Rate Constants For Complex Formation Research Articles

Investigation and Comparison of Specific Antibodies' Affinity Interaction with SARS-CoV-2 Wild-Type, B.1.1.7, and B.1.351 Spike Protein by Total Internal Reflection Ellipsometry.

SARS-CoV-2 vaccines provide strong protection against COVID-19. However, the emergence of SARS-CoV-2 variants has raised concerns about the efficacy of vaccines. In this study, we investigated the interactions of specific polyclonal human antibodies (pAb-SCoV2-S) produced after vaccination with the Vaxzevria vaccine with the spike proteins of three SARS-CoV-2 variants of concern: wild-type, B.1.1.7, and B.1.351. Highly sensitive, label-free, and real-time monitoring of these interactions was accomplished using the total internal reflection ellipsometry method. Thermodynamic parameters such as association and dissociation rate constants, the stable immune complex formation rate constant (kr), the equilibrium association and dissociation (KD) constants and steric factors (Ps) were calculated using a two-step irreversible binding mathematical model. The results obtained show that the KD values for the specific antibody interactions with all three types of spike protein are in the same nanomolar range. The KD values for B.1.1.7 and B.1.351 suggest that the antibody produced after vaccination can successfully protect the population from the alpha (B.1.1.7) and beta (B.1.351) SARS-CoV-2 mutations. The steric factors (Ps) obtained for all three types of spike proteins showed a 100-fold lower requirement for the formation of an immune complex when compared with nucleocapsid protein.

Kinetics and the difference for extraction of praseodymium and neodymium from nitrate aqueous solution by [A336][NO3] using the single drop technique

The kinetics and the difference for Pr(III) and Nd(III) extraction from nitrate aqueous solution using trialkylmethylammonium nitrate ([A336][NO 3 ]) as extractant were investigated by the single drop technique. The dependence of the extraction rate of Pr(III) and Nd(III) on the concentrations of free Pr(III) and Nd(III) ions, the concentrations of NaNO 3 and H + in aqueous solutions, and the concentrations of [A336][NO 3 ] in kerosene solutions were discussed and the corresponding extraction rate equations for Pr(III) and Nd(III) were obtained. These equations demonstrated that the reaction rate constant of Pr(III) with [A336][NO 3 ] was double than that of Nd(III). The effect of the addition of diethylenetriaminepentaacetic acid (DTPA) on the difference in the extraction rate of Pr(III) and Nd(III) by [A336][NO 3 ] was also investigated. It was revealed that the difference in the complex formation rates of Pr(III) and Nd(III) with DTPA made a significant impact on the difference in the extraction rates of Pr(III) and Nd(III) with [A336][NO 3 ]. The ratio of extraction rates of Pr(III) to Nd(III) with [A336][NO 3 ] was in proportion to the ratio of complex formation rates of Pr(III) to Nd(III) with DTPA. The extraction rate difference for Pr(III) and Nd(III) with [A336][NO 3 ] increased due to a higher complex formation rate constant of DTPA with the free and un-complexed Nd(III) ions in the aqueous nitrate solution than that with Pr(III) ions. Therefore, the addition of DTPA in the aqueous nitrate solution is an effective method to intensify the separation of Pr(III) and Nd(III) in kinetics. The study on the extraction mechanism indicated that both the extraction of Pr(III) and Nd(III) by [A336][NO 3 ] were diffusion controlled, and the reactions obeyed S N 2 mechanism. The present work highlights a possible approach to strengthen the kinetic separation of Pr(III) and Nd(III). • The rate equations of Pr(III) and Nd(III) extracted by [A336][NO 3 ] are obtained. • Difference in the extraction rate of Pr(III) and Nd(III) increased due to the addition of DTPA. • Difference in the complexing rate of DTPA with Pr(III) and Nd(III) play a crucial role to intensify kinetic separation. • Extraction of Pr(III) and Nd(III) by [A336][NO 3 ] obeys S N 2 mechanism.

