Inducement and stabilization of G-quadruplex DNA by a thiophene-containing dinuclear ruthenium(II) complex

G-quadruplex structures are attractive targets for the development of anticancer drugs, as their formation in human telomere could impair telomerase activity, thus inducing apoptosis in cancer cells. In this work, a thiophene-containing dinuclear ruthenium(II) complex, [Ru₂(bpy)₄(H₂bipt)]⁴⁺ {bpy = 2,2′-bipyridine, H₂bipt = 2,5-bis[1,10]phenanthrolin[4,5-f]-(imidazol-2-yl)thiophene}, was prepared and the interaction between the complex and human telomeric DNA oligomers 5′-G₃(T₂AG₃)₃-3′ (HTG21) has been investigated by UV-Vis, fluorescence and circular dichroism (CD) spectroscopy, fluorescence resonance energy transfer (FRET) melting assay, polymerase chain reaction (PCR) stop assay, fluorescent intercalator displacement (FID) titrations, Job plot and color reaction studies. The results indicate that the complex can well induce and stabilize the formation of antiparallel G-quadruplex of telomeric DNA in the presence or absence of metal cations, and the ΔT_m value of the G-quadruplex DNA treated with the complex was obtained to be 12.8 °C even at levels of 50-fold molar of duplex DNA (calf-thymus DNA), suggesting that the complex exhibits higher G-quadruplex DNA selectivity over duplex DNA. The complex shows high interaction ability with G-quadruplex DNA at (1.17 ± 0.12) × 10⁷ M^?1 binding affinity using a 2:1 [complex]/[quadruplex] binding mode ratio. A novel visual method has been developed here for making a distinction between G-quadruplex DNA and duplex DNA by our ruthenium complex binding hemin to form the hemin-G-quadruplex DNAzyme.     

New phosphonate-substituted tricarbocyanines and their interaction with bovine serum albumin

New tricarbocyanine dyes with phosphonate groups (2, 3) were synthesized and their binding constants to bovine serum albumin (BSA) were determined. The binding constants of the synthesized tricarbocyanines and cardiogreen (1) (K_b ~ 10⁵ M^–1) are similar, indicating an insignificant contribution of the Coulomb interaction to the complex formation, which is determined by the polymethine chain interaction with BSA. The fluorescence lifetimes attest to the formation of two types of complexes: the lifetime of the dye complex with BSA with a major contribution (~80%) is 740–800 ps, and a lifetime of ~210 ps corresponds to the complex of dye aggregates with BSA.     

Quadruplex–Duplex Junction: A High-Affinity Binding Site for Indoloquinoline Ligands

A parallel quadruplex derived from the Myc promoter sequence was extended by a stem-loop duplex at either its 5′- or 3′-terminus to mimic a quadruplex–duplex (Q–D) junction as a potential genomic target. High-resolution structures of the hybrids demonstrate continuous stacking of the duplex on the quadruplex core without significant perturbations. An indoloquinoline ligand carrying an aminoalkyl side chain was shown to bind the Q–D hybrids with a very high affinity in the order K_a≈10⁷ m ⁻¹ irrespective of the duplex location at the quadruplex 3′- or 5′-end. NMR chemical shift perturbations identified the tetrad face of the Q–D junction as specific binding site for the ligand. However, calorimetric analyses revealed significant differences in the thermodynamic profiles upon binding to hybrids with either a duplex extension at the quadruplex 3′- or 5′-terminus. A large enthalpic gain and considerable hydrophobic effects are accompanied by the binding of one ligand to the 3′-Q–D junction, whereas non-hydrophobic entropic contributions favor binding with formation of a 2:1 ligand-quadruplex complex in case of the 5′-Q–D hybrid.     

