Potentiometric Study of Dissociation Constants of Dihydroxybenzoic Acids at Reduced Ionic Strengths and Temperatures

The dissociation behavior of two dihydroxybenzoic acid isomers, 2,3-DHBA and 3,4-DHBA, at 281 K and 293 K was determined by potentiometric titrations in 0.01 M NaCl and 0.03 M NaCl. Results showed that the dissociation enthalpy for the carboxylic group in DHBA is close to zero, resulting in dissociation constants that do not vary appreciably with temperature, whereas the dissociation constants for the first hydroxyl group vary significantly with temperature. Increasing ionic strength was found to result in increased values for the second dissociation constant, whereas the effect on the first dissociation constant was less clear.

Effect of Temperature and Solvent Composition on the Acid Dissociation Constants of 2-Pyrrol and 2-Thiophene Aldehyde Phenyl Sulphonyl Hydrazones

Acid dissociation constants, pKa, of 2-pyrrolaldehyde phenylsulphonyl hydrazone, 1, and 2-thiophenealdehyde phenylsulphonyl hydrazone, 2, have been determined spectrophotometrically in ethanol-water media of various compositions over the temperature range 25℃ - 45℃. The obtained results were used in the calculation of the enthalpy, △H°, and the entropy, △S°, of the ionization processes. The slight variations observed in the pKa values of the thiophene compound compared to the pyrrol analogue revealed that neither of the two hetero atoms in the pyrrol or thiophene rings, of the two compounds, is involved in a hydrogen bond chelation. This conclusion was also confirmed through measurements of the dipole moment, IR and NMR spectra.

The Thermodynamic Dissociation Constants of Azathioprine by the Nonlinear Regression and Factor Analysis of Multiwavelength Spectrophotometric pH-Titration Data

The mixed dissociation constant of azathioprine—chemically 6-(3-methyl-5-nitroimidazol-4-yl)sulfanyl-7H- purine at various ionic strengths I of range 0.01-0.2, and at temperatures of 25℃ and 37℃, was determined with the use of two different multiwavelength and multivariate treatments of spectral data, SPECFIT32 and SQUAD(84) nonlinear regression analyses and INDICES factor analysis according to a general rule. First, the number of components is determined, and then the spectral responses and concentrations of the components are calculated. Concurrently, the experimental determination of the thermodynamic dissociation constant was in agreement with its computational prediction of the PALLAS programme based on knowledge of the chemical structures of the drug. The factor analysis in the INDICES programme predicts the correct number of two light-absorbing species L- and HL. The thermodynamic dissociation constant of azathioprine was estimated by nonlinear regression of {pKa, I} data, = 8.07(1) at 25℃ and 7.84(1) at 37℃, where the figure in brackets is the standard deviation in last significant digits. The reliability of the dissociation constants of azathioprine was proven with goodness-of-fit tests of the multiwavelength spectrophotometric pH-titration data.

Determination of the dissociation constants of polyepoxysuccinic acid

The dissociation constants of polyepoxysuccinic acid(PESA)were investigated in this study.Based on the potentiometric titration and the BEST program,the dissociation constants of PESA were determined.Considering the complexity of the dissociation of PESA in aqueous solution,several models were constructed to simulate the dissociation process of PESA.By comparison,the dissociation constants of PESA were obtained with model 4.The species distribution of PESA in aqueous solution as a function of pH was also presented according to the experimental and calculation results.It showed that the H_(2)L model with five basic structure units to describe the dissociation of PESA was reasonable,and the relevant constants had less error and better matching between the experimental and calculation data.The corresponding values of pK_(ai) were 4.68 and 4.92,respectively,for H2L at 35℃ with ionic strength of 0.1 mol/L.

