Synthesis, characterization, fluorescence and computational studies of new Cu, Ni and Hg complexes with emissive thienylbenzoxazolyl-alanine ligands

The interaction of four fluorescent compounds containing thiophene and benzoxazole moieties combined with an alanine residue with alkaline, alkaline-earth, transition and post-transition metal ions was explored. The highly fluorescent heterocyclic alanine derivatives are strongly quenched in the solid state after complexation with the paramagnetic metal ions Cu²⁺ and Ni²⁺, and with the diamagnetic Hg²⁺. Absorption and steady-state fluorescence titrations reveal a selective interaction with Cu²⁺, Ni²⁺ and Hg²⁺. In all cases the formation of mononuclear or dinuclear metal complexes in solid state and in solution are postulated. DFT calculations on the mercury(II) complexes confirm the formation of dinuclear species. Our results suggest that one metal ion is coordinated by the chelate group formed by the amine and the protonated carboxylic groups present in the amino acid residue while a second metal ion is directly linked to the chromophore. As parent compound, L4 shows no interaction with Cu²⁺ and Ni²⁺ salts. However, the interaction with Hg²⁺ induces a strong quenching and a red shift of the fluorescence emission.     

Chemical and functional characterization of metal-binding pseudotripeptides with different functionalized N-alkyl residues Dedicated to Professor Dr. Ernst-Gottfried Jaeger on occasion of his 65th birthday

Pseudotripeptide ligands with 4 different N-functionalized glycine residues were qualitatively, semiquantitatively and quantitatively tested for their complexation of the bivalent transition metal ions Zn²⁺, Cu²⁺, Co²⁺, Ni²⁺ and Mn²⁺. The functional side chains have different length and different groups available for complexation. MALDI-MS and ESI-MS were used for more qualitative or semiquantitative estimation of the complex formation tendencies. The found ranking differs by these two methods only for Zn²⁺ and Ni²⁺. For one of the pseudotripeptide ligands, the ligand L1, complex formation with certain transition metal was estimated quantitatively by potentiometric titration. The Zn-complex of that ligand polarizes bound water strongly, resulting in a low pK_a-value. Complexes of pseudotripeptide ligand L1 with certain metal ions were tested for their hydrolytic activity. The pseudo first order rate constants of the hydrolysis of the substrates 4-nitrophenyl acetate and bis(4-nitrophenyl)phosphate were compared to complexes with the same metal ions formed with a very well studied ligand from the literature, the 1,4,7,10-tetraaza cyclododecane (cyclen). The hydrolysis of the phosphate ester occurs very slowly compared to the acetate ester. No correlation exists between the estimated pK_a values of complexes formed from ligand L1 with different metal ions and the phosphate ester hydrolysis. The Ni ions give totally different hydrolytic activities for pseudotripeptide ligand L1 and cyclen. With one exception, the Ni-cyclen complex, all other complexes have only a low or moderate catalytic activity.     

