Complexation Constants Research Articles

Influence of Molecular Level Interactions of Ofloxacin and Methylxanthines in Pharmacotherapy

ObjectiveThe main objective of this investigation was to identify the solubility phase interactions of methylxanthines caffeine and theophylline with ofloxacin one of these combinations is used to treat asthma associated with secondary infections.Materials and Methods Solubility studies of ofloxacin were carried out in presence of different concentrations 0 to 4times10-2 moleslitre of caffeine and theophylline at 37deg C for 24 h. Solid ofloxacin-methylxanthines complexes were prepared by co-precipitation method. In vitro transport studies were performed and the samples were analyzed by HPLC method. In situ intestinal absorption of drugs was performed in Wistar Albino rats by rat gut technique.Results Solubility of ofloxacin was enhanced in presence of methylxanthines. Stability constants of caffeineofloxacin and theophylline-ofloxacin complexes were found to be 10.5 and 9.3 litremol respectively. These complexes belong to class lsquotype Arsquo and sub-class lsquoAL rsquo. Melting point FTIR and DSC results confirm the formation of complexes between methylxanthines and ofloxacin. In vitro transport studies reveal that methylxanthines were not readily available for transportation in presence of ofloxacin due to their complex form. Transport of theophylline 0.01 min-1 is twice faster than that of the caffeine 0.0046 min-1. In situ absorption studies indicated that the absorption of methylxanthines was differed in presence of ofloxacin and vice versa and their absorption followed zero order kinetics. Absorption rate constants of caffeine theophylline and ofloxacin were 4.44 5.25 and 3.7 h-1 respectively.Conclusion Regression and statistical results supported the significant interactions between ofloxacin and methylxanthines which are noteworthy for pharmacotherapy

Molecular mechanism of chromogenic substrate hydrolysis in the active site of human carboxylesterase-1

Human carboxylesterases are involved in the protective processes of detoxification during the hydrolytic metabolism of xenobiotics. Knowledge of the molecular mechanisms of substrates hydrolysis in the enzymes active site is necessary for the rational drug design. In this work, the molecular mechanism of the hydrolysis reaction of para-nitrophenyl acetate in the active site of human carboxylesterase was determined using modern methods of molecular modeling. According to the combined method of quantum mechanics/molecular mechanics calculations, the chemical reaction occurs within four elementary steps, including two steps of the acylation stage, and two steps of the deacylation stage. All elementary steps have low energy barriers, with the gradual lowering of the intermediate energies that stimulates reaction in the forward direction. The molecular docking was used to estimate the binding constants of the enzyme-substrate complex and the dissociation constant of enzyme-product complexes. The effective kinetic parameters of the enzymatic hydrolysis in the active site of carboxylesterase are determined by numerical solution of the differential kinetic equations.

Determination of affinities of lanthanide-binding proteins using chelator-buffered titrations.

The recent discoveries of the first proteins that bind lanthanides as part of their biological function not only are relevant to the emerging field of lanthanide-dependent biology, but also hold promise to revolutionize the technologically critical rare earths industry. Although protocols to assess the thermodynamics of metal-protein interactions are well established for "traditional" metal ions in biology, the characterization of lanthanide-binding proteins presents a challenge to biochemists due to the lanthanides' Lewis acidity, propensity for hydrolysis, and high-affinity complexes with biological ligands. These properties necessitate the preparation of metal stock solutions with very low buffered "free" metal concentrations (e.g., femtomolar to nanomolar) for such determinations. Herein we describe several protocols to overcome these challenges. First, we present standardization methods for the preparation of chelator-buffered solutions of lanthanide ions with easily calculated free metal concentrations. We also describe how these solutions can be used in concert with analytical methods including UV-visible spectrophotometry, circular dichroism spectroscopy, Förster resonance energy transfer (FRET), and sensitized terbium luminescence, in order to accurately determine dissociation constants (Kds) of lanthanide-protein complexes. Finally, we highlight how application of these methods to lanthanide-binding proteins, such as lanmodulin, has yielded insights into selective recognition of lanthanides in biology. We anticipate that these protocols will facilitate discovery and characterization of additional native lanthanide-binding proteins, will motivate the understanding of their biological context, and will prompt their applications in biotechnology.

