Terms Of Association Constants Research Articles

Electrochemical behavior of superoxide anion radical towards quinones: a mechanistic approach

Among the reactive oxygen species, the superoxide anion radical (O 2 ·− ) has a fundamental role in several biological functions. Consequently, its interaction with several chemical moieties is equally important both from a biological and chemical point of view. Considering this importance, the present study demonstrates the electrochemical generation of O 2 ·− followed by its interaction with four different quinones. The fate of electrochemically generated O 2 ·− was probed in the presence of a quinone while monitoring the changes in the shape of its voltammogram. The nature of interaction was interpreted in terms of electrochemical mechanism and the interaction moiety was identified both from voltammetric and computational data (Mullikan charges). The chemical coupled reactions of the quinones with O 2 ·− were identified as hydrogen atom transfer, protonation and electron transfer in different cases. The strength of interaction was quantified in terms of association constant (Kao) and IC50 values for antiradical activity of quinones and compared with standards of vitamin C and quercetin. The kinetics of the interaction reaction was quantitated having second-order rate constant values 0.1 to 3.0 × 103 M−1 s−1.

Acid-base responsive probes for mercury(II) ions in aqueous solution

A simple ratiometric optical method for mercuric ion sensing has been developed using an ensemble of an indicator, such as 7-hydroxy-4-methylcoumarin (7H4MC) and bromothymol blue (BTB), and a receptor, such as N,N-bis(p-methoxybenzyl)-3,6-dioxaoctane-1,8-diamines (bis-pMDODA) and cryptand [2.2.2] (C222), in aqueous solution. The equilibrium between indicator and receptor is perturbed by adding Hg2+ ions (0.8–80 μM), due to the formation of a receptor-metal complex, resulting in a change of the UV/Vis spectra. Both sensitivity and selectivity are explained in terms of association constants between receptor and metal ions and equilibrium constants of the reaction between indicator and receptor. Indicator bromothymol blue has also presented a color variation from light blue to yellow upon coordination of Hg2+ to the receptor, which allows a metal naked-eye detection. The detection limit of the system was found to be about 3 μM and the relative standard deviation for five measurements of 20 μM concentration was 5.6%. BTB/7H4MC arrangement showed a particular selectivity over different interfering cations, suffering only interference from Cu2+ ions for Hg2+ sensing.

Spectroscopic study of charge transfer complexation between doxepin and π–acceptors and its application in quantitative analysis

Charge transfer complexes of the doxepin with π – acceptors such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetracyanoquinodimethane (TCNQ) and p-chloranilic acid (p-CA) have been studied spectrophotometrically. The stoichiometry of the reactions was studied by Job's method of continuous variations which show that the complexation ratio was 1:1 (acceptor: donor) for DDQ, TCNQ and p-chloranilic acid with doxepin base. The data were discussed in terms of association constant (K), molar absorptivity (ε), free energy change (ΔG), oscillator strength (f), transition dipole moment (μEN), resonance energy (RN) and ionization potential (ID). Three simple, fast and sensitive spectrophotometric methods were proposed for the quantification of doxepin hydrochloride in pure form and drug formulations. The methods (A, B and C) were based on the reaction of doxepin base as n-donor with DDQ, TCNQ and pCA resulting in coloured species which absorbed maximally at 570nm, 840nm and 530nm, respectively. Beer's law was obeyed in the concentration range of 5–45μgmL−1, 40–360μgmL−1, and 40–400μgmL−1 for the methods A, B, and C, respectively. A series of variables were studied to optimize the reaction conditions. The developed methods were successfully applied for the determination of doxepin in its formulations with high accuracy and precision and without interference from common excipients.

<b>Investigation of 1-alkanols in organised solutions</b>

Conductometric behaviour of 1-alkanols (C5-C10) in organised solutions of sodium dodecylbenzenesulfonate (SDBS) is investigated. Interaction of each alkanol with anionic surfactant is reflected in terms of association constants, Kc. It is observed that self-assembly of SDBS is induced by the alkanol addition. The depression in critical micelle concentration (CMC) of SDBS caused by each alkanol is translated to partition coefficient, Kc by using interaction coefficient. The dimensionless partition coefficient, Kx is utilized to highlight the energy efficiency of the solubilization process. The results indicate that even longer chain alkanols prefer interfacial area for their residence. The relative solubility of each alkanol is enhanced with increasing SDBS concentration. Such basic information could be vital for development of nano-scale assemblies for specific delivery of water soluble drugs.KEY WORDS: SDBS, Alcohols, Partition coefficient, Relative solubility, Conductometry Bull. Chem. Soc. Ethiop. 2011, 25(3), 469-474.

Structural Changes of eIF4E upon Binding to the mRNA 5′ Monomethylguanosine and Trimethylguanosine Cap

Recognition of the 5' cap by the eukaryotic initiation factor 4E (eIF4E) is the rate-limiting step in the ribosome recruitment to mRNAs. The regular cap consists of 7-monomethylguanosine (MMG) linked by a 5'-5' triphosphate bridge to the first transcribed nucleoside, while some primitive eukaryotes possess a N (2), N (2),7-trimethylguanosine (TMG) cap structure as a result of trans splicing. Mammalian eIF4E is highly specific to the MMG form of the cap in terms of association constants and thermodynamic driving force. We have investigated conformational changes of eIF4E induced by interaction with two cap analogues, 7-methyl-GTP and N (2), N (2),7-trimethyl-GTP. Hydrogen-deuterium exchange and electrospray mass spectrometry were applied to probe local dynamics of murine eIF4E in the apo and cap-bound forms. The data show that the cap binding induces long-range conformational changes in the protein, not only in the cap-binding pocket but also in a distant region of the 4E-BP/eIF4G binding site. Formation of the complex with 7-methyl-GTP makes the eIF4E structure more compact, while binding of N (2), N (2),7-trimethyl-GTP leads to higher solvent accessibility of the protein backbone in comparison with the apo form. The results suggest that the additional double methylation at the N (2)-amino group of the cap causes sterical effects upon binding to mammalian eIF4E which influence the overall solution dynamics of the protein, thus precluding formation of a tight complex.

