrast to the negligible RT adsorption at a bare gold electrode, the modification by CNTs notably enhanced the amount of adsorption, and almost all of the adsorbed RT molecules were found to be electroactive. On the basis of the frequency response from the binding of adsorbed RT to solution Hb and the diminished electroactivity of adsorbed RT after the formation of the electrochemically inactive RT-Hb adduct, the average binding molar ratio of adsorbed RT to Hb was estimated to be 23.9:1, and the association constant (Ka) for the binding was estimated to be 2.87 × 106 (frequency) and 3.92 × 106 (charge) L mol,1, respectively. Comparable results were obtained from fluorescence quenching measurements in mixed solutions containing RT of fixed concentration and Hb of varying concentrations, demonstrating that the interfacial RT here behaved equivalently in the RT-Hb binding activity compared to that in solution. This work may have presented a new and general protocol involving CNTs to study many other electroactive natural antioxidants or drugs that are at the interface or in solution, their binding with proteins or other biomolecules, and changes of their antioxidant activity after the binding. [source]


Helix Formation in Synthetic Polymers by Hydrogen Bonding with Native Saccharides in Protic Media

CHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2006
Minoru Waki
Abstract Water-soluble poly(m -ethynylpyridine)s were designed to realize saccharide recognition in protic media. UV/Vis, 1H NMR, and fluorescence measurements revealed that the polymer forms a helical higher order structure by solvophobic interactions between the ethynylpyridine units in the protic medium. The resulting pore in the helix behaves like a binding pocket in proteins, by taking advantage of inwardly directed hydrogen-bonding functional groups of the polymers. Molecular recognition of native saccharides by the polymers was investigated by circular dichroism (CD). The chirality of the saccharide was transferred to the helical sense of the polymers, accompanied by the appearance of induced CDs (ICDs) in the absorptive region of the polymers. In MeOH/water (10/1), mannose and allose showed intense ICDs, and the apparent association constant between the polymer and D -mannose was 14,M,1. [source]


Host,Guest Interaction of Chaperonin GroEL and Water-Soluble CdTe Quantum Dots and its Size-Selective Inclusion

CHEMPHYSCHEM, Issue 15 2008
Chaoqing Dong
Abstract Some nanoparticles, such as quantum dots (QDs), are widely used in the biological and biomedical fields due to their unique optical properties. However, little is currently known about the interaction between these nanoparticles and biomolecules. Herein, we systemically investigated the interaction between chaperonin GroEL and water-soluble CdTe QDs based on fluorescence correlation spectroscopy (FCS), capillary electrophoresis, and fluorescence spectrometry. We observed that some water-soluble CdTe QDs were able to enter the inner cavity of GroEL and formed an inclusion complex after the activation of chaperonin GroEL with ATP. The inclusion of GroEL was size-selective to QDs and only small QDs were able to enter the inner cavity. The inclusion could suppress the fluorescence quenching of the QDs. Meanwhile, we evaluated the association constant between chaperonin GroEL and CdTe QDs by FCS. Our results further demonstrated that FCS was a very useful tool for study of the interaction of QDs and biomolecules. [source]


Interaction of Co(II) with Bovine Serum Albumin under UV C Irradiation

CHINESE JOURNAL OF CHEMISTRY, Issue 8 2007
He-Dong Zhang
Abstract The interaction of Co(II) with BSA under UV C (253.7 nm) irradiation under physiological conditions has been studied by UV-vis spectrum, ultraviolet second-derivative spectroscopy and fluorescence spectrum. The quenching rate constant kq and the association constant Ka were calculated according to Stern-Volmer equation based on the quenching of the fluorescence of BSA by Co(II). [source]


Estimation of Ion-Pairing Constants in Plasticized Poly(vinyl chloride) Membranes Using Segmented Sandwich Membranes Technique