Potentiometric studies on bioactive material species of ternary complexes in SLS-Water mixtures

Chemical speciation plays key role in understanding the physiological and biological aspects of metal ions and ligands in human body. The complexation of essential metal ions with Glycylglycine (GG) and Histamine (Him) in Sodium Lauryl Sulphate (SLS) has been presented in this study. The complexes formed in the present study are M L X H 2 , M L 2 X H 2 and M L X 2 H 3 . The Skewness values in the present study are from 0.05 to 0.84 for Ca(II), 0.03 to 0.93 for Mg(II) and 0.45 to 1.34 for Zn(II) and the χ2 values obtained the present study are ranging from 7.8250.37 for Ca(II), 6.1837.45 for Mg (II) and15.69143.67 for Zn(II). A very low standard deviation in log β values indicates the precision of the parameters. The advanced principles of mathematics and fuzzy logic have been employed to correlate chemical problems, which is called chemo metrics. In the present investigation potentiometer is used for analysis and the results obtained in the study have been processed through Gran plot method, SCPHD is used for best fit chemical model and MINIQUAD75 is used for refinement of results. An experiment has been conducted to understand the distribution of species, effect of surfactant on stability constants of ternary complexes, effect of influential parameters like concentration of alkali, acid, ligands and metal ions on the stability constants.

Conformational selection of vasopressin upon V1a receptor binding

The neuropeptide vasopressin (VP) and its three G protein-coupled receptors (V1aR, V1bR and V2R) are of high interest in a wide array of drug discovery programs. V1aR is of particular importance due to its cardiovascular functions and diverse roles in the central nervous system. The structure–activity relationships underpinning ligand-receptor interactions remain however largely unclear, hindering rational drug design. This is not least due to the high structural flexibility of VP in its free as well as receptor-bound states. In this work, we developed a novel approach to reveal features of conformational selectivity upon VP-V1aR complex formation. We employed virtual screening strategies to probe VP’s conformational space for transiently adopted structures that favor binding to V1aR. To this end, we dissected the VP conformational space into three sub-ensembles, each containing distinct structural sets for VP’s three-residue C-terminal tail. We validated the computational results with experimental nuclear magnetic resonance (NMR) data and docked each sub-ensemble to V1aR. We observed that the conformation of VP’s three-residue tail significantly modulated the complex dissociation constants. Solvent-exposed and proline trans-configured VP tail conformations bound to the receptor with three-fold enhanced affinities compared to compacted or cis-configured conformations. The solvent-exposed and more flexible structures facilitated unique interaction patterns between VP and V1aR transmembrane helices 3, 4, and 6 which led to high binding energies. The presented “virtual conformational space screening” approach, integrated with NMR spectroscopy, thus enabled identification and characterization of a conformational selection-type complex formation mechanism that confers novel perspectives on targeting the VP-V1aR interactions at the level of the encounter complex – an aspect that opens novel research avenues for understanding the functionality of the evolutionary selected conformational properties of VP, as well as guidance for ligand design strategies to provide more potent and selective VP analogues.

Determination of Stability constants Nickel binary and ternary complexes in aquesous DMSO by Pontentiometric method

The stability constants of some of the binary and ternary system involving Nickel(II) have been studied. Bis-(pyridyl) benzilidene acts as a primary ligand. Glycine, L-alanine, L-phenylalanine, L-leucine and L-valine were acts as a secondary ligand. The stability constants of complex are calculated by SCOGS computer program. The reported values are ± 0.02 log unit accuracy. The protonation constants values increase with an increasing percentage of dimethyl sulphoxide. Protanation constants values for the glycine, L-valine ternary complexes higher stability constants indicate that inductive effect is predominant than steric effect. Whereas L-Phenylalanine ternary system has high stability constants than other ternary complexes due to stacking interaction between phenyl ring of phenylalanine and phenyl ring of primary ligand. The order of stability constant of ternary systems is Ni(bpb)(phe)2 > Ni(bpb)(leu)2 > Ni(bpb)(val)2 > Ni(bpb)(gly)2.

Computer simulated approach to the formation of mixed coordinated complexes of DOPA with toxic metal ions

The potentiometric method is adopted to determine formation constants of complexes and used with a new numerical scheme. This study deals with the coordination behaviour of L-DOPA ((3,4-dihydroxyphenyl)alanine) in Hg(II)/Pb(II)/Cd(II)-DOPA-L-alanine/glycine/L-phenylalanine (1:1:1) systems. DOPA is recognized as a neurotransmitter, and used in the treatment of Parkinson’s disease. All the selected ligands are biologically active and play a vital role in the physiological processes. All the systems were investigated pH-metrically at 20±1°C and 30±1°C at three different ionic strengths in aqueous medium. Stability Constants of Generalized Species’ computer program was used to obtain refined values of formation constants. Thermodynamic parameters (logk μ→ 0, ΔG o, ΔH o, ΔS o) for biligand systems were calculated. The speciation of different species in mixed coordinated systems was obtained up to a specific pH in particular system. High values of formation constants support the formation of stable ternary complexes. Biligand complexes involving alanine were found to be the most stable, while those of phenylalanine were the least with lowest concentration at a particular pH in specific conditions. Results obtained are helpful in understanding the physiological behaviour of these systems.