Interactions of DNA with divalent metal ions. I. 31P-nmr studies

³¹P-nmr has been used to investigate the specific interaction of three divalent metal ions, Mg²⁺, Mn²⁺, and Co⁺², with the phosphate groups of DNA. Mg²⁺ is found to have no significant effect on any of the ³¹P-nmr parameters (chemical shift, line-width, T₁, T₂, and NOE) over a concentration range extending from 20 to 160 mM. The two paramagnetic ions, Mn²⁺ and Co²⁺, on the other hand, significantly change the ³¹P relaxation rates even at very low levels. From an analysis of the paramagnetic contributions to the spin–lattice and spin–spin relaxation rates, the effective internuclear metal–phosphorus distances are found to be 4.5 ± 0.5 and 4.1 ± 0.5 Å for Mn²⁺ and Co²⁺, respectively, corresponding to only 15 ± 5% of the total bound Mn²⁺ and Co²⁺ being directly coordinated to the phosphate groups (inner-sphere complexes). This result is independent of any assumptions regarding the location of the remaining metal ions which may be bound either as outer-sphere complexes relative to the phosphate groups or elsewhere on the DNA, possibly to the bases. Studies of the temperature effects on the ³¹P relaxation rates of DNA in the absence and presence of Mn²⁺ and Co²⁺ yielded kinetic and thermodynamic parameters which characterize the association and dissociation of the metal ions from the phosphate groups. A two-step model was used in the analysis of the kinetic data. The lifetimes of the inner-sphere complexes are 3 × 10^?7 and 1.4 × 10^?5 s for Mn²⁺ and Co²⁺, respectively. The rates of formation of the inner-sphere complexes with the phosphate are found to be about two orders of magnitude slower than the rate of the exchange of the water of hydration of the metal ions, suggesting that expulsion of water is not the rate-determining step in the formation of the inner-sphere complexes. Competition experiments demonstrate that the binding of Mg²⁺ ions is 3–4 times weaker than the binding of either Mn²⁺ or Co²⁺. Since the contribution from direct phosphate coordination to the total binding strength of these metal ion complexes is small (～15%), the higher binding strength of Mn²⁺ and Co²⁺ may be attributed either to base binding or to formation of stronger outer-sphere metal–phosphate complexes. At high levels of divalent metal ions, and when the metal ion concentration exceeds the DNA–phosphate concentration, the fraction of inner-sphere phosphate binding increases. In the presence of very high levels of Mg²⁺ (e.g., 3.1M), the inner-sphere ? outer-sphere equilibrium is shifted toward ～100% inner-sphere binding. A comparison of our DNA results and previous results obtained with tRNA indicates that tRNA and DNA have very similar divalent metal ion binding properties. A comparison of the present results with the predictions of polyelectrolyte theories is presented.     

Chemical and functional characterization of metal-binding pseudotripeptides with different functionalized N-alkyl residues Dedicated to Professor Dr. Ernst-Gottfried Jaeger on occasion of his 65th birthday

Summary Pseudotripeptide ligands with 4 different N-functionalized glycine residues were qualitatively, semiquantitatively and quantitatively tested for their complexation of the bivalent transition metal ions Zn²⁺, Cu²⁺, Co²⁺, Ni²⁺ and Mn²⁺. The functional side chains have different length and different groups available for complexation. MALDI-MS and ESI-MS were used for more qualitative or semiquantitative estimation of the complex formation tendencies. The found ranking differs by these two methods only for Zn²⁺ and Ni²⁺. For one of the pseudotripeptide ligands, the ligand L1, complex formation with certain transition metal was estimated quantitatively by potentiometric titration. The Zn-complex of that ligand polarizes bound water strongly, resulting in a low pK _a -value. Complexes of pseudotripeptide ligand L1 with certain metal ions were tested for their hydrolytic activity. The pseudo first order rate constants of the hydrolysis of the substrates 4-nitrophenyl acetate and bis(4-nitrophenyl)phosphate were compared to complexes with the same metal ions formed with a very well studied ligand from the literature, the 1,4,7,10-tetraaza cyclododecane (cyclen). The hydrolysis of the phosphate ester occurs very slowly compared to the acetate ester. No correlation exists between the estimated pK _a values of complexes formed from ligand L1 with different metal ions and the phosphate ester hydrolysis. The Ni ions give totally different hydrolytic activities for pseudotripeptide ligand L1 and cyclen. With one exception, the Ni-cyclen complex, all other complexes have only a low or moderate catalytic activity. Dedicated to Professor Dr. Ernst-Gottfried Jaeger on occasion of his 65th birthday.     