ELECTROANALYSIS, Issue 17-18 2009
Vladimir
Abstract Segmented sandwich membrane method was used to determine ion-pairing constants for four cationic sites: tris-(2,3,4-dodecyloxy)benzenetrimethylammonium, tris-(2,3,4-dodecyloxy)benzenedimethyloctylammonium, tris-(2,3,4-dodecyloxy)benzenemethyldioctylammonium, and dimethyldioctadecylammonium with chloride, bromide, nitrate, benzene sulfonate, trichloroacetate, thiocyanate, perchlorate and picrate, as well as ion-pairing constants for two anionic sites: tetraphenylborate and tris-(2,3,4-octyloxy)benzenesulfonate with dimedrol, quinine, anapriline, and amantadine cations in poly(vinyl chloride) membranes plasticized with 2-nitrophenyl octyl ether. Ion association constants of anions with quaternary ammonium sites regularly increase from picrate to chloride along with reduction of the anion radius and with improvement of site exchanger center steric accessibility. Ion association constants of amine cations with tris-(2,3,4-octyloxy)benzenesulfonate are several orders higher than those with tetraphenylborate and regularly increase from tertiary amine to primary one. [source]


Facilitated Transfer of Alkali Metal Ions by a Tetraester Derivative of Thiacalix[4]arene at the Liquid,Liquid Interface

ELECTROANALYSIS, Issue 12 2008
Akgemci, Emine Guler
Abstract The facilitated transfer of alkali metal ions (Na+, K+, Rb+, and Cs+) by 25,26,27,28-tetraethoxycarbonylmethoxy-thiacalix[4]arene across the water/1,2-dichloroethane interface was investigated by cyclic voltammetry. The dependence of the half-wave transfer potential on the metal and ligand concentrations was used to formulate the stoichiometric ratio and to evaluate the association constants of the complexes formed between ionophore and metal ions. While the facilitated transfer of Li+ ion was not observed across the water/1,2-dichloroethane interface, the facilitated transfers were observed by formation of 1,:,1 (metal:ionophore) complex for Na+, K+, and Rb+ ions except for Cs+ ion. In the case of Cs+ a 1,:,2 (metal:ionophore) complex was obtained from its special electrochemical response to the variation of ligand concentrations in the organic phase. The logarithms of the complex association constants, for facilitated transfer of Na+, K+, Rb+, and Cs+, were estimated as 6.52, 7.75, 7.91 (log ,1°), and 8.36 (log ,2°), respectively. [source]


Numeric Simulation of Ion-Site Association Effects in Ion-Selective Electrode Response

ELECTROANALYSIS, Issue 15-16 2003
Konstantin
Abstract Effects in ion-selective electrode response and selectivity, caused by ion-site association in membranes are studied by means of numeric simulations based on a generalized model, which does not rely on a certain degree of the dissociation of electrolytes in membranes. The variability of the experimental values of the potentiometric selectivity coefficients is considered in view of the association in membranes. The reasons why the "dissociation approach" often fits experimental data are also discussed. A novel version of segmented sandwich membrane method is proposed for direct potentiometric measurements of ion-site association constants in membranes, and preliminary experimental results are presented. [source]


Chiral separation of N -imidazole derivatives, aromatase inhibitors, by cyclodextrin-capillary zone electrophoresis.

ELECTROPHORESIS, Issue 16 2004
Mechanism of enantioselective recognition
Abstract Baseline separation of ten new, substituted [1-(imidazo-1-yl)-1-phenylmethyl)] benzothiazolinone and benzoxazolinone derivatives with one chiral center was achieved using cyclodextrin-capillary zone electrophoresis (CD-CZE). A method for the enantiomeric resolution of these compounds was developed using neutral CDs (native ,-, ,-, ,-CDs or ,-, ,-, ,-hydroxypropyl (HP)-CDs) as chiral selectors. Operational parameters including the nature and concentration of the chiral selectors, pH, ionic strength, organic modifiers, temperature, and applied voltage were investigated. The use of neutral CDs provides enantiomeric resolution by inclusion of compounds in the CD cavity. The HP-,-CD and HP-,-CD were found to be the most effective complexing agents and allowed efficient enantiomeric resolutions. Optimal separation of N -imidazole derivatives was obtained using 50 mM phosphate buffer at pH 2.5 containing either HP-,-CD or HP-,-CD (7.5,12.5 mM) at 25°C, with an applied field of 0.50 kV·cm,1 giving resolution factors Rs superior to 1.70 with migration times of the second enantiomer less than 13 min. The same enantiomer migration order observed for all molecules can be related to a close interaction mechanism with CDs. The influence of structural features of the solutes on Rs and tm was studied. The lipophilic character (log kw) of the solutes and the apparent and averaged association constants of inclusion complexes for four compounds with the six different CDs led us to rationalize the enantioseparation mechanisms. The conclusions were corroborated with reversed-phase high-performance liquid chromatography (HPLC) on chiral stationary phases (CSPs) based on CDs. [source]