Effect of temperature on the complexation of triscarbonatouranyl(VI) with calcium and magnesium in NaCl aqueous solution.

The complex formation of triscarbonatouranyl(VI) UO2(CO3)34- with the alkaline earth metal ions Mg2+ and Ca2+ in 0.10 mol kgw-1 NaCl was studied at variable temperatures: 5-30 °C for Mg2+ and 10-50 °C for Ca2+. Under appropriate conditions, the ternary complexes (MnUO2(CO3)3(4-2n)- with n = 1 for Mg, n = {1; 2} for Ca) were identified by time-resolved laser-induced luminescence spectrometry. Their pure spectral components at 50 °C for CanUO2(CO3)3(4-2n)- and 30 °C for MgUO2(CO3)32- were recovered by multivariate curve resolution alternating least-squares analysis. Approximation models were tested to fit the experimental data-the equilibrium constants of complexation measured at different temperatures-and deduce the thermodynamic functions, i.e., enthalpy, entropy, and heat capacity. The weak influence of temperature on complexation constants induces large uncertainties in terms of thermodynamic functions. Assuming the enthalpy is constant with temperature using the Van't Hoff equation, the first stepwise complexation of UO2(CO3)34- by Ca2+ is estimated to be slightly endothermic, with , while the second stepwise complexation of CaUO2(CO3)32- by Ca2+ with is slightly exothermic, . In contrast to Ca2+, the complexation of UO2(CO3)34- by Mg2+ is slightly exothermic, with . These values are not significantly different from zero inasmuch as the uncertainties are important due to a weak dependence of log10 K° values. The entropic character of the complexation is verified as for the first stepwise complexation of UO2(CO3)34- by Ca2+, for the second stepwise complexation of CaUO2(CO3)32- by Ca2+, and for the complexation of UO2(CO3)34- by Mg2+. The energetics of complexation and sensitivity analysis of the model estimates with temperature are discussed. The uranium speciation in the case of the safety of nuclear waste management, using the present thermodynamic functions, provides support to the assessment of underground nuclear waste repositories.

An expanded halogen bonding scale using astatine.

As a non-covalent interaction, halogen bonding is now acknowledged to be useful in all fields where the control of intermolecular recognition plays a pivotal role. Halogen-bond basicity scales allow quantification of the halogen bonding of referential donors with organic functional groups from a thermodynamic point of view. Herein we present the pKBAtI basicity scale to provide the community an overview of halogen-bond acceptor strength towards astatine, the most potent halogen-bond donor element. This experimental scale is erected on the basis of complexation constants measured between astatine monoiodide (AtI) and sixteen selected Lewis bases. It spans over 6 log units and culminates with a value of 5.69 ± 0.32 for N,N,N′,N′-tetramethylthiourea. On this scale, the carbon π-bases are the weakest acceptors, the oxygen derivatives cover almost two-thirds of the scale, and sulphur bases exhibit the highest AtI basicity. Regarding the applications of 211At in targeted radionuclide therapy, stronger labelling of carrier agents could be envisaged on the basis of the pKBAtI scale.

Hydrogen Bond-Based Macrocyclic and Tripodal Neutral Ionophores for Highly Selective Polymeric Membrane Sulfate-Selective Electrodes.

Four hydrogen bond-based macrocyclic and tripodal neutral receptors with increasing conformational complementarity with sulfate were used for the first time as ionophores to develop polymeric membrane sulfate-selective electrodes. Optimizing the membrane composition such as ionophores, lipophilic additives, and plasticizers yielded ISEs which showed Nernstian response to sulfate with the best selectivity so far and improved detection limits (a slope of -29.8 mV/dec in the linear range of 1 × 10-6-1 × 10-1 M with a detection limit of 5 × 10-7 M), which led to the success of the determination of sulfate in drinking water samples and neomycine tablets. The anion-ionophore complex constants in the membrane phase were determined and correlated with the selectivity sequence of the ISEs. Studies on the influence of pH of the sample solution demonstrated that the developed ISEs can be operated in a wide pH range of 3-8 with fast response and rapid (in 1 min) and long lifetime. The success of these ionophores represents a feasible strategy for overcoming the "Hofmeister series" by employing a combination of complementarity and hydrogen bonds.