The thermodynamics of the complexation of lanthanides by 1-hydroxy-4,7-disulfo-2-naphthoic acid

The thermodynamic parameters, log β, ΔH and ΔS, for formation of lanthanide-1-hydroxy-4,7- disulfo-2-naphthoic acid complexes have been determined at 25 °C in 0.10 M NaClO₄ solutions by potentiometric and calorimetric titrations. Under the experimental conditions, the data can be explained with the formation of LnL⁻, LnL₂⁵⁻ and LnHL complexes (H₂L²⁻ = 1-hydroxy-4,7-disulfo-2- naphthoic acid anion). At pH < 3 the LnHL complex is the major species, whereas by increasing pH the formation of LnL_n³⁻⁴ⁿ complexes becomes more important. The data are compared to the comparable data for complexing by aromatic carboxylic acids.     

A multinuclear nmr relaxation study of the interaction of divalent metal ions with l-aspartic acid

Carbon-13 spin-lattice relaxation times, T₁, have been measured for aqueous solutions of L-aspartic acid, L-alanine, O-phospho-L-serine, and 2-mercapto-L-succinic acid in the presence of the paramagnetic metal ions, Cu²⁺ and Mn²⁺, and Mg²⁺ as a diamagnetic control, at ambient temperature and neutral pH. Nitrogen-15, oxygen-17 and proton relaxation times were also obtained for L-aspartic acid and phosphorus-31 relaxation times for O-phospho-L-serine under similar conditions. The structures of these complexes in solution were determined from the various metal ion-nuclei distances calculated from the paramagaetically-induced relaxation. These results indicate that the Cu²⁺ interaction with L-aspartic acid is through α-amino and β-carboxyl groups while Mn²⁺ coordinates most strongly through α-and β-carboxyl groups, with the possibility of a weak interaction through the amino group.An examination of the coordination of these divalent metal ions to an analog of L-aspartic acid in which the β-carboxyl group is replaced by a phosphate group (O-phospho-L-serine) indicated that Cu²⁺ coordination is now probably through the α-amino and phosphate groups, while this analog is a monodentate ligand for Mn²⁺ coordinating through the phosphate group. Removal of the β-carboxyl group (L-alanine) also results in Cu²⁺ coordination through the α-carboxyl and α-amino groups, and the same ligand interactions are observed with Mn²⁺. Replacement of the α-amino group of L-aspartic acid with an - SH group (2-mercapto-L-succinate) is sufficient to eliminate any specific coordination with either Cu²⁺ or Mn²⁺.     

Chelates of tetracycline with first row transition metal perchlorates

2:1 adducts of tetracycline (tc) with 3d metal perchlorates (M = Cr ³⁺, Mn ²⁺, Fe ²+, ^Fe3+, ^Co2+, Ni ²⁺, Cu ²⁺, Zn ²⁺) are synthesized by boiling under reflux mixtures of tc free base and metal salt in ethanol— triethyl orthoformate. Characterization studies suggest that the new complexes are monomeric chelates involving bidentate tetracycline ligands, chelating via the amide group oxygen and the C3O oxygen of the ring A tricarbonylmethane. The complexes also contain unidentate coordinated −OClO ₃ ligands, as weil as ionic ClO ₄^-. The M ³⁺ (Cr, Fe) complexes are hexacoordinated of the [M(tc) ₂(OClO ₃)2 ₃]ClO ₄ type (MO ₆ chromophores), with two bidentate chelating tc and two unidentate perchlorato ligands in the first coordination sphere of the central metal ion. In the M2+ (Mn, Fe, Co, Ni, Cu, Zn) complexes, the inner coordination sphere of the metal ion is occupied by two bidentate chelating tc and only one −OClO ₃ ligand, and the coordination number is five, i.e. [M(tc) ₂OClO ₃]ClO ₄ (MO ₅absorbing species).     

The interaction of transition metal ions with adenosine 5'-monophosphate

Several complexes of Mn²⁺, Fe²⁺, Zn²⁺, Pd²⁺, Cd²⁺, Pb²⁺, Ce²⁺, Pr³⁺, Sm³⁺, Tb³⁺, and Uo₂₂₊ metal ions with adenosine 5'-monophosphate have been isolated. The stoichiometry of all the complexes prepared were 1:1 metal to mononucleotide, respectively, and they were characterized by elemental analysis, infrared, electronic and fluorescence spectroscopy, conductivity, and magnetic measurements. Similarities among the spectra of the substances with structurally known metal-mononucleotide complexes suggest that the metal ions interact directly or indirectly with the N₇ of the purine ring and the phosphate group of the mononucleotide.     