Formation of copper complexes in landfill leachate and their toxicity to zebrafish embryos

ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 5 2000
Jonathon K. Fraser
Abstract Toxic metal organic complexeshavenot been found in natural waters, although someorganicacids form bioavailable lipophilic and metabolite-type metal complexes. Landfill leachates usually contain organic acids and in the urban environment these leachates, when mixed with storm waters containing Cu, could be a source of toxic Cu organic complexes in streams and estuaries. We investigated the formation of Cu complexes in the leachate from an active urban landfill and found that some of the complexes formed were toxic to zebrafish embryos. High and low nominal molecular weight (NMWT) fractions; >5,000 Da and <700 Da, of leachate both formed Cu complexes with almost identical Cu complexing characteristics but the toxicity was due solely to the low NMWT complexes formed in the <700 Da fraction. Chemical equilibrium modelling with MINTEQA2 and H and Cu complex conditional association constants and ligand concentrations obtained from pH and Cu titrations with a Cu ion-selective electrode and van den Berg,Ruzic analyses of the titration data was used to calculate the copper speciation in the embryo test solutions. This calculated speciation, which was confirmed by measurements of Cu2+ in the test solutions, enabled the toxicity due to the free Cu ion and to the Cu complexes to be distinguished. [source]


The Use of N -Type Ligands in the Enantioselective Liquid,Liquid Extraction of Underivatized Amino Acids

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 27 2010
Bastiaan J. V. Verkuijl
Abstract The first palladium based extraction system using chiral N -based ligands in the enantioselective liquid,liquid extraction (ELLE) of underivatized amino acids, is presented. The system shows the highest selectivity for the ELLE of methionine with metal complexes as hosts reported to date. Furthermore, the host can be prepared in situ from commercially available compounds. The dependency of the system on parameters such as pH, organic solvent, and temperature has been established. The intrinsic selectivity was deduced by determination of the association constants of the palladium complex with the tryptophan enantiomers. [source]


From Theozymes to Artificial Enzymes: Enzyme-Like Receptors for Michael Additions with Oxyanion Holes and Active Amino Groups

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 29 2007
Luis Simón
Abstract Different artificial enzymes, based on the theozyme concept, have been designed for Michael additions of pyrrolidine to ,,,-unsaturated lactams. These molecules each have skeleton able to mimic a structure called an "oxyanion hole", as is present in many enzymes. Amine groups are also responsible for the catalytic activities of these receptors, since they support the important proton-transport step. The requirement for the amine groups was established from the reaction mechanism and from theoretical calculations. The catalytic activities of the receptors are discussed, taking into account their relative association constants with the substrate: kcat/kuncat values of up to 104 were obtained. The catalytic activities of the receptors are compared with those found in natural enzymes and catalytic antibodies. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]


Deprotonation of Indole Derivatives in Aqueous Cationic Surfactants

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2004
Nicoletta Spreti
Abstract Deprotonations of 5-nitroindole, 1a, and its 2-carboxylate ion, 2a, have been monitored in 0.01, 0.1, and 0.5 M NaOH in micellar solutions of cetyl trialkylammonium bromide, alkyl = Me, Et, nPr, nBu, CTABr, CTEABr, CTPABr, CTBABr. Extents of deprotonation (% f) have been fitted using the pseudophase model of micellar effects with interionic competition described by ion exchange or by independent association constants. Both treatments indicate that base dissociation constants in dilute OH, are lower than in water by factors of ca. 3,11, and decrease with increasing bulk of the head groups, and that these factors increase modestly as the OH, concentration increases to 0.5 M. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]


