Estimation Of Association Constants Research Articles

Synthesis of cyclodextrins-metronidazole inclusion complexes and incorporation of metronidazole - 2-hydroxypropyl-β-cyclodextrin inclusion complex in chitosan nanoparticles

Inclusion complexes (ICs) of metronidazole (MTZ) with β-cyclodextrin (BCD) and its derivative 2-hydroxypropyl-β-cyclodextrin (HPBCD) were prepared to improve its apparent solubility. A 1:1 stoichiometry has been proposed for both ICs according to the Job's plots. The estimation of association constants and thermodynamic parameters demonstrated that the incorporation of MTZ in both cavities of cyclodextrins (CDs) is a spontaneous exothermic process. Nuclear magnetic resonance spectroscopy (NMR), two-dimensional (2D) NMR, Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) investigations were used to ascertain the structure of ICs and that prove their formation. Thermal analysis showed that the thermal stability of MTZ was improved in the presence of both CDs. The in vitro drug release of ICs showed a considerable increase in the dissolution rate in comparison with the pure MTZ. An improvement of antibacterial activity was observed after encapsulation, especially for MTZ-HPBCD IC. Then chitosan (CS) nanoparticles (NPs) were prepared from the preformed MTZ-HPBCD IC by ionic gelation method. FTIR confirmed that MTZ-HPBCD IC has successfully entrapped in CS NPs. Transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM) images and dynamic light scattering (DLS) show that the NPs have a spherical shape with a uniform diameter. The in vitro release profile of NPs showed a controlled-release of MTZ suggesting that such MTZ-HPBCD IC-loaded CS NPs can be an efficient alternative to modify the MTZ release and increase its solubility.

Computational Studies on Two Supramolecular Structures: Cyclodextrins and Rotaxanes

Supramolecular structures are formed by holding two or more molecules without covalent bonds. Although rotaxanes are not supramolecular species but real molecules, the interactions existing between the fragments composing rotaxanes are of the same type of those in supramolecular species. Rotaxanes and cyclodextrins are the two structures being considered in this review. The contribution of the authors in the study of these two classes of compounds by computational techniques will be presented. Aspects like geometry of the complex (bimodality), estimation of association constants, conformational changes produced by the complexation, enantiodifferentiation, changes in the molecular reactivity, formation of aggregates, shuttling of the rotaxanes, and the energetic aspects and the geometry of cyclodiastereomeric rotaxanes are also discussed. Keywords: monosubstituted benzene, MD simulations, NMR, Enantiodifferentiation, Cyclodiastereomeric [3]rotaxanes, Cyclodextrine

Estimation of association constants between oral malodor components and various native and derivatized cyclodextrins

The association constants of 33 oral malodorous compounds and odor precursors (9 organic acids, 7 amine-containing bases, 11 organic neutral and aromatic compounds, and 6 amino acids) with native and derivatized cyclodextrins were measured using one or more of a variety of techniques including affinity capillary electrophoresis, nuclear magnetic resonance titrations, and head-space gas chromatography. With the exception of formic acid and urea, which had binding constants that were too small to measure, all analytes showed significant binding to at least one of the cyclodextrins studied. In most cases, the native cyclodextrins exhibited the most stable complexes with these analytes. However, with cationic analytes under acidic conditions, the negatively charged sulfated and carboxymethyl cyclodextrins had higher association constants. The six amino acid precursor molecules only bound significantly with the sulfated cyclodextrins. In addition, several analyte–cyclodextrin combinations were observed to form insoluble complexes, indicating that these cyclodextrins are particularly effective at extracting these compounds from aqueous solution.

Estimation of Association Constants of Macrocyclic Ethers by Potentiometry. Part V. Na+ Binding in Dioxane/Water (80/20)

The various association constants, βnm and Knm , of NaCl and NaClO4 with [12]crown-4, [15]crown-5, [18]crown-6, benzo[15]crown-5, benzo[12]crown-4, dibenzo[18]-crown-6 and dinaphtho[18]crown-6 have been estimated in a mixture of dioxane/water (80/20) using a sodium ion selective electrode (ISE). The constants have been estimated using where P′, the mole fraction of complexed Na+in the presence of identical cation/ligand concentrations, is obtained for different aggregation–dissociation possibilities. The association constants of the complexes are reported and discussed in terms of the multiple composition due to strong dioxane/water binding. The binding mechanism is shown to depend on the free cation and counter ion that are formed by different ratio macrocycle–metal complexes in dioxane/water (80/20) solutions.