A potentiometric determination of the dissociation constants of glycine in {14.926 mass% urea+water} mixed solvent at 278.15 to 318.15K

First and second thermodynamic dissociation constants of glycine at five temperatures from 278.15K to 318,15K in {14.926 mass％ urea + water} mixed solvent have been determined from precise emfmeasurements of hydrogen-silver chloride electrodes in two cells, (A) and (B), without liquid junc- tion, by the new method of polynomial approximation proposed on the basis of Pitzer's electro- lytic solution theory in our previous papers, as compared with traditional extrapolation on the basis of the extended Debye-(?) equation. The results obtained from both methods are in agreement within experimental error. The thermodynamic quantities △G^0, △H^0, △S^0 and △C_p^0 were derived from the dependence of pK^0 values on temperature, and pK_1 = 367.60 (K / T) - 0.3973 + 3.108× 10^(-3) (T/K), whereas pK_2=2028.77 (K/ T)+ 2.8791-2.110×10^(-3) (T/K). The results have been discussed in terms of the solute-solvent interaction and have been compared with those results in water and in other mixed solvents.

Determination of the acid dissociation constants of N-carboxymethyl-N-(p-hydroxy phenyl carbamoylmethyl)-2,3-dihydroxy- 5-carbomethoxy benzylamine and the stability constants of its metal complexes

The acid dissociation constants of N-carboxymethyl-N-(p-hydroxy phenyl carbamoyl- methyl)-2,3-dihydroxy-5-carbomethoxy benzylamine(CHDCB) and the stability constants of its 1:1 complexes with alkaline earth, Cd(Ⅱ), Co(Ⅱ), Ni(Ⅱ), Cu(Ⅱ), Zn(Ⅱ), Fe(Ⅲ),Th(Ⅳ)and U(Ⅵ)ions have been determined at 25.0±0.1℃ and at an ionic strength of 0.1(KNO_3)by pH titration method. The probable coordination sites have also been discussed.

Acid dissociation constants and cytotoxicity test of a series of omega-aminoalkyl phosphates

We synthesised a series of ω-aminoalkyl sodium hydrogen phosphates （AAP-n-Na, n =3, 4, 5, 6, purity 〉 99%）, which have potential applications as bioactive cosmetic ingredients and surface modifiers of bone minerals （i.e. hydroxyapatites）. Results from Fourier transformed infrared （FTIR）, nuclear magnetic resonance （NMR） and high resolution mass spectroscopy, and elemental analysis all matched their chemical structures. The acid dissociation constants （pKa＇s） of each AAP-n （acid form of AAP-n-Na, n ; 2-6） were measured by potentiometric titration, showing a general increasing trend with an increase in the chain length of AAP-n. However, the pKa3 constant, which corresponds to the deprotonation of the ammonium group in AAP-n-Na, displayed an unusual decrease when n = even. This odd-even effect can be explained by the pairwise self-association of AAP-n-Na molecules in water where intermolecular hydrogen bonding in case of n=even is weaker than that in case of n=odd. All AAP-n-Na at concentrations up to 0.1% （w/v） were non-toxic to L929 fibroblasts and MG 63 osteoblast-like cells in terms of cell growth and morphology, These basic data were important for applications of AAP-n and their salts in biomedical engineering.

Overview of the detection methods for equilibrium dissociation constant KD of drug-receptor interaction

Drug-receptor interaction plays an important role in a series of biological effects, such as cell pro- liferation, immune response, tumor metastasis, and drug delivery. Therefore, the research on drug-re- ceptor interaction is growing rapidly. The equilibrium dissociation constant （KD） is the basic parameter to evaluate the binding property of the drug-receptor. Thus, a variety of analytical methods have been established to determine the KD values, including radioligand binding assay, surface plasmon resonance method, fluorescence energy resonance transfer method, affinity chromatography, and isothermal ti- tration calorimetry. With the invention and innovation of new technology and analysis method, there is a deep exploration and comprehension about drug-receptor interaction. This review discusses the differ- ent methods of determining the KD values, and analyzes the applicability and the characteristic of each analytical method. Conclusively, the aim is to provide the guidance for researchers to utilize the most appropriate analytical tool to determine the KD values.

THE DISSOCIATION CONSTANT OF ARACHIDIC ACID BY IR AND LB TECHNIQUES

The dissociation constant of arachidic acid an a subphase containing Ag+ ion was measured by IR and LB techniques.

