Studies on the protonation constants and solvation of alpha-amino acids in dioxan-water mixtures

To gain more information about the effect of solvent on alpha-amino acids, the stoichiometric protonation constants of 10 alpha-amino acids (glycine, DL-alanine, DL-valine, L-leucine, L-isoleucine, DL-phenylalanine, L-serine, L-threonine, L-asparagine, and L-glutamine) in different dioxan-water mixtures have been determined potentiometrically using a combined pH electrode system calibrated in concentration units of hydrogen ion at 25 degrees C with an ionic strength of 0.10 M. For all amino acids studied, it was observed that the log K(1) values relating to the protonation equilibria of the anionic form are almost unaltered by the change in solvent composition. However, the log K(2) values corresponding to the formation equilibria of cationic form increase with the increase in dioxan content. The variation of these constants is discussed on the basis of specific solute-solvent interactions and structural changes of amino acids from water to dioxan-water media. The zwitterionic to neutral form ratio of these acids in dioxan-water mixtures is also discussed.     

Influence of dielectric constant on protonation equilibria of maleic acid and L-asparagine in acetonitrile water-mixtures

Abstract

The protonation constants of maleic acid and L-asparagine have been studied pH-metrically in various concentrations (0–50% v/v) of acetonitrile–water mixtures maintaining an ionic strength of 0.16 mol L^-1 at 30⁰C. The protonation constants have been calculated using the computer program MINIQUAD75 and are selected based on statistical parameters. Linear variation of step-wise protonation constants (log K) with the reciprocal of the dielectric constant of the solvent mixture has been attributed to the dominance of the electrostatic forces.     

Effect of dielectric constant of medium on protonation equilibria of ethylenediamine

Abstract

The effect of dielectric constant of medium on protonation equilibria has been studied by determining protonation constants of ethylenediamine pH metrically in various concentrations (0–60%v/v) of acetoni-trile– and ethylene glycol–water mixtures, at an ionic strength of 0.16mol L^?1 and at 303.0 K. MINIQUAD75 computer program has been used for the calculation of protonation constants. Linear and non-linear variations of step-wise protonation constants with reciprocal of dielectric constant of the solvent mixtures have been attributed to the dominance of the electrostatic and non-electrostatic forces, respectively. The trend is explained on the basis of solute–solute and solute–solvent interactions, solvation, proton transfer processes and dielectric constants of the media.     

Effect of dielectric constant on protonation equilibria of 2, 3-dihydroxybenzoic acid and malonic acid in 1, 2-propanediol-water mixtures

Abstract

The protonation constants of 2,3-diydroxybenzoic acid (2, 3-DHBA) and malonic acid (MA) at 303.0 ± 0.1 K and 0.16 mol L^-1 ionic strength in various concentrations (0–60% v/v) of 1,2-propanediol–water-mixtures were determined by pH-metric method. The protonation constants were calculated with MINIQUAD75 computer program. Selection of the best fit chemical models of the acid–base equilibria was based on statistical parameters. The log K values were found to increase with the increase in percentage of 1,2-propanediol and vary linearly with the reciprocal of the dielectric constant of the medium. This has been attributed to the dominance of electrostatic forces. Distributions of species and effect of influential parameters on the protonation constants are also presented.     

The protonation equilibria of selected glycine dipeptides in ethanol-water mixture: solvent composition effect

Knowledge of the protonation constants of small dipeptide is important, interesting and necessary for complete understanding of the physiochemical behavior of dipeptide. In this study, the protonation constants of some aliphatic dipeptides (Gly–Gly, Gly–Val, Gly–Leu, Gly–Thr, Gly–Phe and Gly–Met) were studied in water and ethanol–water mixtures [20% ethanol–80% water, 40% ethanol–60% water, 60% ethanol–40% water, (v/v)] at 25 ± 0.1°C under nitrogen atmosphere and ionic strength at 0.10 mol dm⁻³ by potentiometry. The constants of the systems were calculated by using BEST computer program, and distribution species diagrams were produced using the SPE computer program. The protonation constants were influenced by changes in solvent composition, and their variations were discussed in terms of solvent and structural properties. The concentration distribution of the various species in ethanol–water mixtures was evaluated.     