Effects of Discriminator Changes on Open Complex Formation, Stabilization, and Transcription Initiation

The discriminator is the region of promoter DNA between the key −10 recognition region for RNA polymerase (RNAP) and the transcription start site (TSS). The length, sequences of the discriminator, and the interactions between RNAP mobile elements and promoter DNA the discriminator direct, are major determinants of the lifetime and the structure of the RNAP-promoter open complex (OC). The OC lifetime and rate of formation can both vary by ∼4 orders of magnitude for different promoter sequences, and together regulate the rate of initiation and escape of RNAP from these promoters. We are investigating a series of discriminators including those of the long-lived λPR OC (GGTTGC) and the short-lived rrnBP1 OC (GCGCCACC), in the context of the upstream λPR promoter and a modified λPR transcribed region. Fast kinetic methods with filter binding detection are used to determine overall rate constants for open complex formation and dissociation, and to dissect contributions to these rate constants from initial equilibria involving closed or open complexes (CC or OC), and the rate-determining isomerizations step between them. Our results to date indicate that the discriminator affects primarily the steps that stabilize the initial OC, and has only limited effects on the DNA closing rate constant or on the overall second order rate constant for OC formation. From previous studies, interactions of in-cleft (ss) DNA and downstream (ds) DNA with RNAP change in the process of stabilization of the initial OC. Regions 1.1 and 1.2 of σ70 and downstream mobile elements of core RNAP are involved, and the orientation of the downstream duplex is changed. We are investigating these interactions and conformational changes for different discriminators in more detail, and the relationship to initiation and promoter escape of RNAP.

Employing Luminescence to Determine Eu-DTPA Complex Formation Rate Constants in Lactate and Citrate Media: Experiment and Aggregate-Species Kinetic Modelling

ABSTRACT The formation kinetics for DTPA binding to europium ions in aqueous media were determined using the change in photoluminescence. The influence of lactic acid and citric acid on these kinetics were also investigated. The total formation rate constant of DTPA complexation with europium ions was strongly dependent upon the pH and DTPA speciation, with values ranging from 5 × 103 M–1s–1 to >1 × 106 M–1s–1 over the pH range 1.5 to 2.3. The ligand-metal binding in the presence of lactate and citrate ions showed an inverse kinetic behavior, with strongly decreasing reaction rates at higher buffer concentrations and pH.

Kinetic Influence of Surface Charge Transfer Reactions Preceded by Non‐Electrochemical Processes on the Response in Cyclic Voltammetry

AbstractA general analytical explicit expression for the response of a surface charge transfer preceded by a non‐electrochemical process has been deduced. This solution is applicable to any electrochemical technique independent of the rates of both reactions. Interesting simplified cases are obtained for very common experimental situations in which the rates of electrochemical and non‐electrochemical steps are similar. These expressions have been applied to cyclic voltammetry in order to analyze the kinetic influences on the current‐potential response and to propose procedures for determining the rate and equilibrium constants of the mechanism. Moreover, it has been obtained an “apparent” rate constant of the overall process that reduces to a pure non‐electrochemical rate‐limiting process or to a pure electrochemical rate limiting one, under the right conditions. Experimental verification of the theoretical framework has been done with the analysis of the host‐guest complexation between a surface confined electroactive ferrocene moiety and a solution soluble cyclodextrin in a water‐ethanol media. From the dynamic analysis of the influence of the scan rate on the cyclic voltammetry curves, values have been obtained for the binding constant ( ) and rate constants for the inclusion complex formation and dissociation showing the great stability of the electro‐inactive ferrocene‐cyclodextrin complex.

Coordination Chemistry of Cu2+ Complexes of Small N-Alkylated Tetra-azacyclophanes with SOD Activity.