Structural and thermodynamic insights into the binding mode of five novel inhibitors of lumazine synthase from Mycobacterium tuberculosis

FEBS JOURNAL, Issue 20 2006
Ekaterina Morgunova
Recently published genomic investigations of the human pathogen Mycobacterium tuberculosis have revealed that genes coding the proteins involved in riboflavin biosynthesis are essential for the growth of the organism. Because the enzymes involved in cofactor biosynthesis pathways are not present in humans, they appear to be promising candidates for the development of therapeutic drugs. The substituted purinetrione compounds have demonstrated high affinity and specificity to lumazine synthase, which catalyzes the penultimate step of riboflavin biosynthesis in bacteria and plants. The structure of M. tuberculosis lumazine synthase in complex with five different inhibitor compounds is presented, together with studies of the binding reactions by isothermal titration calorimetry. The inhibitors showed the association constants in the micromolar range. The analysis of the structures demonstrated the specific features of the binding of different inhibitors. The comparison of the structures and binding modes of five different inhibitors allows us to propose the ribitylpurinetrione compounds with C4,C5 alkylphosphate chains as most promising leads for further development of therapeutic drugs against M. tuberculosis. [source]


Study of association thermodynamics between crystal violet and sodium bis(2-ethylhexyl)sulfosuccinate and kinetics of basic fading of crystal violet in microemulsions

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2008
Zhi Yun Chen
The thermodynamics of the association between 4,4,,4,-tris(dimethyl-amino)triphenylmethyl chloride (crystal violet or CV) and sodium bis(2-ethylhexyl)-sulfosuccinate (aerosol OT or AOT) in water/AOT/n -decane microemulsion and the kinetics of the basic hydrolysis of CV in a water-in-oil microemulsion were investigated by UV,vis spectroscopic measurements. An association model of CV and AOT was used to analyze the experimental data to obtain the association constants at various temperatures. By taking the association into account, the "actual" rate constants and the activation energies of the basic hydrolysis of CV in the media of water/AOT/oil were obtained. The difference in thermodynamics and kinetics between the two media of water/AOT/n -decane and water/AOT/isooctane is discussed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 294,300, 2008 [source]


Characterization of non-covalent complexes of rutin with cyclodextrins by electrospray ionization tandem mass spectrometry

JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 6 2004
Mingquan Guo
Abstract Electrospray ionization tandem mass spectrometry (ESI-MSn) and the phase solubility method were used to characterize the gas-phase and solution-phase non-covalent complexes between rutin (R) and ,-, ,- and ,-cyclodextrins (CDs). The direct correlation between mass spectrometric results and solution-phase behavior is thus revealed. The order of the 1 : 1 association constants (Kc) of the complexes between R and the three CDs in solution calculated from solubility diagrams is in good agreement with the order of their relative peak intensities and relative collision-induced dissociation (CID) energies of the complexes under the same ESI-MSn condition in both the positive and negative ion modes. Not only the binding stoichiometry but also the relative stabilities and even binding sites of the CD,R complexes can be elucidated by ESI-MSn. The diagnostic fragmentation of CD,R complexes, with a significant contribution of covalent fragmentation of rutin leaving the quercetin (Q) moiety attached to the CDs, provides convincing evidence for the formation of inclusion complexes between R and CDs. The diagnostic fragment ions can be partly confirmed by the complexes between Q and CDs. The gas-phase stability order of the deprotonated CD,R complexes is ,-CD,R > ,-CD,R > ,-CD/R; ,-CD seems to bind R more strongly than the other CDs. Copyright © 2004 John Wiley & Sons, Ltd. [source]


Influence of response factors on determining equilibrium association constants of non-covalent complexes by electrospray ionization mass spectrometry

JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 5 2003
Valérie Gabelica
Abstract A method for determining the equilibrium association constant of a complexation reaction A + B , AB by electrospray ionization mass spectrometry is described. The method consists in measuring the relative intensities of the peaks corresponding to A and to AB in equimolar A,B solutions at different concentrations C0. The results are fitted by a non-linear least-squares procedure, with the two variable parameters being the equilibrium association constant Ka and a factor R, defined by I(AB)/I(A) = R × [AB]/[A]. The factor R is the ratio between the response factors of AB and A, and corrects for the relative electrospray responses of the complex and the free substrate A, mass discrimination of instrumental origin and/or moderate in-source dissociation. The method is illustrated with the following two systems: complexes between a double-stranded 12-base pair oligonucleotide and minor groove binders, and cyclodextrin complexes with ,,,-dicarboxylic acids. For the oligonucleotide complexes, it is found that the response of the complex is not dramatically different to the response of the free oligonucleotide duplex, as the double helix conformation is disturbed by the drug only to a minor extent. In the case of cyclodextrin complexes, these complexes were found to have a much higher response than free cyclodextrin. This may be due to the fact that cyclodextrin is neutral in solution, whereas the complex is charged, but it can also stem from the fact that a significant proportion of the complex is in a non-inclusion geometry. The present method requires the exact determination of the concentrations of the reactants and is applicable to 1 : 1 complexes. Copyright © 2003 John Wiley & Sons, Ltd. [source]


RPA repair recognition of DNA containing pyrimidines bearing bulky adducts,

JOURNAL OF MOLECULAR RECOGNITION, Issue 3 2008
Irina O. Petruseva
Abstract Recognition of new DNA nucleotide excision repair (NER) substrate analogs, 48-mer ddsDNA (damaged double-stranded DNA), by human replication protein A (hRPA) has been analyzed using fluorescence spectroscopy and photoaffinity modification. The aim of the present work was to find quantitative characteristics of RPA,ddsDNA interaction and RPA subunits role in this process. The designed DNA structures bear bulky substituted pyrimidine nitrogen bases at the inner positions of duplex forming DNA chains. The photoreactive 4-azido-2,5-difluoro-3- pyridin-6-yl (FAP) and fluorescent antracenyl, pyrenyl (Antr, Pyr) groups were introduced via different linker fragments into exo-4N of deoxycytidine or 5C of deoxyuridine. J-dU-containing DNA was used as a photoactive model of undamaged DNA strands. The reporter group was a fluorescein residue, introduced into the 5,-phosphate end of one duplex-forming DNA strand. RPA,dsDNA association constants and the molar RPA/dsDNA ratio have been calculated based on fluorescence anisotropy measurements under conditions of a 1:1 RPA/dsDNA molar ratio in complexes. The evident preference for RPA binding to ddsDNA over undamaged dsDNA distinctly depends on the adduct type and varies in the following way: undamaged dsDNA,<,Antr-dC-ddsDNA,<,mmdsDNA,<,FAPdU-, Pyr-dU-ddsDNA,<,FAP-dC-ddsDNA (KD,=,68,±,1; 25,±,6; 13,±,1; 8,±,2, and 3.5,±,0.5,nM correspondingly) but weakly depends on the chain integrity. Interestingly the bulkier lesions not in all cases have a greater effect on RPA affinity to ddsDNA. The experiments on photoaffinity modification demonstrated only p70 of compactly arranged RPA directly interacting with dsDNA. The formation of RPA,ddsDNA covalent adducts was drastically reduced when both strands of DNA duplex contained virtually opposite located FAP-dC and Antr-dC. Thus RPA requires undamaged DNA strand presence for the effective interaction with dsDNA bearing bulky damages and demonstrates the early NER factors characteristic features underlying strand discrimination capacity and poor activity of the NER system toward double damaged DNA. Copyright © 2008 John Wiley & Sons, Ltd. [source]


IgG binding kinetics to oligo B protein A domains on lipid layers immobilized on a 27,MHz quartz-crystal microbalance