Fluorescence studies on some hydroxypyridines including compounds of the vitamin B6 group

1. The variations with pH (from 36n-sulphuric acid to 10n-sodium hydroxide) of the excitation and fluorescence wavelengths and fluorescence intensity of 2-, 3- and 4-hydroxypyridine and their O- and N-methyl derivatives were investigated. 2. 4-Hydroxy- and 4-methoxy-pyridine were non-fluorescent at all pH values. 3. The cations and dipolar ions of the 3-hydroxypyridine derivatives and the anion of 3-hydroxypyridine were fluorescent, but the neutral forms were not. 4. All the forms of the 2-hydroxypyridine derivatives were fluorescent. 5. Pyridoxol, pyridoxal and its 5-phosphate, pyridoxamine and pyridoxic acid and its lactone were studied similarly. All these compounds, except pyridoxal 5-phosphate, were more fluorescent than 3-hydroxypyridine. 6. The most fluorescent forms of these compounds are the anions, except for pyridoxol, where the dipolar ion was the most fluorescent form. The least fluorescent forms are the neutral molecules. The dipolar ions were appreciably fluorescent in all cases. 7. The most fluorescent form examined was the dianion of pyridoxic acid lactone. 8. The cations were all fluorescent except the cations of 2- and 3-methoxypyridine. All the cations showed excited-state ionization. The excited pK(a) values of these cations were determined and the results are discussed with reference to Weller's (1952) equation relating ground- and excited-state dissociation constants. 9. The pK(a) values for all ionizations undergone by the compounds examined were determined from fluorescence data. 10. Stokes shifts for the various ionic and neutral species of the compounds examined were calculated and are discussed.     

STUDIES IN THE PHYSICAL CHEMISTRY OF AMINO ACIDS,PEPTIDES, AND RELATED SUBSTANCES : XI. THE SOLUBILITY OF CYSTINE IN THE PRESENCE OF IONS AND ANOTHER DIPOLAR ION

1. As an introduction to the relations that obtain in biochemical systems containing several components, some ionic, some dipolar ionic, the solubility of cystine has been investigated in the presence of glycine and neutral salts. 2. Both glycine and sodium chloride increase cystine solubility at all concentrations. The interaction between cystine and ions is, however, diminished with increase in glycine concentration, and the interaction between cystine and glycine with increase in ionic strength. 3. Sodium sulfate also increases the solubility of cystine, but at concentrations greater than one molal its solvent action is smaller than its salting-out effect, which is greater at all concentrations than that of sodium chloride, and greater the higher the glycine concentration. 4. These interactions are defined by an equation giving the solubility ratio of cystine in terms of salting-out constants, constants related to the electric moments of cystine, and to the ionic strength and dielectric constant of the solution. 5. The higher the concentration of glycine and therefore the dielectric constant of the solution, the smaller that part of the interaction between ions and dipolar ions which depends upon Coulomb forces and the greater appears the salting-out effect. 6. Conversely, the greater the ionic strength and the salting-out effect the smaller the interaction between dipolar ions in solution.     

N-(2-Carboxyethyl)chitosans: regioselective synthesis,characterisation and protolytic equilibria

N-(2-Carboxyethyl)chitosans were obtained by reaction of low molecular weight chitosan with a low degree of acetylation and 3-halopropionic acids under mild alkaline media (pH 8-9, NaHCO3) at 60 degrees C. The chemical structure of the derivatives obtained was determined by 1H and 13C NMR spectroscopies. It was found that alkylation of chitosan by 3-halopropionic acids proceeds exclusively at the amino groups. The products obtained are described in terms of their degrees of carboxyethylation and ratio of mono-, di-substitution and free amine content. The protonation constants of amino and carboxylate groups of a series of N-(2-carboxyethyl)chitosans were determined by pH-titration at ionic strength 0.1 M KNO3 and 25 degrees C.     

Modeling ATP protonation and activity coefficients in NaClaq and KClaq by SIT and Pitzer equations

The acid-base properties of Adenosine 5'-triphosphate (ATP) in NaCl and KCl aqueous solutions at different ionic strengths (0相似文献   

Diverse associations in the ternary systems of β-cyclodextrin, simple carbohydrates and phenyl derivatives of inorganic oxoacids