A new tetraaza-pyridinophane macrocycle (L1) N-alkylated with two isopropyl and one methyl groups symmetrically disposed has been prepared and its behavior compared with those of the unsubstituted pyridinophane (L3) and the related compound with three methyl groups (L2). The protonation studies show that, first, a proton binds to the central methylated amine group of L1, while, second protonation leads to a reorganization of the protons that are at this stage attached to the lateral isopropylated amines. The X-ray structure of [HL1]+ agrees with the UV-vis and NMR studies as well as with the results of DFT calculations. The stability of the Cu2+ complexes decreases on increasing the bulkiness of the alkyl substituents of the amine groups. The crystal structures of [CuL1Cl](ClO4) and [CuL1(H2O)](ClO4)2·H2O show square pyramidal coordination geometries with the ligands disposed in a bent L-shaped conformation. Kinetic studies indicate that the rates of both complexation and ligand dissociation decrease with the bulkiness of the substituents, so that the stability changes are surely the results of compensating effects, complex formation dominating over complex dissociation. The pH dependence of the rate constants for complex formation cannot be explained by consideration of rapid pre-equilibria involving the different protonated forms of the ligand, and it has been interpreted in terms of a mechanism involving an acid-base equilibrium for a reaction intermediate. NBT SOD studies show that the Cu2+ complex of the bulkiest L1 ligand is the one having the highest activity (IC50 = 0.26(5) μM, kcat = 13.7 × 106 M-1 s-1) which can be associated with the poorer σ-donor ability of the tertiary amino groups, and the rigidity of the system, caused by the bulky isopropyl groups.

Reliable Strategy for Analysis of Complex Biosensor Data

When using biosensors, analyte biomolecules of several different concentrations are percolated over a chip with immobilized ligand molecules that form complexes with analytes. However, in many cases of biological interest, e.g., in antibody interactions, complex formation steady-state is not reached. The data measured are so-called sensorgram, one for each analyte concentration, with total complex concentration vs time. Here we present a new four-step strategy for more reliable processing of this complex kinetic binding data and compare it with the standard global fitting procedure. In our strategy, we first calculate a dissociation graph to reveal if there are any heterogeneous interactions. Thereafter, a new numerical algorithm, AIDA, is used to get the number of different complex formation reactions for each analyte concentration level. This information is then used to estimate the corresponding complex formation rate constants by fitting to the measured sensorgram one by one. Finally, all estimated rate constants are plotted and clustered, where each cluster represents a complex formation. Synthetic and experimental data obtained from three different QCM biosensor experimental systems having fast (close to steady-state), moderate, and slow kinetics (far from steady-state) were evaluated using the four-step strategy and standard global fitting. The new strategy allowed us to more reliably estimate the number of different complex formations, especially for cases of complex and slow dissociation kinetics. Moreover, the new strategy proved to be more robust as it enables one to handle system drift, i.e., data from biosensor chips that deteriorate over time.

Nanoscopic Approach to Quantification of Equilibrium and Rate Constants of Complex Formation at Single-Molecule Level.

Equilibrium and rate constants are key descriptors of complex-formation processes in a variety of chemical and biological reactions. However, these parameters are difficult to quantify, especially in the locally confined, heterogeneous, and dynamically changing living matter. Herein, we address this challenge by combining stimulated emission depletion (STED) nanoscopy with fluorescence correlation spectroscopy (FCS). STED reduces the length-scale of observation to tens of nanometres (2D)/attoliters (3D) and the time-scale to microseconds, with direct, gradual control. This allows one to distinguish diffusional and binding processes of complex-formation, even at reaction rates higher by an order of magnitude than in confocal FCS. We provide analytical autocorrelation formulas for probes undergoing diffusion-reaction processes under STED condition. We support the theoretical analysis of experimental STED-FCS data on a model system of dye-micelle, where we retrieve the equilibrium and rates constants. Our work paves a promising way toward quantitative characterization of molecular interactions in vivo.

Influence of pH and ionic strength on electrostatic properties of ferredoxin, FNR, and hydrogenase and the rate constants of their interaction

Ferredoxin (Fd) protein transfers electrons from photosystem I (PSI) to ferredoxin:NADP+-reductase (FNR) in the photosynthetic electron transport chain, as well as other metabolic pathways. In some photosynthetic organisms including cyanobacteria and green unicellular algae under anaerobic conditions Fd transfers electrons not only to FNR but also to hydrogenase—an enzyme which catalyzes reduction of atomic hydrogen to H2. One of the questions posed by this competitive relationship between proteins is which characteristics of thylakoid stroma media allow switching of the electron flow between the linear path PSI-Fd-FNR-NADP+ and the path PSI-Fd-hydrogenase-H2. The study was conducted using direct multiparticle simulation approach. In this method protein molecules are considered as individual objects that experience Brownian motion and electrostatic interaction with the surrounding media and each other. Using the model we studied the effects of pH and ionic strength (I) upon complex formation between ferredoxin and FNR and ferredoxin and hydrogenase. We showed that the rate constant of Fd-FNR complex formation is constant in a wide range of physiologically significant pH values. Therefore it can be argued that regulation of FNR activity doesn’t involve pH changes in stroma. On the other hand, in the model rate constant of Fd-hydrogenase interaction dramatically depends upon pH: in the range 7–9 it increases threefold. It may seem that because hydrogenase reduces protons it should be more active when pH is acidic. Apparently, regulation of hydrogenase’s affinity to both her reaction partners (H+ and Fd) is carried out by changes in its electrostatic properties. In the dark, the protein is inactive and in the light it is activated and starts to interact with both Fd and H+. Therefore, we can conclude that in chloroplasts the rate of hydrogen production is regulated by pH through the changes in the affinity between hydrogenase and ferredoxin.