Interaction of Adenosine 5′-Triphosphate with Metal Ions X-ray Structure of Ternary Complexes Containing Mg(II), Ca(II), Mn(II), Co(II), ATP and 2,2′ -Dipyridylamine

Abstract

The X-ray structures of the isomorphous Mg²⁺, Ca²⁺, Mn²⁺ and Co²⁺ complexes of ATP have been determined. The metal ions are wrapped in hexa-coordination by the α, β and γ phosphate groups of two ATP molecules thus blocking the interaction of the metal ions with the adenine base. A second metal ion which is fully hydrated, M(H₂O)²⁺ ₆, is engaged in a strong hydrogen bond with the γ phosphate group of ATP and suggests a possible step in facilitating the cleavage between the β and γ phosphates in phosphoryl transfer reactions.     

Temperature and pressure dependent equilibrium studies of 1, 4, 8, 11-tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane nickel(ii) in coordinating solvents

In coordinating solvents, the complex 1, 4, 8, 11- tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane nickel(II) bisperchlorate exists as an equilibrium mixture involving four coordinate R,S,R,S-[Ni(tmc)]²⁺ and five coordinate R,S,R,S-[Ni(tmc)(solvent)]²⁺ species. Spectrophotometric measurements of this equilibrium in a number of solvents have been conducted over a range of temperatures and pressures. The stability order for the five coordinate complex in the solvents investigated is CH₃CN>DMF>DMSO>C₆H₅CN> H₂O>ClCH₂CN at 25 °C. Differences in stability are considered in terms of the measured thermodynamic parameters ΔH° and ΔS°. Both steric and electronic factors were found to influence solvent coordination with the macrocyclic complex.For the equilibrium in CH₃CN, C₆H₅CN, DMF and H₂O, reaction volumes, ΔV°, of −3.2±0.5, −4.2±0.5, −0.2±0.5 and −0.5±0.5 cm³ mol⁻¹ respectively have been determined. Each is significantly smaller than the corresponding solvent molar volume. The ΔV° for the equilibrium in CH₃CN is comparable with the previously determined activation volume for exchange of this solvent on R, S, R, S- [Ni(tmc)(CH₃CN)]²⁺. The equilibrium and measured volume parameters are discussed in relation to the mechanism for solvent exchange.     

Metal ion complexes of d-biotin in solution. Stability of the stereoselective thioether coordination

By spectrophotometry and ¹H nmr, several of the stability constants of the thioether complexes between Mg²⁺, Ca²⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺, or Ag⁺ and d-biotin (Bio), tetrahydrothiophene (Tht), and dimethyl sulfide (Dms) have been measured in 50% aqueous ethanol, 96% N,N-dimethylformamide (DMF), 98% d₆-dimethyl sulfoxide, or in D₂O. With decreasing concentration of water, the thioether interaction increases with the biologically important metal ions, whereas, e.g., Ag⁺ behaves in the opposite way. The stability of these complexes is, in general, quite small: for example, with d-biotin in 96% DMF (I = 1.0; 25°C) log K_M(Bio)^M = 0.03 and 1.64 for Cu²⁺ and Ag⁺, respectively; in D₂O (I = 0.5 for Ag⁺, all others 2–5; 27°C) log K_M(Bio)^M ? ?1.0, ?1.4, ?1.2, ?0.9, or 4.20 for Mg²⁺, Ca²⁺, Zn²⁺, Cd²⁺, or Ag⁺. In those cases where the difference log K_M(Tht)^M ? log K_M(Bio)^M can be calculated, it is in the order of 0.3 log units; this observation, as well as the chemical shifts measured, confirm the earlier suggestion that the interaction at the sulfur of biotin is stereoselective: the metal ion coordinates from “below” the tetrahydrothiophene ring of biotin to the sulfur atom, i.e., trans to the urea ring. It is emphasized that despite the low stability of these complexes with the biologically meaningful metal ions, the extent of the interaction is enough to create specific structures.     