Complexes of cyclen side-bridged with a methylene-bis(phosphinate) group

A macrobicyclic cyclen-based ligand containing a geminal bis(phosphinate) N,N′-bridge was synthesized via a Mannich-type reaction. The other two macrocycle amine groups were modified with two acetate pendants, yielding ligand H4L. The cyclen-bis(phosphinate) intermediate, the ligand salt Na0.5(H5L)(H3.5L)Cl·20H2O and CuII complex Li2[Cu(L)]·4H2O were characterized by single-crystal X-ray diffraction. The ligand protonation constants and the stability constants of H4L complexes with Cu(II) and Zn(II) and lanthanide(III) ions were determined by potentiometry. The ligand protonation constants and the stability constants of H4L complexes with Cu(II) and Zn(II) ions are similar to those of H4dota. Conversely, the stabilities of its Ln(III) complexes are ~ 7–10 orders of magnitude lower than those of H4dota complexes due to steric restrictions of the methylene-bis(phosphinate) side bridge. Therefore, these findings indicate that the ligand H4L is suitable for the complexation of transition metal ions.

Double-sided and single-sided polished 6H-SiC wafers with subsurface damage layer studied by Mueller matrix ellipsometry

The complex optical constants and the subsurface damage layer of uniaxial doped 6H-SiC wafers are measured using Mueller matrix spectroscopic ellipsometry. A comparison is made between measurements on a single-sided polished wafer that can be treated as a semi-infinite substrate and on a double-sided polished wafer that is studied with the partial-wave theory. The refractive indices and extinction coefficients for ordinary and extraordinary directions are determined below the bandgap after point-by-point fitting of experimental Mueller matrices. The thickness of the subsurface damage layer caused by mechanical polishing and chemical mechanical polishing determined by ellipsometry is consistent with the result of transmission electron microscope. The ellipsometry results show that the analysis based on a double-sided polished wafer is more informative about the optical properties of 6H-SiC than that of the single-sided polished wafer as it renders information properties about the bulk properties of the material, for example, allowing the determination of the very weak absorption coefficient (k≈10−4) due to doping.

Enantioseparation of three constitutional isomeric 2-(methylphenyl)propanoic acids by countercurrent chromatography

2-(3-Methylphenyl)propanic acid and 2-(4-methylphenyl)propanoic acid were successfully enantioseparated by countercurrent chromatography using hydroxypropyl-β-cyclodextrin (HP-β-CD) as a chiral selector. 2-(2-Methylphenyl)propanoic acid was also studied to compare the enantioseparation ability of three isomeric 2-(methylphenyl)propanoic acids. Totally 20 mg of 2-(3-methylphenyl)propanic acid and 20mg of 2-(4-methylphenyl)propanic acid were enantioseparated individually by countercurrent chromatography. Recovery for the (±)-2-(3-methylphenyl)propanic acid enantiomer was in the range of 85%-90% with 98.0%-98.8% purity and recovery for the (±)-2-(4-methylphenyl)propanic acid enantiomer was in the range of 80%-83% with 97.0%-98.0% purity. The enantioseparation factor in countercurrent chromatography for 2-(4-methylphenyl)propanic acid and 2-(3-methylphenyl)propanic acid were 1.31 and 1.26, and the peak resolution in HPLC reached 2.2 and 1.4. However, no enantioseparation could be found for 2-(2-methylphenyl)propanic acid. In addition, the inclusion complexes were investigated by UV spectrophotometer. The inclusion formation constant of inclusion complex between 2-(4-methylphenyl)propanic acid, 2-(3-methylphenyl)propanic acid, 2-(2-methylphenyl)propanic acid and HP-β-CD were determined as 121.73 mol/L, 78.12 mol/L and 53.18 mol/L, respectively. The present results showed that enantiorecognition was greatly affected by substituted positions of methyl group on the benzene ring. Combined with our previous results, the steric hindrance had a significant effect on inclusion interaction between HP-β-CD and racemic 2-(substitutedphenyl)propanoic acids. No enantiorecognition could be achieved for 2-(substitutedphenyl)propanoic acids with ortho-substituent group on benzene, while the influence of meta- and para- group on enantiorecognition varies with different substituent groups on benzene ring.