Enhanced visible-light photocatalytic activity of a TiO2 hydrosol assisted by H2O2: Surface complexation and kinetic modeling

Although the complexation of H2O2 onto TiO2 particles can form a colored surface with enhanced visible-light photocatalytic activity, the properties and behavior of this complex (>Ti-OOH) have not been thoroughly characterized. In this study, rather than using TiO2 powder, a nanoscale TiO2 hydrosol was used because it is easily measured using spectrometric methods. The results indicated that the addition of H2O2 to the TiO2 hydrosol greatly enhanced the photocatalytic degradation of methylene blue (MB) under visible light irradiation. Although the presence of O2 slightly affected the photocatalytic activity, the presence of dissolved H2O2 markedly enhanced the photocatalytic activity. Furthermore, the results demonstrated that the consumption of dissolved H2O2 may account for the decrease in MB degradation after repeated cycles. The kinetics of the formation of >Ti-OOH were examined using UV–vis spectroscopy and the stopped-flow spectral system, and the adsorption–desorption constant (714M−1) and the complex formation rate constants ((1.01±0.13)×104M−1s−1 and 14.1±1.2s−1) are reported for the first time. Based on the MB degradation and >Ti-OOH formation results, a kinetic model composed of well-known key reactions was established. By fitting the experimental data to the model, the forward and backward rate constants for the photo-excitation of >Ti-OOH were determined to be (3.5±0.9)×10−5s−1 and (6.1±2.0)×1013M−1s−1, respectively.

Determination of equilibrium and rate constants for complex formation by fluorescence correlation spectroscopy supplemented by dynamic light scattering and Taylor dispersion analysis.

The equilibrium and rate constants of molecular complex formation are of great interest both in the field of chemistry and biology. Here, we use fluorescence correlation spectroscopy (FCS), supplemented by dynamic light scattering (DLS) and Taylor dispersion analysis (TDA), to study the complex formation in model systems of dye-micelle interactions. In our case, dyes rhodamine 110 and ATTO-488 interact with three differently charged surfactant micelles: octaethylene glycol monododecyl ether C12E8 (neutral), cetyltrimethylammonium chloride CTAC (positive) and sodium dodecyl sulfate SDS (negative). To determine the rate constants for the dye-micelle complex formation we fit the experimental data obtained by FCS with a new form of the autocorrelation function, derived in the accompanying paper. Our results show that the association rate constants for the model systems are roughly two orders of magnitude smaller than those in the case of the diffusion-controlled limit. Because the complex stability is determined by the dissociation rate constant, a two-step reaction mechanism, including the diffusion-controlled and reaction-controlled rates, is used to explain the dye-micelle interaction. In the limit of fast reaction, we apply FCS to determine the equilibrium constant from the effective diffusion coefficient of the fluorescent components. Depending on the value of the equilibrium constant, we distinguish three types of interaction in the studied systems: weak, intermediate and strong. The values of the equilibrium constant obtained from the FCS and TDA experiments are very close to each other, which supports the theoretical model used to interpret the FCS data.

Kinetic size-exclusion chromatography with mass spectrometry detection: an approach for solution-based label-free kinetic analysis of protein-small molecule interactions.