AN EQUILIBRIUM AND RATE STUDY OF THE INTERACTION OF AQUEOUS CHROMIUM(III) ION WITH ADENINE

Abstract

Potentiometric titration data obtained at 50° for aqueous solutions containing aquochromium(III) ions and adenine were analyzed to yield values for the acid dissociation constants of adenine and the formation constants of 1:1 and 1:2 mononuclear complexes. There is also evidence for a 2:2 dimer for which elemental analysis data suggest a structure of the form (HL=neutral adenine): [(H₂ O)₂ (HL)Cr-μ(OH)₃ -Cr(HL)(H₂ O)₂]³⁺. The kinetic studies indicate that the 1:1 complex is formed in excess adenine with a half-time of about two hours at pH=4.4 and 60°. The proposed mechanism involves an outer-sphere associative intermediate of appreciable stability (K_A ～ 200) followed by rate-determining substitution of adenine for ligand water to produce the 1:1 complex. A much slower subsequent reaction yields the dimer as the final stable product. The formation of the 1:1 complex is an equilibration and both forward and reverse rate constants were determined. The pH dependence data indicate that the reactive species in the system are HL, Cr(H₂ O) ₅OH²⁺, and Cr(H₂ O)₄ (OH)₂ ⁺.     

Oligoamide Foldamers as Helical Chloride Receptors—the Influence of Electron‐Withdrawing Substituents on Anion‐Binding Interactions

The anion‐binding properties of three closely related oligoamide foldamers were studied using NMR spectroscopy, isothermal titration calorimetry and mass spectrometry, as well as DFT calculations. The ¹H NMR spectra of the foldamers in [D₆]acetone solution revealed partial preorganization by intramolecular hydrogen bonding, which creates a suitable cavity for anion binding. The limited size of the cavity, however, enabled efficient binding by the inner amide protons only for the chloride anion resulting in the formation of a thermodynamically stable 1:1 complex. All 1:1 chloride complexes displayed a significant favourable contribution of the entropy term. Most likely, this is due to the release of ordered solvent molecules solvating the free foldamer and the anion to the bulk solution upon complex formation. The introduction of electron‐withdrawing substituents in foldamers 2 and 3 had only a slight effect on the thermodynamic constants for chloride binding compared to the parent receptor. Remarkably, the binding of chloride to foldamer 3 not only produced the expected 1:1 complex but also open aggregates with 1:2 (host:anion) stoichiometry.     

Molecular dynamics study on the interaction of a mithramycin dimer with a decanucleotide duplex

The complex of a minor groove binding drug mithramycin (MTR) and the self-complementary d(TAGCTAGCTA) 10-mer duplex was investigated by molecular dynamics (MD) simulations using the AMBER 7.0 suite of programs. There is one disaccharide and trisaccharide segment projecting from opposite ends of an aglycone chromophore of MTR. A MTR dimer complex (MTR)2Mg2+ is formed in the presence of a coordinated ion Mg2+. A NMR solution structure of two (MTR)2Mg2+ complexes bound with one DNA duplex, namely, the 2:1 duplex complex, was taken as the starting structure for the MD simulation. The partial charge on each atom was calculated using the multiple-RESP fitting procedure, and all of the missing parameters in the Parm99 force field used were adapted comparably from the literature. The length of the MD simulation was 5 ns, and the binding free energy for the formation of a 1:1 or 2:1 duplex complex was determined from the last 4 ns of the simulation. The binding free energies were decomposed to components of the contributions from different energy types, and the changes in the helical parameters of the bound DNA duplex plus the glycosidic linkages between sugar residues of the bound MTR dimer were determined. It was found that binding of the first (MTR)2Mg2+ complex with the DNA duplex to form a 1:1 duplex complex does not cause stiffening of the duplex especially in the unoccupied site of the duplex. However, the overall flexibility of the DNA duplex is reduced substantially once the second (MTR)2Mg2+ complex is bound with the unoccupied site to form the 2:1 duplex complex. The van der Waals interactions were found to be dominant in the central part of the DNA duplex where sugar residues from each bound (MTR)2Mg2+ complex were inwardly pointing and the corresponding minor groove was widened.     