Complex formation reactions of phenylboronic, phenylphosphonic, phenylarsonic and 4-aminophenyl arsonic acids with β-cyclodextrin (cycloheptaamylose, β-CD) and some simple carbohydrates (mannitol, sorbitol, glucose) have been studied using spectrophotometric, potentiometric methods and solubility measurements, supplemented with HPLC and IR analyses of the solid samples. Equilibrium constants have been determined at ionic strength of 0.2 M (NaCl) and 25 °C. β-CD forms the most stable complexes with the neutral, undissociated forms of the acids, the stability constants are as follows: phenylboronic acid: 320 ± 36, phenylphosphonic acid: 108 ± 25, phenylarsonic acid: 97 ± 4 and 4-aminophenyl arsonic acid: 107 ± 10. The stability constants for the β–CD-complexes of the ionic forms are much lower. Ternary complexes of low stability could be detected in the case of phenylphosphonic acid and sorbitol with the undissociated form and with glucose and the dianion. In more concentrated solutions phenylboronic acid forms insoluble complexes with mannitol, sorbitol and β-CD. The solid phases obtained in the ternary systems are predominantly mixtures of ester type 3:1 complexes with the carbohydrate and 1:1 inclusion complex with the β-CD. No significant interaction has been found with glucose. The phenomena can be explained by the differences in the structures of the components and by the changes in the H-bonding network of β-CD on the complex formation.     

Effect of ionic liquid on the kinetics of peroxidase catalysis

The effect of a water-miscible ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), on the horseradish peroxidase (HRP)-catalyzed oxidation of 2-methoxyphenol (guaiacol) with hydrogen peroxide (H2O2) was investigated. HRP maintains its high activity in the aqueous mixtures containing various concentrations of the ionic liquid and even in 90% (v/v) ionic liquid. In order to minimize the effect of solution viscosity on the kinetic constants of HRP catalysis, the enzymatic reactions in the subsequent kinetic study were performed in water-ionic liquid mixtures containing 25% (v/v) ionic liquid at maximum. As the concentration of [BMIM][BF4] increased for the oxidation of guaiacol by HRP, the K(m) value increased with a slight decrease in the k(cat) value: The K(m) value increased from 2.8 mM in 100% (v/v) water to 22.5 mM in 25% (v/v) ionic liquid, indicating that ionic liquid significantly weakens the binding affinity of guaiacol to HRP.     

Interactions of Some Divalent Metal Ions with Thymine and Uracil Thiosemicarbazide Derivatives

The study of interactions between metal ions and nucleobases, nucleosides, nucleotides, or nucleic acids has become an active research area in chemical, biological, and therapeutic fields. In this respect, the coordination behavior of nucleobase derivatives to transition metals was studied in order to get a better understanding about DNA-metal interactions in in vitro and in vivo systems. Two nucleobase derivatives, 3-benzoyl-1-[3-(thymine-1-yl)propamido]thiourea and 3-benzoyl-1-[3-(uracil-1-yl)propamido]thiourea, were synthesized and their dissociation constants were determined at different temperatures and 0.3 ionic strength. Potentiometric studies were carried out on the interaction of the derivatives towards some divalent metals in 50% v/v ethanol-water containing 0.3 mol.dm^?3 KCl, at five different temperatures. The formation constants of the metal complexes for both ligands follow the order: Cu²⁺ > Ni²⁺ > Co²⁺ > Zn²⁺ > Pb²⁺ > Cd²⁺ > Mn²⁺. The thermodynamic parameters were estimated; the complexation process has been found to be spontaneous, exothermic, and entropically favorable.     

The stability constants of copper(II) complexes with some alpha-amino acids in dioxan-water mixtures

In this study, the overall stability constants of copper(II) complexes with some alpha-amino acids (glycine, dl-alanine, dl-valine, l-leucine, l-asparagine, l-glutamine) were determined by potentiometric titration in water, 25% dioxan-75% water, 35% dioxan-65% water, 50% dioxan-50% water, and 60% dioxan-40% water. The titrations were performed at 25 degrees C, under nitrogen atmosphere, and the ionic strength of the medium was maintained at 0.10 M by using sodium perchlorate. The formation curves of their complexes (n-p[L]) were obtained by means of the titration data. Then the stability constants were determined in relation to these curves. The mol ratio of copper(II) to alpha-amino acid was also determined and it was found that the complexes were CuL(2) type. Another important result obtained was that the tendency of amino acids to form complexes with copper(II) was greater in dioxan-water mixtures compared to water.     

Determination of limiting mobilities and dissociation constants of 21 amino acids by capillary zone electrophoresis at very low pH

The dependence of the effective electrophoretic mobility on pH of the background electrolyte was experimentally determined by capillary zone electrophoresis (CZE) for cationic forms of amino acids. The pH of the background electrolytes was in the highly acidic range, 1.6-2.6 pH units, to ensure a high degree of protonation of the amino acids. Poly(vinyl alcohol) was added to the background electrolytes to avoid possible adsorption of the analytes at the inner capillary wall. Non-linear regression of the experimental data was applied to obtain the parameters of the relevant regression functions--the actual mobilities and mixed dissociation constants corresponding to the actual ionic strength. The extended Onsager and Debye-Hückel law was used to calculate the limiting mobilities and thermodynamic dissociation constants. The comparison of the experimental electropherogram with the computer prediction by PeakMaster using the determined data is presented for the selected sample of amino acids.     