Studying the kinetics of reversible protein-small molecule binding is a major challenge. The available approaches require that either the small molecule or the protein be modified by labeling or immobilization on a surface. Not only can such modifications be difficult to do but also they can drastically affect the kinetic parameters of the interaction. To solve this problem, we present kinetic size-exclusion chromatography with mass spectrometry detection (KSEC-MS), a solution-based label-free approach. KSEC-MS utilizes the ability of size-exclusion chromatography (SEC) to separate any small molecule from any protein-small molecule complex without immobilization and the ability of mass spectrometry (MS) to detect a small molecule without a label. The rate constants of complex formation and dissociation are deconvoluted from the temporal pattern of small molecule elution measured with MS at the exit from the SEC column. This work describes the concept of KSEC-MS and proves it in principle by measuring the rate constants of interaction between carbonic anhydrase and acetazolamide.

CYCLIC VOLTAMMETRY OF THE SECOND ORDER CE MECHANISM AT THE THIN MERCURY FILM COVERED STATIONARY PLANAR ELECTRODE

A theoretical model of electrode reaction influenced by the kinetics of complex formation and dissociation is developed for cyclic voltammetry. It is assumed that the electro-reductions of free metal ions and the complex are reversible and totally irreversible, respectively, and that the ligand is not present in great excess. The results show that the stripping peak current and potential depend on the relative bulk concentration of ligand, particularly within the range 1 < c*L,tot / c*M.tot < 3, and on the rate constant of complex formation.

Investigation on the Synergistic Complexation of Ni(II) with 1,10-Phenanthroline and Dithizone at Hexane-Water Interface Using Centrifugal Liquid Membrane-Spectrophotometry

Complex formation of Ni(II) and 1,10-phenanthroline (C12H8N2/Phen) with the addition of dithizone (C13H12N4S/HDz) at the hexane-water interface has been studied by direct measurement spectrophotometry using the centrifugal liquid membrane (CLM) method. Ni(II) ion with Phen formed a cationic complex of Ni(C12H8N2)2 2+ or NiPhen2 2+ . That complex dissolved in the aqueous phase and had two UV absorption spectrum maxima wavelengths, λmax 270 and 292 nm. Observation of complex formation was performed variations of pH and ligand concentration. The pH caused protonation that affected the amount of the formed complex. With the variations of ligand concentrations, the greater was the concentration of ligands the greater was the formed complex. Based on the Batch method, the HDz ligand addition into the NiPhen2 2+ cationic complex produced ion association complex of Ni(C13H11N4S)2(C12H8N2) or NiDz2Phen at λmax 403 nm, and was extracted in the organic phase. Measurement results using CLM method showed that NiDz2Phen complex was formed at hexane-water interface with λmax 523 nm. Comparison of Phen with HDz ligand concentrations affected the initial formation rate of NiDz2Phen complex. The greater concentration of Phen ligand increased the initial rate of formation for synergistic complex. The obtained data using CLM method indicated that the synergistic complex formation rate constant of NiDz2Phen at the interface, k was 0.30/s.

Chemodynamics of Metal Complexation by Natural Soft Colloids: Cu(II) Binding by Humic Acid

The chemodynamics of Cu(II) complexation by humic acid is interpreted in terms of recently developed theory for permeable charged nanoparticles. Two opposing electric effects are operational with respect to the overall rate of association, namely, (i) the conductive enhancement of the diffusion of Cu(2+), expressed by a coefficient f(el), which accounts for the accelerating effect of the negative electrostatic field of the humic particle on the diffusive transport of metal ions toward it, and (ii) the ionic Boltzmann equilibration with the bulk solution, expressed by a factor f(B), which quantifies the extent to which Cu(2+) ions accumulate in the negatively charged particle body. These effects are combined in the probability of outer-sphere metal-site complex formation and the covalent binding of the metal ion by the complexing site (inner-sphere complex formation) as in the classical Eigen mechanism. Overall "experimental" rate constants for CuHA complex formation, k(a), are derived from measurements of the thermodynamic stability constant, K*, and the dissociation rate constant, k(d)*, as a function of the degree of metal ion complexation, θ. The resulting k(a) values are found to be practically independent of θ. They are also compared to theoretical values; at an ionic strength of 0.1 mol dm(-3), the rate of diffusive supply of metal ions toward the particles is comparable to the rate of inner-sphere complex formation, indicating that both processes are significant for the observed overall rate. As the ionic strength decreases, the rate of diffusive supply becomes the predominant rate-limiting process, in contrast with the general assumption made for complexes with small ligands that inner-sphere dehydration is the rate-limiting step. The results presented herein also resolve the discrepancy between experimentally observed and predicted dissociation rate constants based on the above assumption.