JOURNAL OF MOLECULAR RECOGNITION, Issue 2 2007
Hideyuki Mitomo
Abstract Although molecular recognitions between membrane receptors and their soluble ligands have been analyzed using their soluble proteins in bulk solutions, molecular recognitions of membrane receptors should be studied on lipid membranes considering their orientation and dynamics on membrane surfaces. We employed Staphylococcal Protein A (SpA) oligo B domains with long trialkyl-tags from E. coli (LppBx, x,=,1, 2, and 5) and immobilized LppBx on lipid layers using hydrophobic interactions from the trialkyl-tag, while maintaining the orientation of B domain-chains on a 27,MHz quartz-crystal microbalance (QCM; AT-cut shear mode). The binding of IgG Fc regions to LppBx on lipid layers was detected by frequency decreases (mass increases) on the QCM. The maximum amount bound (,mmax), association constants (Ka), association and dissociation rate constants (k1 and k,1, respectively) were obtained. Binding kinetics of IgG to LppB2 and LppB5 were quite similar, showing a simple 1:1 binding of the IgG Fc region to the B domain, when the surface coverage of LppB2 and LppB5 on the lipid surface is low (1.4%). When LppB5 was immobilized at the high surface coverage of 3.5%, the complex bindings of IgG such as one IgG bound to one or two LppB5 on the membrane could be observed. IgG-LppB1 binding was largely restricted because of steric hindrance on lipid surfaces. This gives a suggestion why Protein A has five IgG binding domains. Copyright © 2006 John Wiley & Sons, Ltd. [source]


Self-assembly of daisy chain oligomers from heteroditopic molecules containing secondary ammonium ion and crown ether moieties

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 4 2010
Harry W. Gibson
Abstract MALDI-TOF MS of the heteroditopic compound 2 -(benzylammoniomethyl)dibenzo-24-crown-8 hexafluorophosphate (4) revealed oligomeric "daisy chain" species up to the hexamer. Similar results were obtained for 2-(6,-hydroxyhexylammoniomethyl)dibenzo-24-crown-8 hexafluorophosphate (8). The complexations of two substituted dibenzylammonium salts, 2,2,-dimethyldibenzylammonium hexaflurophosphate (9a) and 2,2,,5-trimethoxydibenzylammonium hexafluorophosphate (9b), with dibenzo-24-crown-8 were examined as models for slippage systems; association constants are reported for these systems. A crystal structure is reported for the new dimethylbenzylammonoium pseudorotaxane. The trimethoxy analog is shown to be capable of slippage formation of a rotaxane, albeit in low yield. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 975,985, 2010 [source]


Biocomplementary interaction behavior in DNA-like and RNA-like polymers

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 23 2009
Chih-Chia Cheng
Abstract A series of nucleobased polymers and copolymers were synthesized through atom transfer radical polymerization (ATRP). Biocomplementary DNA- and RNA-like supramolecular complexes are formed in dilute DMSO solution through nucleobase recognition. 1H NMR titration studies of these complexes in CDCl3 indicated that thymine-adenine (T-A) and uracil-adenine (U-A) complexes form rapidly on the NMR time scale with high association constants (up to 534 and 671 M,1, respectively) and result in significant Tg increase. WAXD and differential scanning calorimetry analyzes in the bulk state indicate the presence of highly physical cross-linked structures and provide further details into the nature of the self-assembly of these systems. Furthermore, this study is of discussion on the difference in the hydrogen bond strength between T-A and U-A base pairs within polymer systems, indicating that the strength of hydrogen bonds in RNA U-A pairs is stronger than that in DNA T-A base pairs. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6388,6395, 2009 [source]


Preparation of molecularly imprinted polymer microspheres via atom transfer radical precipitation polymerization

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2009
Baiyi Zu
Abstract The first combined use of atom transfer radical polymerization (ATRP) and precipitation polymerization in the molecular imprinting field is described. The utilized polymerization technique, namely atom transfer radical precipitation polymerization (ATRPP), provides MIP microspheres with obvious molecular imprinting effects towards the template, fast template binding kinetics and an appreciable selectivity over structurally related compounds. The living chain propagation mechanism in ATRPP results in MIP spherical particles with diameters (number-average diameter Dn , 3 ,m) much larger than those prepared via traditional radical precipitation polymerization (TRPP). In addition, the MIP microspheres prepared via ATRPP have also proven to show significantly higher high-affinity binding site densities on their surfaces than the MIP generated via TRPP, while the binding association constants Ka and apparent maximum numbers Nmax of the high-affinity sites as well as the specific template bindings are almost the same in the two cases. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3257,3270, 2009 [source]