Electro-olfactogram and multiunit olfactory receptor responses to complex mixtures of amino acids in the channel catfish, Ictalurus punctatus

In vivo electrophysiological recordings from populations of olfactory receptor neurons in the channel catfish, Ictalurus punctatus, clearly showed that both electro-olfactogram and integrated neural responses of olfactory receptor cells to complex mixtures consisting of up to 10 different amino acids were predictable with knowledge of (a) the responses to the individual components in the mixture and (b) the relative independence of the respective receptor sites for the component stimuli. All amino acid stimuli used to form the various mixtures were initially adjusted in concentration to provide approximately equal response magnitudes. Olfactory receptor responses to both multimixtures and binary mixtures were recorded. Multimixtures were formed by mixing equal aliquots of 3-10 different amino acids. Binary mixtures were formed by mixing equal aliquots of two equally stimulatory solutions. Solution 1 contained either one to nine different neutral amino acids with long side-chains (LCNs) or one to five different neutral amino acids with short side-chains (SCNs). Solution 2, comprising the binary mixture, consisted of only a single stimulus, either a LCN, SCN, basic, or acidic amino acid. The increasing magnitude of the olfactory receptor responses to mixtures consisting of an increasing number of neutral amino acids indicated that multiple receptor site types with highly overlapping specificities exist to these compounds. For both binary mixtures and multimixtures composed of neutral and basic or neutral and acidic amino acids, the receptor responses were significantly enhanced compared with those mixtures consisting of an equal number of only neutral amino acids. These results demonstrate that receptor sites for the basic and acidic amino acids, respectively, are highly independent of those for the neutral amino acids, and suggest that a mechanism for synergism is the simultaneous activation of relatively independent receptor sites by the components in the mixture. In contrast, there was no evidence for the occurrence of mixture suppression.     

Effect of electrophilic and nucleophilic substituents on the protonation microequilibria of tyrosine derivatives.

The microscopic protonation constants of 10 tyrosine-like, unusual amino acids used in the syntheses of opioid peptides have been determined by using a combined pH-metric-spectrophotometric method, at 0.10 mol dm-3 (NaCl) ionic strength and 25.0 degrees. The role of the different electrophilic and nucleophilic substituents on the individual basicity of the aliphatic amine and phenolic hydroxylate basic centers is discussed in detail. The interactivity parameters between these two groups correlate fairly well with the structure of the skeleton and the distance between the two basic centers, but they were found to be substituent-independent. This finding made it possible to extend the calculations to compounds having non-overlapping protonation equilibria.     

Study of the hydrolysis and ionization constants of Schiff base from pyridoxal 5''-phosphate and n-hexylamine in partially aqueous solvents. An application to phosphorylase b.

Formation and hydrolysis rate constants as well as equilibrium constants of the Schiff base derived from pyridoxal 5'-phosphate and n-hexylamine were determined between pH 3.5 and 7.5 in ethanol/water mixtures (3:17, v/v, and 49:1, v/v). The results indicate that solvent polarity scarcely alters the values of these constants but that they are dependent on the pH. Spectrophotometric titration of this Schiff base was also carried out. We found that a pKa value of 6.1, attributed in high-polarity media to protonation of the pyridine nitrogen atom, is independent of solvent polarity, whereas the pKa of the monoprotonated form of the imine falls from 12.5 in ethanol/water (3:17) to 11.3 in ethanol/water (49:1). Fitting of the experimental results for the hydrolysis to a theoretical model indicates the existence of a group with a pKa value of 6.1 that is crucial in the variation of kinetic constant of hydrolysis with pH. Studies of the reactivity of the coenzyme (pyridoxal 5'-phosphate) of glycogen phosphorylase b with hydroxylamine show that this reaction only occurs when the pH value of solution is below 6.5 and the hydrolysis of imine bond has started. We propose that the decrease in activity of phosphorylase b when the pH value is less than 6.2 must be caused by the cleavage of enzyme-coenzyme binding and that this may be related with protonation of the pyridine nitrogen atom of pyridoxal 5'-phosphate.     