Ortho-aminobenzoic acid as a fluorescent probe for the interaction between peptides and micelles

Ortho-aminobenzoic acid ( o-Abz) has been used as a fluorescent probe in internally quenched fluorescent peptides for continuous protease assays. We investigated the fluorescent properties of the probe in order to verify if it can be used to monitor the interaction of peptides with micelles. Abz-aminoacyl-monomethyl amides (Abz-Xaa-NHCH 3, where Xaa=Arg, Phe, Leu and Glu) were synthesized. Quantum yield, spectral position, anisotropy and lifetime decay were analyzed in the presence and absence of sodium dodecyl sulfate (SDS) micelles. Significant changes in the fluorescence parameters were observed for Abz-Arg-NHCH 3 in comparison to Abz-Glu-NHCH 3, indicating a strong electrostatic component in the compound's interaction with the negative charged micelles. The change in fluorescence parameters, observed when the probe is bound to hydrophobic amino acids Abz-Phe-NHCH 3 and Abz-Leu-NHCH 3, is probably due to insertion of those compounds into micelles. Abz-NHCH 3 fluorescence is less affected by the presence of micelles, indicating that the occurrence of interaction is dependent on the properties of the amino acid to which the fluorophore is attached. The quenching data with acrylamide confirmed these results. Titration curves allowed the estimation of association constants between Abz compounds and SDS, according to a single partition model. Although the results cannot be strictly applied to the titration with charged compounds, it was verified that the association constant for the isolated Abz-NHCH 3 is significantly lower than those for Abz-Phe-NHCH 3 and Abz-Leu-NHCH 3. It is concluded that the Abz group is a sensitive and convenient fluorescent probe to monitor peptide binding to amphiphilic aggregates. That conclusion is supported by measurements with the peptide Abz-Leu-Arg-Phe-NH 2. © 1998 Elsevier Science B.V. All rights reserved.

Determination of Association Constants in Cyclodextrin or Vancomycin-Modified Micellar Capillary Electrophoresis

The estimation of association constants of chiral analytes to chiral selectors is often useful in determining the nature and extent of interactions that exist between them. These determinations also provide insight into the mechanisms that are involved in the enantiorecognition of these analytes. Simple models have been developed previously to estimate association constants of analytes binding through a 1:1 stoichiometry with a ligand. These models have also been extrapolated to explain the binding behavior of analytes in systems containing two ligands. In the present study, the binding behavior of some chiral analytes was studied in micellar capillary electrophoresis (CE) systems modified with hydroxypropyl-β-cyclodextrin (HPBCD) or vancomycin as the chiral selectors. The pseudophase or three phase model was used as the basis for the determination of the absolute binding constants of the solutes to the selector and to the micellar phases.

Cyclodextrin/weak-electrolyte complexation: interpretation and estimation of association constants from phase solubility diagrams.

Cyclodextrin/weak-electrolyte complexation: interpretation and estimation of association constants from phase solubility diagrams.

The molecular basis for complexation of adriamycin with flavin mononucleotide and flavin adenine dinucleotide

The interaction of Adriamycin with FMN or FAD was characterized using proton nuclear magnetic resonance chemical shift, linewidth, and T 1 relaxation rate measurements. Adriamycin produced saturable, concentration-dependent upfield shifts of the proton signals of both FMN and FAD. Differential changes in the magnitude of the upfield chemical shifts of the various proton signals of the flavin nucleotides in the presence of Adriamycin were also observed. The greatest change in chemical shift produced by Adriamycin occurred for the overlapping H-6, H-9 proton signals of the isoalloxazine ring of FMN followed by the 8α, 7α methyl group and aliphatic side chain signals, respectively. Qualitatively similar results were observed for FAD where changes in chemical shift in the presence of Adriamycin were such that H-6 > H-9 > 8α,7α-CH 3 > adenosine H-8 > adenosine H-1′. Concomitant with the Adriamycin-induced changes in chemical shift were increases in the linewidth ( ν 1 2 ) and longitudinal relaxation rate, T 1 −1. Variable temperature studies demonstrated the reversibility of Adriamycin-flavin nucleotide interaction and the presence of rapid exchange between free and complexed species. These results suggest complex formation between Adriamycin and FMN or FAD via π-π ring stacking interactions occurring between the anthracycline ring of Adriamycin and the isoalloxazine ring of the flavins. Analysis of spectroscopic changes revealed the predominance of 1:1 stoichiometry of complex formation and allowed estimation of association constants and thermodynamic parameters. The results indicate that Adriamycin is effective in forming specific complexes with each flavin nucleotide and with comparable association constants.