Undular Diffusion in Nonlinear Sigma Models.

We discuss general features of charge transport in nonrelativistic classical field theories invariant under non-Abelian unitary Lie groups by examining the full structure of two-point dynamical correlation functions in grand-canonical ensembles at finite charge densities (polarized ensembles). Upon explicit breaking of non-Abelian symmetry, two distinct transport laws characterized by dynamical exponent z=2 arise. While in the unbroken symmetry sector, the Cartan fields exhibit normal diffusion, the transversal sectors governed by the nonlinear analogs of Goldstone modes disclose an unconventional law of diffusion, characterized by a complex diffusion constant and undulating patterns in the spatiotemporal correlation profiles. In the limit of strong polarization, one retrieves the imaginary-time diffusion for uncoupled linear Goldstone modes, whereas for weak polarizations the imaginary component of the diffusion constant becomes small. In models of higher rank symmetry, we prove absence of dynamical correlations among distinct transversal sectors.

Biophysical insight into the binding mechanism of doxofylline to bovine serum albumin: An in vitro and in silico approach

Insight into the mechanistic binding of bovine serum albumin (BSA) with doxofylline can layout pivotal enlightenment with relevance to pharmacokinetics and pharmacodynamics properties. Herein, many spectroscopic techniques and computational methods had been employed to interpret the structural and binding dynamics of BSA-doxofylline interaction. Doxofylline quenched the intrinsic fluorescence of BSA by static quenching. The stoichiometry and the binding constant of the BSA-doxofylline complex were 1:1 and in the order of 103 M−1. It was also concluded that the binding process was spontaneous and exothermic, primarily based on the thermodynamic study. Circular dichroism and three-dimensional excitation-emission matrix fluorescence results concluded pronounced conformational and microenvironmental changes in BSA structure on binding with doxofylline. The influence of metal ions and vitamins on the binding affinity of the BSA-doxofylline system were also explored. The in vitro findings were further supported by in silico analysis. With a score value of −6.25 kcal/mol, molecular docking showed strong interactions. Molecular dynamics simulation interpretation also suggested the stable binding with lower deviation in the values of RMSD and RMSF obtained by uninterrupted long simulation run. These studies will propose the optimum potency of distribution of the doxofylline into the bloodstream for asthma treatment.

Application of capillary electrophoresis technique for the enantioseparation of bioactive ferrocene-based compounds versus DFT calculated data.

Herein, a series of bioactive ferrocene-modified N-heterocycles with alkyl linkers was prepared in good to quantitative yields starting from easy accessible ferrocene alcohols and heterocycles under acidic or neutral (for imidazole) conditions in racemic forms. The analytical resolution of a number of bioactive racemic ferrocene azoles 1-6 (where azole = imidazole, pyrazole, and benzotriazole derivatives) into enantiomers was first carried out by CE using sulfobuthylether-β-CD (captisol) as a chiral selector. The analytical approaches to highly enantiomeric-enriched ferrocene derivatives are based on the formation of their inclusion complexes. The best chiral separation was achieved using zone CE in a quartz capillary. The ACE was used to evaluate the stability constants of captisol complexes with enantiomeric forms of two ferrocene derivatives 1, FcCHMe-imidazole, and 6, FcCHMe-benzotriazole. The optimal conditions for the resolution of the studied (R, S)-ferrocene compounds 1, 2, and 6 were predicted on the basis of the performed quantum chemical calculations and then implemented by the electrophoretic method. A high correlation between density functional theory calculation results and experimental electrophoresis data were obtained. Successful enantioseparation of racemic mixtures is of great importance for the characterization and further applications of drug candidates in enantiopure forms and in the development of clinical treatment. The advantages of the CE procedure make it possible to have important practical value and significance for determining the purity and enantiomeric excess of other ferrocene-containing compounds.

Potentiometric Studies on Bioactive Material Species of Ternary Complexes

Complexation of essential metal ions with glycylglycine and histamine in cetyltrimethylammonium bromide (CTAB) has been presented in this study, which targets insight in the distribution of species, effect of surfactants on stability constants of ternary complexes, and effect of concentration of alkali, acids, ligands, and metal ions on the stability constants. The studied complexes are MLXH2, ML2XH2, and MLX2H3. The advanced principles of mathematics and fuzzy logic have been employed to correlate chemical problems, known as chemo metrics. A potentiometer has been used for analysis, and the results obtained have been processed through gran plot method, SCPHD used for best fit chemical model. MINIQUAD75 has been used for refinement of the results.