The kinetics of lanthanide complexation by EDTA and DTPA in lactate media

The interaction of trivalent lanthanide and actinide cations with polyaminopolycarboxylic acid complexing agents in lactic acid buffer systems is an important feature of the chemistry of the TALSPEAK process for the separation of trivalent actinides from lanthanides. To improve understanding of metal ion coordination chemistry in this process, the results of an investigation of the kinetics of lanthanide complexation by ethylenediamine-N,N,N',N'-tetraacetic acid (EDTA) and diethylenetriamine-N,N,N',N'',N''-pentaacetic acid (DTPA) in 0.3 M lactic acid/0.3 M ionic strength solution are reported. Progress of the reaction was monitored using the distinctive visible spectral changes attendant to lanthanide complexation by the colorimetric indicator ligand Arsenazo III, which enables the experiment but plays no mechanistic role. Under the conditions of these experiments, the reactions occur in a time regime suitable for study by stopped-flow spectrophotometric techniques. Experiments have been conducted as a function of EDTA/DTPA ligand concentration, total lactic acid concentration, and pH. The equilibrium perturbation reaction proceeds as a first order approach to equilibrium over a wide range of conditions, allowing the simultaneous determination of complex formation and dissociation rate constants. The rate of the complexation reaction has been determined for the entire lanthanide series (except Pm(3+)). The predominant pathway for lanthanide-EDTA and lanthanide-DTPA dissociation is inversely dependent on the total lactate concentration; the complex formation reaction demonstrates a direct dependence on [H(+)]. Unexpectedly, the rate of the complex formation reaction is seen in both ligand systems to be fastest for Gd(3+). Correlation of these results indicates that in 0.3 M lactate solutions the exchange of lanthanide ions between lactate complexes and the polyaminopolycarboxylate govern the process.

Revealing Equilibrium and Rate Constants of Weak and Fast Noncovalent Interactions

Rate and equilibrium constants of weak noncovalent molecular interactions are extremely difficult to measure. Here, we introduced a homogeneous approach called equilibrium capillary electrophoresis of equilibrium mixtures (ECEEM) to determine k(on), k(off), and K(d) of weak (K(d) > 1 μM) and fast kinetics (relaxation time, τ < 0.1 s) in quasi-equilibrium for multiple unlabeled ligands simultaneously in one microreactor. Conceptually, an equilibrium mixture (EM) of a ligand (L), target (T), and a complex (C) is prepared. The mixture is introduced into the beginning of a capillary reactor with aspect ratio >1000 filled with T. Afterward, differential mobility of L, T, and C along the reactor is induced by an electric field. The combination of differential mobility of reactants and their interactions leads to a change of the EM peak shape. This change is a function of rate constants, so the rate and equilibrium constants can be directly determined from the analysis of the EM peak shape (width and symmetry) and propagation pattern along the reactor. We proved experimentally the use of ECEEM for multiplex determination of kinetic parameters describing weak (3 mM > K(d) > 80 μM) and fast (0.25 s ≥ τ ≥ 0.9 ms) noncovalent interactions between four small molecule drugs (ibuprofen, S-flurbiprofen, salicylic acid and phenylbutazone) and α- and β-cyclodextrins. The affinity of the drugs was significantly higher for β-cyclodextrin than α-cyclodextrin and mostly determined by the rate constant of complex formation.

Metal-complex formation and DNA interaction of 5, 10,15,20-tetrakis(1-methyl-4-pyridiyl)-porphine: Study of the mechanistic aspects