Metal-promoted shift of equilibrium between the two fluorescent forms of a synthetic macrocycloureide in view of designing chemical sensors

The emergence of the fiber-optic sensor era has stimulated investigations to create synthetic fluororeceptors capable of signaling the binding of metal ions. To fulfill this purpose, a new concept is presented: the metal-promoted shift of equilibrium between the two fluorescent forms of a synthetic receptor able to dimerize and to specifically recognize a metal ion by multiple non-covalent bonds. The thermodynamics of the dimerization itself and the dimer-zinc interactions are analyzed in terms of association constants. It is demonstrated that finely tuning the equilibrium between the monomeric and dimeric forms by controlling the pH, permits the alternate selective recognition and signaling of zinc or cadmium.

Interaction between Squash Inhibitors and Bovine Trypsinogen

Squash seeds proteinase inhibitors form stoichiometric complexes with bovine trypsinogen. In terms of association constants (Ka), the interaction is weak. The inhibitors bind to the zymogen with Ka values of approx. 10(4)M-1 i.e. 2 X 10(7) times weaker than to bovine beta-trypsin. Squash inhibitor with Lys at the P1 position binds to trypsinogen with a Ka value 2.1-fold higher than the inhibitor with Arg at P1. The Ile-Val binding cleft and the Ca2+ binding site of trypsinogen are cooperatively linked to the inhibitor binding site. Although these three sites are spatially separated, either binding of calcium ion or Ile-Val dipeptide to trypsinogen increase the Ka values 3-fold and more than 100-fold, respectively. In the presence of Ile-Val trypsinogen resynthetizes extremely slowly (about 10(4) times slower than beta-trypsin) the reactive site peptide bond in squash inhibitors.

A study on the molecular complexes of crown ethers with polynitro fluorenones

The interaction of four crown ethers, benzo-15-crown-5, dibenzo-18-crown-6, dicyclohexyl-18-crown-6 and dibenzo-24-crown-8 with three nitrofluorenones has been studied using electronic absorption, infrared, 1 H NMR and 13 C NMR spectral methods. It is shown that association constants of 1 :1 complexes vary linearly with the number of oxygen's in the ethers. The molecular binding properties are discussed collectively in terms of association constants, charge transfer transition energies, intensities of these bands, enthalpies of complexation and electron affinities of the acceptors. Evidence has been presented that the dominant contributions to charge transfers arise from π — π interaction. Possible overlap structures have been proposed based on infrared, and NMR spectra of the complexes.

THE SPECIFICITY OF THE HUMAN UTERINE RECEPTOR

The aim of the present study was to assess the contribution of different parts of natural and synthetic steroid molecules in their binding to high affinity oestradiol receptor preparations obtained from whole human uteri. Fifty-five compounds were used in the study of which 38 contained the steroid nucleus. The affinity (in terms of association constants) of the compounds for the receptor was determined from competitive studies with radioactive oestradiol. As a consequence the compounds could be grouped according to their association constants for the receptor. The contribution of the individual functional groups of the steroid molecule to the binding process was analysed. The preliminary quantitative evaluation of the contribution was derived from the equation: log K=logdeltaKs + sigmalogdeltaKF where K8 is the contribution of the basic 1,3,5-(10)-oestratriene skeleton, KF the associated functional groups and K the affinity constant for the entire molecule. The main positive contribution in the binding is provided by skeleton and the 3-hydroxyl group. It is concluded that functional groups present at either the 3 or 17 position act independently of each other in the binding process. The possible synergism between the functional groups and the steroid skeleton is discussed.

The binding of phytohemagglutinin M to rat spleen lymphocytes. Quantitative studies.

Phytohemagglutinin M (PHAM) has been purified from the commercial mixture of proteins produced by Phaseolus vulgaris, using a Sepharose-thyroglobulin column. The protein gave one band on gel electrophoresis and two bands on SDS-gel electrophoresis (mol. wt. 33 700 and 32 100, respectively). Molecular weight determination by ultracentrifugation gave a value of 61 200 +/- 700. The protein had a minimum sugar content of 16%. Binding studies of PHAM to purified rat spleen lymphocytes have been performed at 0, 25, and 37 degrees C. It was shown that the cells bound about the same amount of lectin at 0 and 37 degrees C, but less protein was bound at 25 degrees C. The binding phenomenon showed saturability at all temperatures. Data were analyzed by Scatchard plots and two kinds of binding sites were found. High-affinity sites and low-affinity sites have been characterized in terms of association constants and (apparent) number. It was also shown that cells treated with trypsin or sodium azide bound less lectin. Bound concanavalin A did not appear to affect the amount of bound PHAM, but its influence was reflected in the value of the association constant for the binding of PHAM. Unlabelled PHAM was shown to displace radioactive PHAM from the cells, but could not remove bound concanavalin A. The significance of these results is discussed in terms of the fluid plasma membrane model and cellular metabolism.