Determining Stability Constants of Complexes Between Ester Betulin Derivatives and β-Cyclodextrin by Affinity Capillary Electrophoresis

Inclusion complexes of ester betulin derivatives (pentacyclic lupane triterpenoids) with β-cyclodextrin (β-CD) were studied by affinity capillary electrophoresis. Stability constants were calculated from dependencies of viscosity-corrected effective electrophoretic mobilities on β-CD concentration (0-10 mM) in background electrolytes using nonlinear regression fitting. Logarithms of stability constants at 25 °C for 1:1 and 1:2 β-CD complexes of betulin 3,28-diphthalate, betulin 3,28-disuccinate and betulin 3,28-disulfate (95 % confidence interval) are 4.25 (4.16-4.32) and 7.27 (6.73-7.50), 4.38 (4.26-4.48) and 7.58 (6.90-7.84), 4.04 (4.00-4.08) and 5.91 (4.60-6.20), respectively

A Conductometric Study of Complexation Reactions between Kryptofix5 with Al3+, Fe3+, Cu2+, and Ca2+ Metal Cations in Acetonitrile–Ethanol Binary Solvent Solutions

In the current study, the results of the conductometric work of the complexation reaction of Kryptofix5 (Scheme 1) with Al3+, Cu2+, Ca2+, and Fe3+ cations are studied in acetonitrile–ethanol (AN–EtOH) binary non-aqueous solvents. In all cases, the achieved results reveal that the stoichiometry of the complexes formed between the ligand and Al3+, Cu2+, Ca2+, and Fe3+ metal cations is 1 : 1 [ML] complex. GENPLOT (the computer program) calculates the stability constants of complexes of (Kryptofix5 · M)n+. The selectivity order of Kryptofix5 ligand for the studied metal cations in pure AN and AN–EtOH (AN = 50 mol % ) binary mixed solvent at 15°C is found to be Ca2+ > Fe3+ > Cu2+ > Al3+ and Cu2+ > Fe3+ > Ca2+ > Al3+, respectively. As a result, the selectivity order of the ligand for the metal cations shows independence from the nature and composition of the mixed binary solvent. By using the Van’t Hoff plots, the thermodynamic parameters ( $$\Delta H_{{\text{c}}}^{^\circ }$$ , $$\Delta S_{{\text{c}}}^{^\circ }$$ ) value for 1 : 1 complexes are calculated from the temperature due to the stability constants of the complexes. The achieved findings reveal that the thermodynamics of the complexation reaction is affected by the nature and composition of the mixed solvents, and the complexation processes entropy and enthalpy are stabilized in most cases.

TETRAPHENYLPORPHINE AS A MODEL SYSTEM FOR STUDYING THE INFLUENCE OF THE NATURE OF THE SOLVENT ON THE THERMODYNAMICS OF AXIAL COORDINATION WITH METALLOPORPHYRINS

The coordination of zinc(II)tetraphenylporphine with pyridine N-oxides in methylene chloride was studied by electron spectroscopy. Linear correlations are observed between the logarithms of the stability constants of complexes, the shifts of the maximum absorption bands of Zn-TPP in electronic spectra during complexation, the basicity of the ligand, and the σ-constants of the substituent Gammet. In the case of pyridines in CHCl 3 , CCl 4 , and 1,2-dichloroethane, complexation is isoequilibrium compensation, but with N-oxides of pyridines, the replacement of chloroform (isoenthalpic process) with methylene chloride (isoequilibrium anticompensation process) leads to drastic changes in the thermodynamics of coordination. The change of the isoentalpic behavior of pyridine N-oxides in chloroform to the anticompensation behavior in methylene chloride in coordination with Zn-TPP may indicate a change in the mechanism of complexation. It is suggested that this may be due to a change in the type of hybridization of the oxygen atom of the N→O group of the ligand depending on the polarity of the solvent. In favor of this hypothesis is the fact that coordination anilines with Zn-TPP in chloroform (as in the case of N-oxides is possible rehybridization of nitrogen atom of the amino group) is also isoenthalpic process, but in the methylene chloride becomes isoequilibrium unlike the N-oxides of pyridines with positive value β. We found good linear correlations between the rate constants of horseradish peroxidase reactions in aqueous solutions with anilines and the stability constants of Zn-TPP complexes with the same ligands in chloroform. We hope that this work will help clarify important patterns in this area of biochemistry, in particular thermodynamic features of peroxidase functioning.