Theoretical insights into the metal chelating and antimicrobial properties of the chalcone based Schiff bases

The emergence of multi-drug resistant pathogens in infectious disease conditions accentuates the need for the design of new classes of antimicrobial agents that could defeat the multidrug resistance problems. As a new class of molecules, the Heterocyclic Schiff base is of considerable interest, owing to their preparative accessibility, structural flexibilities, versatile metal chelating properties, and inherent biological activities. In the present study, CAM-B3LYP/LANL2DZ and M062X/DEF2-TZVP level of density functional method is used to explore the complexation of chalcone based Schiff base derivatives by Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ metal ions. The HL(1-3)-Co²⁺, HL(1-3)-Ni²⁺ and HL(1-3)-Zn²⁺ complexes formed the distorted tetrahedral geometry. Whereas, the HL(1-3)-Cu²⁺ complexes prefers distorted square-planar geometry. The BSSE corrected interaction energies of the studied complexes reveals that Cu²⁺ ion forms the most stable complexes with all three chalcone based Schiff bases. Of the three Schiff bases studied, the HL2 Schiff base acts as a potent chelating agent and forms the active metal complexes than the HL1 and HL3 Schiff bases. Further, the strength of the interaction follows the order as Cu^2+?>?Ni^2+?>?Co^2+?>?Zn²⁺. The QTAIM analysis reveals that the interaction between the metal ions and coordinating ligand atoms are electrostatic dominant. The metal interaction increases the π-delocalisation of electrons over the entire chelate. Hence, the antimicrobial activity of the metal complexes is more effective than the free Schiff bases. Moreover, the HL(1-3)-Cu²⁺ complexes shows higher antimicrobial activities than the other complexes studied.     

Effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+ and Zn2+) and water coordination on the structure and properties of l-histidine and zwitterionic l-histidine

Interactions between metal ions and amino acids are common both in solution and in the gas phase. The effect of metal ions and water on the structure of l-histidine is examined. The effect of metal ions (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) and water on structures of His·M(H₂O)m, m = 0.1 complexes have been determined theoretically employing density functional theories using extended basis sets. Of the five stable complexes investigated the relative stability of the gas-phase complexes computed with DFT methods (with one exception of K⁺ systems) suggest metallic complexes of the neutral l-histidine to be the most stable species. The calculations of monohydrated systems show that even one water molecule has a profound effect on the relative stability of individual complexes. Proton dissociation enthalpies and Gibbs energies of l-histidine in the presence of the metal cations Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ were also computed. Its gas-phase acidity considerably increases upon chelation. Of the Lewis acids investigated, the strongest affinity to l-histidine is exhibited by the Cu²⁺ cation. The computed Gibbs energies ΔG are negative, span a rather broad energy interval (from ?130 to ?1,300 kJ/mol), and upon hydration are appreciably lowered.     

Estimation of pseudopeptide metal ion complexation tendencies by special MS methods, HPCE and circular dichroism