Stabilities of Scandium(III)- and Yttrium(III)-Tiron Chelates and their hydrolytic behavior

The interactions of Sc³⁺ and Y³⁺ ions with disodium 1,2-dihydroxybenzene-3,5-disulfonate (Na₂H₂L, where H₂L_2- = Tiron) were investigated in aqueous solution by means of potentiometric and spectroscopic methods. The coordination of Tiron to Sc³⁺ and Y³⁺ takes place through two phenolic oxygen atoms of catecholate ion in different stoichiometries. Thus, the binding of Tiron to Sc³⁺ occurs either in 1 : 1 or 1 : 2 molar ratios; they have resulted by the formation of [ScL]^- and [ScL₂]^5- type complexes, respectively. On the other hand, Y³⁺ ion behaves like Th⁴⁺ and Ln³⁺ ions toward Tiron. It forms [YL]^-type complex in 1 : 1 molar ratio; but in 1 : 2 or in higher molar ratios, only unique [Y₂L₃]^6- type complex formation takes place. The formation constants of [ScL]^-, [ScL₂]^5-, [YL]^-, and [Y₂L₃]^6- complexes were determined by analysis of the potentiometric data in ionic medium of 0.1 M KNO₃ or NaClO₄ at 25°C. The hydrolytic reactions of Sc(III) and Y(III) complexes with Tiron were determined from potentiometric data and the formation constants of [ScL(OH)]^2- and [YL(OH)]^2- were also calculated.From Koordinatsionnaya Khimiya, Vol. 31, No. 1, 2005, pp. 62–68.Original English Text Copyright © 2005 by Aydin, Türkel, Özer.     

Synthesis of Double-Armed Benzo-15-crown-5 and Their Complexation Thermodynamics with Alkali Cations

A series of double-armed benzo-15-crown-5 lariats (3–8) have been synthesized by the reaction of 4′, 5′-bis(bromomethyl)-benzo-15-crown-5 (2) with 4-hydroxybenzaldehyde, phenol, 4-chlorophenol, 4-methoxyphenol, 2-hydroxybenzaldehyde, and 4-acetamidophenol in 43 ~ 82% yields, respectively. The complex stability constants (K _S) and thermodynamic parameters for the stoichiometric 1:1 and/or 1:2 complexes of benzo-15-crown-5 1 and double-armed crown ethers 3–8 with alkali cations (Na⁺, K⁺, Rb⁺) have been determined in methanol–water (V/V=8:2) at 25 °C by means of microcalorimetric titrations. As compared with the parent benzo-15-crown-5 1, double-armed crown ethers 3–8 show unremarkable changes in the complex stability constants upon complexation with Na⁺, but present significantly enhanced binding ability toward cations larger than the crown cavity by the secondly sandwich complexation. Thermodynamically, the sandwich complexations of crown ethers 3-8 with cations are mostly enthalpy-driven processes accompanied with a moderate entropy loss. The binding ability and selectivity of cations by the double-armed crown ethers are discussed from the viewpoints of the electron density, additional binding site, softness, spatial arrangement, and especially the cooperative binding of two crown ether molecules toward one metal ion.     

Complex formation of gallium(III) with 8-hydroxy-7-iodoquinoline-5-sulphonic acid (ferron)

The complex formation of gallium(III) with 8-hydroxy-7-iodoquinoline-5-sulphonic acid (ferron) (L) was studied by potentiometry; and the dissociation constants of the ligand were redetermined. A new computer program made it possible to make allowance for a considerable number of equilibria. In the pH range 2–10, the main species were as follows (the logarithm of the absolute stability constants at 25.0 ±0.1° for ionic strength 0.1 M are given in parentheses): GaHL (11.3), GaL (14.7), Ga(OH)₂L (32.3), GaL₂ (23.9), GaOHL₂ (31.0) and GaL₃ (29.6). For 1:1, 1:2 and 1:3 mixtures of metal with ferron, the mole fractions of the various complexes as a function of pH were calculated; the effect of pH on the conditional constants of the species GaL, GaL₂ and GaL₃ was also established.     