The macrocyclic porphyrin 5,10,15,20-tetrakis(1-methyl-4-pyridiyl)-porphine is studied in its ability to coordinate Cu(II) even at very low pH values and to interact, as a copper complex, with calf-thymus (CT-DNA). The kinetics and equilibria for metal-ligand complexes formation are spectrophotometrically studied, particularly focussing on the mechanistic information provided by the kinetic approach. The rate constants of complex formation is much lower than that of water exchange at Cu(II); this behavior is ascribed to an equilibrium between two porphyrin populations, only one of them being reactive. Concerning the interaction of the copper–porphyrin complex (D) with CT-DNA, it has been found that the complex binds to G–C base pairs by intercalation while forms external complex with the A–T base pairs. The kinetic results agree with a reaction mechanism that takes into account the slow shuffling from an AT-bound form (DAT) to a GC-bound form (DGC) of the copper complex (D), finally leading to a more stable DGC* intercalated form. Kinetic and equilibrium parameters for the copper complex binding to the nucleic acid are obtained, and the binding mechanism is discussed. A mechanism is proposed where D reacts simultaneously with (G–C) and (A–T) base pairs. The resulting bound forms interconvert according to a “shuffling” process, which involves formation of an intermediate (DGC) form. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 42: 79–89, 2010

Tweezering the Core of Dendrimers: Medium Effect on the Kinetic and Thermodynamic Properties

We have investigated the complex formation between dendritic guests and a molecular tweezer host by NMR, absorption, and emission spectroscopy as well as electrochemical techniques. The dendrimers are constituted by an electron-acceptor 4,4'-bipyridinium core appended with one (DnB(2+)) or two (Dn(2)B(2+)) polyaryl-ether dendrons. Tweezer T comprises a naphthalene and four benzene components bridged by four methylene groups. Medium effects on molecular recognition phenomena are discussed and provide insight into the conformation of dendrimers: change in solvent polarity from pure CH(2)Cl(2) to CH(2)Cl(2)/CH(3)CN mixtures and addition of tetrabutylammonium hexafluorophosphate (NBu(4)PF(6), up to 0.15 M), the supporting electrolyte used in the electrochemical measurements, have been investigated. The association constants measured in different media show the following trend: (i) they decrease upon increasing polarity of the solvent, as expected for host-guest complexes stabilized by electron donor-acceptor interactions; (ii) no effect of generation and number of dendrons (one for the DnB(2+) family and two for the Dn(2)B(2+) family) appended to the core is observed in higher polarity media; and (iii) in a low-polarity solvent, like CH(2)Cl(2), the stability of the inclusion complexes is higher for DnB(2+) dendrimers than for Dn(2)B(2+) ones, while within each dendrimer family it increases by decreasing dendron generation, and upon addition of NBu(4)PF(6). The last result has been ascribed to a partial dendron unfolding. Kinetic investigations performed in lower polarity media evidence that the rate constants of complex formation are slower for symmetric Dn(2)B(2+) dendrimers than for the nonsymmetric DnB(2+) ones, and that within the Dn(2)B(2+) family, they decrease by increasing dendron generation. The dependence of the rate constants for the formation and dissociation of the complexes upon addition of NBu(4)PF(6) has also been investigated and discussed.

Effect of divalent cations on the kinetics of Fe(III) complexation by organic ligands in natural waters

We have investigated the kinetics of Fe(III) complexation by several organic ligands including fulvic acid, citrate and ethylenediaminetetraacetic acid (EDTA). Particular attention was given to examination of the effect of competitive divalent cations (Me: Ca 2+ and Mg 2+) at concentrations typical of seawater on the complexation rate. All experiments were conducted in 0.5 M NaCl solution buffered with 2 mM bicarbonate at pH 8.0 in the absence and presence of Me (25 μM–250 mM). The rate constants of complex formation determined by using the competitive ligand (5-sulfosalicylic acid) method combined with visible spectrophotometry ranged from 3.3 × 10 4 to 3.2 × 10 6 M −1 s −1. The mechanism of complexation was then examined based on a kinetic model. When EDTA was used as a ligand, Me at concentrations comparable to the ligand markedly retarded the rate of iron complex formation due to the predominance of an adjunctive pathway (where iron–ligand complex is formed via direct association of iron to Me–ligand complex). In contrast, the competing effect of Me on iron complexation by citrate and fulvic acid was observed only when the Me concentration was in excess of the ligand by more than a factor of 10–1000. The kinetic model suggests that iron complexation by fulvic acid occurs predominantly via a disjunctive pathway (where iron complexation by ligand occurs after dissociation of Me from Me–ligand complex) at concentrations of divalent cations and natural organic matter typical of natural waters including seawater and freshwater.