To obtain more insight into catalytic mechanisms of metallo enzymes and specific metal complexation by proteins we use linear and cyclic pseudopeptides as mimetics. Knowledge about tendencies of complex formation of different ligands with selected transition metal ions is an indispensable prerequisite for the development of homo- and hetero-dinuclear metallo enzyme mimetics. Three pseudotripeptide ligands were investigated with respect to formation tendency and properties of complexes with the transition metal ions Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺ and Mn²⁺. To study complexation tendencies we applied different methods. One of the important prerequisites for the application in a screening of series of peptide ligands is the necessity for a minimal amount of substance. We used and compared certain masspectrometric methods for the estimation of a rank order of complexation of certain transition metal ions. We also applied spectrophotometric titration, circular dichroism measurements, capillary electrophoresis and pH-rate profile of catalytic activity in the attempt to evaluate complex formation tendencies. Except for the spectrophotometric pH-titration and the pH-profile of catalytic activity all methods were applicable, but each method has its advantages and disadvantages depending on the separation effect of the ligand from the metal complex, and depending on the spectroscopic properties of ligand and complex. The results regarding complex formation are compared to each other. Comparison of pairs by MALDI-TOF- and ESI-MS allows an estimation of the rank order of complexation tendency of one ligand with different metal ions and requires the least amount of substance. The other investigated methods provided additional information on structural properties of the formed complexes; however either they required too much pseudopeptide ligand or were not applicable for all transition metal ions used in this study.     

Estimation of pseudopeptide metal ion complexation tendencies by special MS methods,HPCE and circular dichroism

Summary To obtain more insight into catalytic mechanisms of metallo enzymes and specific metal complexation by proteins we use linear and cyclic pseudopeptides as mimetics. Knowledge about tendencies of complex formation of different ligands with selected transition metal ions is an indispensable prerequisite for the development of homo-and hetero-dinuclear metallo enzyme mimetics. Three pseudotripeptide ligands were investigated with respect to formation tendency and properties of complexes with the transition metal ions Cu²⁺, Zn²⁺ Ni²⁺, Co²⁺ and Mn²⁺. To study complexation tendencies we applied different methods. One of the important prerequisites for the application in a secreening of series of peptide ligands is the necessity for a minimal amount of substance. We used and compared certain masspectrometric methods for the estimation of a rank order of complexation of certain transition metal ions. We also applied spectrophotometric titration, circular dichroism measurements, capillary electrophoresis and pH-rate profile of catalytic activity in the attempt to evaluate complex formation tendencies. Except for the spectrophotometric pH-titration and the pH-profile of catalytic activity all methods, were applicable, but each method has its advantages and disadvantages depending on the separation effect of the ligand from the metal complex, and depending on the spectroscopic properties of ligand and complex. The results regarding complex formation are compared to each other. Comparison of pairs by MALDI-TOF-and ESI-MS allows an estimation of the rank order of complexation tendency of one ligand with different metal ions and requires the least amount of substance. The other investigated methods provided additional information on structural properties of the formed complexes; however either they required too much pseudopeptide ligand or were not applicable for all transition metal ions used in this study.     

Adenylate cyclase orthologues in two filamentous entomopathogens contribute differentially to growth,conidiation, pathogenicity,and multistress responses

Adenylate cyclase (AC) is a core element of cAMP signalling network. Here we show functional diversity and differentiation of Beauveria bassiana AC (BbAC) and Metarhizium robertsii AC (MrAC). Severe growth defects occurred in ΔBbAC and ΔMrAC grown on nutrition-rich SDAY and several minimal media but were largely alleviated by adding cAMP to SDAY. Conidial yield increased greatly in ΔBbAC but decreased in ΔMrAC. During colony growth, ΔBbAC was highly sensitive to oxidation, high osmolarity, cell wall perturbation, carbendazim fungicide, Mn²⁺, Zn²⁺, Fe³⁺, and EDTA but more tolerant to Cu²⁺ while ΔMrAC showed higher osmotolerance, decreased sensitivity to Fe³⁺, and null response to carbendazim or cell wall stress despite similar responses to oxidation and other metal ions. Conidial UV-B resistance decreased by 32 % in ΔBbAC and 22 % in ΔMrAC despite little change in their theromotolerance. Median lethal time (LT₅₀) estimates of ΔBbAC and ΔMrAC against susceptible insects were 10.9 and 1.4 d longer than those from wild-type strains respectively. All the phenotypic changes were restored to wild-type levels by each gene complementation. Taken together, BbAC and MrAC regulated differentially conidiation, pathogenicity, and multistress responses in B. bassiana and M. robertsii, thereby making different contributions to their biocontrol potential.     