Determination and comparison of stability constants of tungsten(VI) and molybdenum(VI) with nitrilotriacetic acid and glutamic acid at different ionic strengths

The formation constants of species formed in the systems H⁺?+?W(VI)?+?nitrilotriacetic acid (NTA) and H⁺?+?NTA have been determined in aqueous solution for pH?=?4–9 at 25°C and different ionic strengths ranging from 0.1 to 1.0?mol?dm^?3 NaClO₄, using potentiometric and spectrophotometric techniques. It was shown that tungsten(VI) forms a mononuclear 1?:?1 complex with NTA of the type WO₃L^3? at pH?=?7.5. The composition of the complex was determined by the continuous variations method. The complexation of molybdenum(VI) with glutamic acid was investigated in aqueous solution ranging in pH from 4 to 9, using polarimetric, potentiometric and spectrophotometric techniques. The composition of the complex was determined by the continuous variations method. It was shown that molybdenum(VI) forms a mononuclear 1?:?1 complex with glutamic acid of the type MoO₃L^2? at pH?=?6.0. The dissociation constants of glutamic acid and the stability constants of the complex were determined at 25°C and at ionic strengths ranging from 0.1 to 1.0?mol?dm^?3 sodium perchlorate. In both complex formation reactions the dependence of the dissociation and stability constants on ionic strength is described by a Debye-Huckel type equation. Finally, a comparison has been made between the patterns of ionic strength dependence for the two complexes and the results have been compared with data previously reported.     

Potentiometry, Stability and Thermodynamics of Diethyltin(IV) Dichloride with Some Selected Biomolecules

The interaction of diethyltin(IV), DET, with selected bioligands having a variety of model functional groups was investigated at 25 °C and ionic strength 0.1 mol·dm^?3 NaCl using the potentiometric technique. The hydrolysis constants of the DET cation and the formation constants of the complexes formed in solution were calculated using the MINIQUAD-75 program. The stoichiometry and stability constants for the complexes formed are reported. The results show the formation of 1:1 and 1:2 complexes with amino acids and DNA constituents. The dicarboxylic acids form 1:1 complexes. The peptides form both 110 complexes and the corresponding deprotonated amide species [Et₂Sn(LH_?1)] (11-1). The participation of different ligand functional groups in binding to organotin is discussed. The concentration distributions of the various complex species were evaluated as a function of pH. The standard thermodynamic parameters ΔH° and ΔS°, calculated from the temperature dependence of the equilibrium constants, were investigated for the interaction of DET with thymine as a representative example of DNA constituents. The effect of ionic strength and solvent on hydrolysis constants of DET, protonation equilibria of thymine and its complex formation with DET were investigated and discussed.     

Kinetics of Gallium Removal from Transferrin and Thermodynamics of Gallium-Binding by Sulfonated Tricatechol Ligands

Abstract

The thermodynamics of complexation of gallium by tricatechol ligand analogues of enterobactin and the kinetics of gallium removal from human serum transferrin (Tf) by one of those ligands have been studied by UV spectrophotometry. The ligands are a sulfonated monomeric catechoylamide, DMBS (N,N-dimethyl-2,3-dihydroxy-5-sulfonatobenzamide), and four sulfonated triscatechoylamides, MECAMS (1,3,5-N,N',N”-tris(5-sulfonato-2,3-dihydroxybenzoyl)-triaminomethylbenzene), Me₃MECAMS (N, N', N-trimethyl-MECAMS), 3,4-LICAMS (N, N', N”-tris(5-sulfonato-2,3-dihydroxybenzoyl)-1,5,10-triazadecane), and (DiP)LICAMS (N, N”-diisopropyl-LICAMS). The individual orthohydroxyl protonation constants are 7.15 (DMBS), and (average values for the three protons of the tricatechols) 7.09 (MECAMS), 7.01 (LICAMS), 7.62 (Me₃MECAMS), and 7.75 ((DiP(LICAMS), in good agreement with average values obtained potentiometrically. The overall formation constants for binding of Ga³⁺ by these ligands are β₁₁₃ = 41.9 for DMBS, β₁₁₀ = 41.1 for LICAMS, 36.6 for (DiP)-LICAMS, and 39.1 for Me₃MECAMS. Gallium is removed from the two metal binding sites of Ga₂Tf in a process by 3,4-LICAMS that is first order in both Tf and ligand at different rates (277M^?1 min^?1 and 17M^?1 min^?1). These are 12 and 3.4 times the corresponding rates of iron removal from Fe₂Tf. The dissociation pathways of the gallium-ligand complexes upon protonation of the ligands were probed by whole spectrum analysis with the non-linear least-squares program REFSPEC. For all three triscatechoylamide complexes, protonation occurs in sequential one-proton reactions wth logK MLH_n (n = 1, 2, 3) equal to 5.93, 5.00, 2.4 for MECAMS: 5.8, 5.7, 3.0 for 3,4-LICAMS; 6.81,6.34, 3.0 for Me₃MECAMS; 6.34, 6.33, 4.3 for (DiP)LICAMS. First- and second-derivative spectra show that for complexes of trimeric ligands the last two protonations result in a complex with a completely dissociated catecholate arm, Ga(cat)₂-catH₂, similar to the Ga(DMBS)₂ complex observed with the monomer. In the linear complexes, the middle ligand arm is detached from the metal first. Addition of a fourth proton resulted in decomposition of the gallium-trimeric ligand complex.     