The binding of thiopental to serum proteins determined by ultrafiltration and equilibrium dialysis.

An ultrafiltration method is described by which it is possible to estimate the protein bound fraction of a drug at its original concentration in a serum sample. The determination is carried out at 37 degrees and pH 7.4. For thiopental no difference was found between the values of protein binding whether they were determined by the ultrafiltration method or by a less time consuming equilibrium dialysis against an equal volume of phosphate buffered isotonic sodium chloride solution. The equilibrium dialysis was used to measure the concentration of bound and unbound thiopental molecules and the binding parameters in a two class binding model were determined. No evidence was found for binding to other proteins than albumin. About one binding site belonging to the first class was found per 1000 albumin molecules whereas an average of about 5 secondary sites were found for each albumin molecule. The association constants for the primary class of binding sites were 3.4 X 10(6)M-1 for albumin and 13.3 X 10(6)M-1 for serum while the values for the secondary class were 2.2 and 1.2 X 10(3)M-1 for albumin and serum respectively. The estimates of association constants and number of binding sites were based on experiments with total thiopental concentrations ranging from 0.4 to 80 microgram/ml. In a phosphate buffered albumin solution with 2 g albumin per 100 ml, a pH increase from 5 to 8 caused an increase in protein binding from 36 to 76%. A Scatchard plot using data from experiments with increasing albumin concentrations resulted in a "plot" with positive slope. The use of a curve like this is discussed and it is concluded that the binding parameters for thiopental are influenced by the albumin concentration.

Solvency and Hydrogen Bonding Interactions in Nonaqueous Systems

A study of the importance of hydrogen-bonding interactions in the formation of complexes is useful in providing knowledge of the physical and chemical properties of drug molecules and hopefully it will eventually lead to rational formulation of drugs into suitable dosage forms. The present report deals with a study of some of the methods available for the experimental measurement of these interactions and the means of determining the association (equilibrium) constants from the experimental results. An attempt is made to develop suitable methods for the quantitative analysis of hydrogen-bonding data so that useful estimates of association constants can be made a priori. The effect of the non-aqueous solvent on the value of the association constant is also discussed and a method is given whereby an estimate of solvent interaction can be calculated.

Influence of DNA secondary structure on DNA-dependent polypeptide synthesis

Polypeptide synthesis in a soluble protein plus ribosome system from Escherichia coli is markedly stimulated by addition of purified RNA polymerase and T2 DNA. Heat-denatured T2 DNA and single-stranded ΦX174 DNA produce no stimulation, suggesting that a bihelical DNA secondary structure is required for activity. Renaturation of denatured T2 DNA by annealing at 55°C or by replication with DNA polymerase restores the ability to stimulate polypeptide formation; ΦX174 DNA also becomes activated upon conversion to a bihelical structure by replication. Moreover, neither of the isolated single strands of phage α DNA is active alone, but renaturation of the two fractions together produces an active material. Enzymically synthesized RNA's directed by single- and double-stranded DNA also differ markedly in their ability to stimulate polypeptide synthesis. The physical basis for this difference may lie in the smaller size of the former RNA, observed in zone sedimentation experiments, since the more slowly sedimenting components of RNA directed by native T2 DNA are least active. Single-stranded DNA strongly inhibits the stimulation of polypeptide synthesis by native T2 DNA in the presence of RNA polymerase. This effect can be explained by the higher affinity of RNA polymerase for the inactive single-stranded DNA. Estimation of association constants by a kinetic method employing DNA as a competitive inhibitor of the synthesis, directed by the copolymer deoxyadenylate-deoxythymidylate, of riboadenylate-ribouridylate copolymer, indicates that the enzyme binds single-stranded DNA up to 230 times more strongly than native bihelical DNA.