Thermodynamics and coordination characteristics of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 extraction complex

The extraction equilibrium of the hydronium-uranium(VI)-dicyclohexano-24-crown-8 complex was carried out in the crown ether1,2-dichloroethaneHCl aqueous solution system at different temperatures. The extraction complex has the overall composition (L)₂·(H₃O⁺·χH₂O)₂·UO₂Cl₄²⁻ (L = dicyclohexano-24-crown-8). The values of the extraction equilibrium constants (K_ex) increase steadily with a decrease in temperature: 13.5 (298 K), 7.96 (301 K), 4.20 (303 K) and 2.07 (305 K). A plot of log K_ex against 1/T shows a straight line. The value of the enthalpy change, ΔH°, was calculated from the slope and equals −212 kJ mol⁻¹. The value of the entropy change, ΔS°, was calculated from ΔH° and K_ex and equals −690 J K⁻¹ mol⁻¹, whereas ΔG° = −6.45 kJ mol⁻¹. Comparing these thermodynamic parameters with those of the dicyclohexano-18-crown-6 isomer A [1] (ΔS° = −314 J K⁻¹ mol⁻¹, ΔH° = −101 kJ mol⁻¹ and ΔG° = −8.37 kJ mol⁻¹), it can be seen that ΔH° and ΔS° are more negative for the former than for the latter, and both are enthalpy-stabilized complexes. The molecular structure of the complex has the feature that there are two H₅O₂⁺ ions in it, in contrast to the H₃O⁺ ions in the dicyclohexano-18-crown-6 isomer A complex [1]. Each of the H₅O₂⁺ ions is held in the crown ether cavity by four hydrogen bonds. The H₅O₂⁺ ion has a central bond. The uranium atom forms UO₂Cl₄²⁻ as a counterion away from the crown ether. The formation of this complex is in good agreement with more negative entropy change and less negative free energy change, as mentioned above.     

Analysis of the interaction of gallic acid and myoglobin by UV-vis absorption spectroscopy

UV-vis absorption spectroscopy has been used to analyze the interaction of myoglobin (Мb) and gallic acid (GA). The binding constants (4.38 × 10⁴ M^–1 at 298.15 K and 0.42 × 10⁴ М^–1 at 308.15K), the number of binding sites (h = 1.0), and the thermodynamic parameters of binding (ΔH, ΔS, and ΔG) have been determined. Hydrogen bonds have been shown to play a major role in the stabilization of the GA–Мb complexes. GA binding led to slight changes in the electronic state of the heme ring of the protein.     

Simultaneous determination of thermodynamic and kinetic parameters of aminopolycarbonate complexes of cobalt(II) and nickel(II) based on isothermal titration calorimetry data

The influence of the different side chain residues on the thermodynamic and kinetic parameters for complexation reactions of the Co²⁺ and Ni²⁺ ions has been investigated by using the isothermal titration calorimetry (ITC) technique supported by potentiometric titration data. The study was concerned with the 2 common tripodal aminocarboxylate ligands, namely, nitrilotriacetic acid and N‐(2‐hydroxyethyl) iminodiacetic acid. Calorimetric measurements (ITC) were run in the 2‐(N‐morpholino)ethanesulfonic acid hydrate (2‐(N‐morpholino) ethanesulfonic acid), piperazine‐N ,N ′‐bis(2‐ethanesulfonic acid), and dimethylarsenic acid buffers (0.1 mol L⁻¹, pH 6) at 298.15 K. The quantification of the metal‐buffer interactions and their incorporation into the ITC data analysis enabled to obtain the pH‐independent and buffer‐independent thermodynamic parameters (K , ΔG , ΔH , and ΔS ) for the reactions under study. Furthermore, the kinITC method was applied to obtain kinetic information on complexation reactions from the ITC data. Correlations, based on kinetic and thermodynamic data, between the kinetics of formation of Co²⁺ and Ni²⁺ complexes and their thermodynamic stabilities are discussed.