Complexation equilibria involving salts in non-aqueous solvents: ion pairing and activity considerations

Complexation of anions, cations and even ion pairs is now an active area of investigation in supramolecular chemistry; unfortunately it is an area fraught with complications when these processes are examined in low polarity organic media. Using a pseudorotaxane complex as an example, apparent K_a2 values (=[complex]/{[salt]_o?[complex]}{[host]_o?[complex]}) for pseudorotaxane formation from dibenzylammonium salts ( 2 ‐X) and dibenzo‐[24]crown‐8 ( 1 , DB24C8) in CDCl₃/CD₃CN 3:2 vary with concentration. This is attributable to the fact that the salt is ion paired, but the complex is not. We report an equilibrium model that explicitly includes ion pair dissociation and is based upon activities rather than molar concentrations for study of such processes in non‐aqueous media. Proper analysis requires both a dissociation constant, K_ipd, for the salt and a binding constant for interaction of the free cation 2 ⁺ with the host, K_a5; K_a5 for pseudorotaxane complexation is independent of the counterion (500 M ^?1), a result of the complex existing in solution as a free cation, but K_ipd values for the salts vary by nearly two orders of magnitude from trifluoroacetate to tosylate to tetrafluoroborate to hexafluorophosphate anions. The activity coefficients depend on the nature of the predominant ions present, whether the pseudorotaxane or the ions from the salt, and also strongly on the molar concentrations; activity coefficients as low as 0.2 are observed, emphasizing the magnitude of their effect. Based on this type of analysis, a method for precise determination of relative binding constants, K_a5, for multiple hosts with a given guest is described. However, while the incorporation of activity coefficients is clearly necessary, it removes the ability to predict from the equilibrium constants the effects of concentration on the extent of binding, which can only be determined experimentally. This has serious implications for study of all such complexation processes in low polarity media.     

Synthesis of Double-Armed Benzo-15-crown-5 and Their Complexation Thermodynamics with Alkali Cations