Ionic states of substrates and transition state analogues at the catalytic sites of N-ribosyltransferases

Purine nucleoside phosphorylase (PNP) and hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) catalyze N-ribosidic bond cleavage in purine nucleosides and nucleotides, with addition of phosphate or pyrophosphate to form phosphorylated alpha-D-ribose products. The transition states have oxacarbenium ion character with a positive charge near 1'-C and ionic stabilization from nearby phosphoryl anions. Immucillin-H (ImmH) and Immucillin-H 5'-PO(4) (ImmHP) resemble the transition state charge when protonated at 4'-N and bind tightly to these enzymes with K(d) values of 20 pM to 1 nM. It has been proposed that Immucillins bind as the 4'-N neutral form and are protonated in the slow-onset step. Solution and solid-state NMR spectra of ImmH, ImmHP, guanosine, and GMP in complexes with two PNPs and a HGPRTase have been used to characterize their ionization states. Results with PNP*ImmH*PO(4) and HGPRTase*ImmHP*MgPP(i) indicate protonation at N-4' for the tightly bound inhibitors. The 1'-(13)C and 1'-(1)H resonances of bound Immucillins showed large downfield shifts as compared to Michaelis complexes, suggesting distortion of 1'-C toward sp(2) geometry. The Immucillins act as transition state mimics by binding with neutral iminoribitol groups followed by 4'-N protonation during slow-onset inhibition to form carbocationic mimics of the transition states. The ability of the Immucillins to mimic both substrate and transition state features contributes to their capture of transition state binding energy. Enzyme-activated phosphoryl nucleophiles bound to PNP and HGPRTase suggest enhanced electrostatic stabilization of the cationic transition states. Distortion of the oxacarbenium ion mimic toward transition state geometry is a common feature of the three distinct enzymatic complexes analyzed here. Substrate complexes, even in catalytically cycling equilibrium mixtures, do not reveal similar distortions.     

Synthesis, spectroscopic characterization and biological activity on newly synthesized copper(II) and nickel(II) complexes incorporating bidentate oxygen-nitrogen hydrazone ligands

We report the synthesis of the hydrazone ligands, 1-(phenyl-hydrazono)-propan-2-one (PHP), 1-(p-tolyl-hydrazono)-propan-2-one (THP), 1-[(4-chloro-hydrazono)]-propan-2-one (CHP), and their Ni(II) and Cu(II) metal complexes. The structure of the ligands and their complexes were investigated using elemental analysis, magnetic susceptibility, molar conductance and spectral (IR, UV, and EPR) measurements. IR spectra indicate that the free ligands exist in the hydrazo-ketone rather than azo-enol form in the solid state. Also, the hydrazo-NH exists as hydrogen bonded to the keto-oxygen either as intra or as intermolecular hydrogen bonding. In all the studied complexes, all ligands behave as a neutral bidentate ligands with coordination involving the hydrazone-nitrogen and the keto-oxygen atoms. The magnetic and spectral data indicate a square planar geometry for Cu²⁺ complexes and an octahedral geometry for Ni²⁺ complexes. The ligands and their metal chelates have been screened for their antimicrobial activities using the disc diffusion method against the selected bacteria and fungi. They were found to be more active against Gram-positive than Gram-negative bacteria. It may be concluded that the antimicrobial activity of the compounds is related to cell wall structure of bacteria.Protonation constant of (PHP) ligand and stability constants of its Cu²⁺ and Ni²⁺ complexes were determined by potentiometric titration method in aqueous solution at ionic strength of 0.1 M sodium nitrate. It has been observed that the hydrazone ligand (PHP) titrated here has one protonation constant. The divalent metal ions Cu²⁺ and Ni²⁺ form with (PHP) 1:1 and 1:2 complexes. The insolubility of (THP) and (CHP) ligands in aqueous medium does not permit the determination of their protonation constants and formation constants of the corresponding complexes in aqueous solution.     

Protonation equilibria of glycine, 1,10-phenanthroline and 2,2-bipyridyl in ethylene glycol–water mixtures

Abstract

The solute–solvent interactions of glycine, 1,10-phenanthroline and 2,2-bipyridyl have been studied in 0–60% v/v ethylene glycol–water media by a pH metric method. The protonation constants were estimated with the computer program MINIQUAD75. Selection of the best fit chemical model of the protonation equilibria is based on the standard deviation in protonation constants and residual analysis using a sum of squares of residuals in all mass-balance equations. The observed linear variation of protonation constants with the inverse of dielectric constant of the solvent mixture can be attributed to the dominance of the electrostatic forces. The distribution of species, protonation equilibria and effects of influential parameters on the protonation constants are also presented.