A series of double-armed benzo-15-crown-5 lariats (3–8) have been synthesized by the reaction of 4′, 5′-bis(bromomethyl)-benzo-15-crown-5 (2) with 4-hydroxybenzaldehyde, phenol, 4-chlorophenol, 4-methoxyphenol, 2-hydroxybenzaldehyde, and 4-acetamidophenol in 43 ~ 82% yields, respectively. The complex stability constants (K _S) and thermodynamic parameters for the stoichiometric 1:1 and/or 1:2 complexes of benzo-15-crown-5 1 and double-armed crown ethers 3–8 with alkali cations (Na⁺, K⁺, Rb⁺) have been determined in methanol–water (V/V=8:2) at 25 °C by means of microcalorimetric titrations. As compared with the parent benzo-15-crown-5 1, double-armed crown ethers 3–8 show unremarkable changes in the complex stability constants upon complexation with Na⁺, but present significantly enhanced binding ability toward cations larger than the crown cavity by the secondly sandwich complexation. Thermodynamically, the sandwich complexations of crown ethers 3-8 with cations are mostly enthalpy-driven processes accompanied with a moderate entropy loss. The binding ability and selectivity of cations by the double-armed crown ethers are discussed from the viewpoints of the electron density, additional binding site, softness, spatial arrangement, and especially the cooperative binding of two crown ether molecules toward one metal ion.Graphical Abstract Synthesis of Double-Armed Benzo-15-crown-5 and Their Complexation Thermodynamics with Alkali CationsYU LIU*, JIAN-RONG HAN, ZHONG-YU DUAN and HENG-YI ZHANG This revised version was published online in July 2005 with a corrected issue number.     

Mass spectrometric studies of alkali metal ion binding on thrombin-binding aptamer DNA

The binding sites and consecutive binding constants of alkali metal ions, (M⁺ = Na⁺, K⁺, Rb⁺, and Cs⁺), to thrombin-binding aptamer (TBA) DNA were studied by Fourier-transform ion cyclotron resonance spectrometry. TBA-metal complexes were produced by electrospray ionization (ESI) and the ions of interest were mass-selected for further characterization. The structural motif of TBA in an ESI solution was checked by circular dichroism. The metal-binding constants and sites were determined by the titration method and infrared multiphoton dissociation (IRMPD), respectively. The binding constant of potassium is 5–8 times greater than those of other alkali metal ions, and the potassium binding site is different from other metal binding sites. In the 1:1 TBA-metal complex, potassium is coordinated between the bottom G-quartet and two adjacent TT loops of TBA. In the 1:2 TBA—metal complex, the second potassium ion binds at the TGT loop of TBA, which is in line with the antiparallel G-quadruplex structure of TBA. On the other hand, other alkali metal ions bind at the lateral TGT loop in both 1:1 and 1:2 complexes, presumably due to the formation of ion-pair adducts. IRMPD studies of the binding sites in combination with measurements of the consecutive binding constants help elucidate the binding modes of alkali metal ions on DNA aptamer at the molecular level.     

Negative Cooperativity in the Molecular Recognition of Excitatory Amino-Acid Derivatives by Synthetic Allosteric 1,1′-Binaphthalene Receptors

The optically active allosteric receptors (−)-(R,R)- 3 and (+)-(R,R)- 4 were synthesized for the molecular recognition of the N-(benzyloxy)carbonyl (N-Cbz)-protected excitatory amino acids aspartic acid (Asp, 1 ) and glutamic acid (Glu, 2 ). These macrocyclic structures consist of two 1,1′-binaphthalene moieties connected by two but-2-yne-1,4-diyl (for (−)-(R,R)- 3 ) or p-xylylene (for (+)-(R,R)- 4 ) bridges between the O-atoms in the minor grooves. Each 1,1′-binaphthalene moiety contains two 2-acetamidopyridin-6-yl (CONH(py)) H-bonding sites in the major groove to bind excitatory amino-acid derivatives via two COOH█bk█⋅⋅⋅█ek█CONH(py) H-bonding arrays and additional secondary electrostatic interactions. The formation of stable complexes with 1 : 2 host-guest stoichiometry was proven by the evaluation of fluorescence binding titrations using a multiple-wavelength nonlinear least-squares curve-fitting procedure, Job plot analysis, and solubilization experiments. Complexation of the first excitatory amino-acid guest at binding site 1 reduces the affinity for the second guest at binding site 2. As measures for the negative cooperativity between the two sites, the ratios of the association constants for the first and second binding events, {K_a(1 : 1)/K_a(1 : 2)}_corr. (corrected for the statistical preference of the 1 : 1 complex formation), were found to adopt values between 1.4 and 2.4, and the Hill coefficients n_H varied between 0.49 and 0.59.     

Synthesis,Spectroscopy, Semi‐empirical and Biological Activities of Organotin(IV) Complexes with o‐Isopropyl Carbonodithioic Acid

New organotin(IV) complexes have been synthesized by treating potassium o‐isopropyl carbonodithioate with R₂SnCl₂/R₃SnCl in 1 : 2/1 : 1 M/L ratio. All complexes have been characterized by IR and NMR (¹H, ¹³C) spectroscopy. IR results shows that ligand acts as bidentate which is also confirmed by semi‐empirical study. NMR data reveals four coordinated geometry in solution. Computed positive heat of formation shows that complex 5 is thermodynamically unstable. UV/visible spectroscopy was used to assess the mode of interaction and binding of the complexes with DNA which shows that complex 5 exhibits higher binding constant as compared to complex 3 . In protein kinase inhibition assay, compound 3 was found most active, while other biological activities shows that triorganotin(IV) complexes are biologically more active as compared to diorganotin(IV) complexes.     

Competitive Binding of Tolmetin to β-Cyclodextrin and Human Serum Albumin: <Superscript>1</Superscript>H NMR and Fluorescence Spectroscopy Studies

The host–guest interaction of tolmetin (TOL) with β-cyclodextrin (β-CD) and the influence of human serum albumin (HSA) on the formation of the inclusion complex were studied by 1D and 2D NMR spectroscopy. The TOL/β-CD inclusion complex formed at a molar ratio of 1:1 with a binding constant value of 2164.5 L·mol^?1. Data analysis showed that the addition of 10 μmol·L^?1 of HSA weakened the strength of TOL binding to β-CD (K _a = 1493 L·mol^?1). The interaction of TOL with HSA in the absence and presence of β-CD was studied by analyzing the fluorescence quenching data. The Stern–Volmer quenching constants and the binding constants are found to be smaller in the presence of β-CD, suggesting that β-CD hinders the strong interaction of TOL with HSA by complex formation. Additionally, the presence of β-CD does not induce conformational and microenvironmental changes on HSA.     

Structural and solution studies of new cadmium(II) complexes with 2,2′-diamino-4,4′-bithiazole

Two new Cd(II) complexes, [Cd(DABT)(CH₃COO)₂] (1) and [Cd(DABT)(NO₃)₂] (2), DABT = 2,2′-diamino-4,4′-bithiazole, have been synthesized and characterized by single crystal X-ray crystallography. In 1, each cadmium is chelated by two nitrogens of one DABT and four oxygens of acetate, whereas 2 is a dinuclear complex and there are two Cd's with eight and six coordination: Cd₁O₆N₂ and Cd₂O₄N₂ units. The counter ion effect of the complex formation reaction between cadmium(II) nitrate and acetate with DABT has been studied by spectroscopy. The formation constants of the 1 : 1 and 1 : 2 (metal ion to ligand) complexes were evaluated by computer fitting of the absorbance–mole ratio data.     

Optical chloride sensor based on [9]mercuracarborand-3 with massively expanded measuring range

Bulk optodes for monovalent ions typically exhibit a relatively narrow measuring range of about two-orders of magnitude. Here, the measuring range was expanded to about six-orders of magnitude concentration change by optimizing a fluorescent chloride optical sensing film based on a plasticized poly(vinyl chloride) film incorporating the halide-selective ionophore [9]mercuracarborand-3 and the H⁺-chromoionophore 9-dimethylamino-5-[4-(15-butyl-1,13-dioxo-2,14-dioxanonadecyl) phenyl-imino]benzo[a]phenox-azine (ETH 5418). This was achieved with the recently established two-step ionophore binding mechanism: the expanded sensing range sequentially makes use of the 1:2 and 1:1 binding stoichiometries between ionophore to chloride. The relevant complex formation constants were found here as log K₁=9.1 and log K₂=2.7, which are consistent with previously reported values. The selectivity of this optode is very good, with an excellent discrimination of thiocyanate and lipophilic anions such as salicylate, nitrate and perchlorate. As with earlier work, the main interferences are the other halides bromide and iodide.