Determination of vanadyl sulfate ion-pair dissociation constant at 298.15 K by Fuoss method

Vanadium battery has fast and large capacity charge and discharge characteristics, and the concentration and stability of the battery electrolyte are the key to the electrolyte performance. In actual, with higher acidity and higher concentration of the vanadium battery electrolyte, the ion-pair is dominant. The ion-pair dissociation constant is one of the important thermodynamic data related to battery performance. The vanadyl sulfate conductivity in aqueous solution was determined using the conductance method at 298.15K. Using the origin data fitting, the limiting molar conductance is obtained, and then by the improved Ostwald dilution law and the Davies equation, the activity coefficient is solved so as to obtain the ionic strength of the true solution. Using the Fuoss method, the dissociation constant of vanadyl sulfate ion-pair was calculated. At last, the vanadyl sulfate limiting molar conductance (Λ 0 ) is 174.8251748 S·dm2·mol?1, and the vanadyl sulfate ion-pair dissociation constant K d is 0.002636367, and then other thermodynamic properties can be studied.

Determination of the thermodynamic dissociation constant of p-nitrophenol in {NaCl+EtOH+H_2O} mixture by ultra-violet spectrophotometry

The thermodynamic ionization constant K° of p-nitrophenol in {10 mass% ethanol-water} mixed solvent is derived from measurements of its absorption spectrum and pH by the two methods: Debye--Huckel extrapolation and polynomial approximation proposed by us in suitable buffer solution at constant ionic strength from 0.1 to 2.0 mol.kg^(-1) at 298.2 ± 0.2 K. The results from both methods are pK° =7.406 and 7.415, respectively. The effect of medium on the ultraviolet spectra of p-nitrophenol has been discussed.

The missing hydrogen ion,part-1:Historical precedents vs.fundamental concepts

The purpose of this review and commentary was to provide an historical and evidence-based account of organic acids and the biochemical and organic chemistry evidence for why cells do not produce metabolites that are acids.The scientific study of acids has a long history dating to the 16th and 17th centuries,and the definition of an acid was proposed in 1884 as a molecule that when in an aqueous solution releases a hydrogen ion(H^(+)).There are three common ionizable functional groups for molecules classified as acids:1)the carboxyl group,2)the phos-phoryl group and 3)the amine group.The propensity by which a cation will associate or dissociate with a negatively charged atom is quantified by the equilibrium constant(K_(eq))of the dissociation constant(K_(d))of the ionization(K_(eq)=K_(d)),which for lactic acid(HLa)vs.lactate(La^(-))is expressed as:K_(eq)=K_(d)=[H^(+)][La^(-)]/[HLa]=4677.3514(ionic strength=0.01 Mol⋅L^(-1),T=25℃).The negative log10 of the dissociation pKd reveals the pH at which half of the molecules are ionized,which for HLa=3.67.Thus,knowing the pKd and the pH of the solution at question will reveal the extent of the ionization vs.acidification of molecules that are classified as acids.

Dissolution Modeling and Experimental Measurement of CaS-H20 Binary System

CaS formed during the retorting process of oil shale has a hazardous influence on surface water quality. Interaction of retorted oil shale with water generates highly alkaline leachate with a high content of sulfur due to the CaS component. A theoretical model describing the behavior of solid calcium sulfide in contact with water was developed. The model was consistent with the measurements showing change in dissolution behavior when solid CaS remained in the solution. Experimental measurements of pH and concentrations of ions were carried out in oxy- gen-free water at 25 ~C using CaS concentrations from 24.2-131.5 mgoL-1 （0.335--1.823 mmol/L）. Analysis of pH and concentrations of ions in the solution and calculations by the developed model showed that the solubility of CaS was estimated as 125.0 mg·L 1 （1.733 mmol/L）, and therefore the solubility product of CaS is 3.41×10^-10 （mo·L^-1）^2 at a temperature of 25℃.

Fluorescence regional integration and differential fluorescence spectroscopy for analysis of structural characteristics and proton binding properties of fulvic acid sub-fractions

Structural characteristics and proton binding properties of sub-fractions（FA3–FA13） of fulvic acid（FA）, eluted stepwise by pyrophosphate buffer were examined by use of fluorescence titration combined with fluorescence regional integration（FRI） and differential fluorescence spectroscopy（DFS）. Humic-like（H-L） and fulvic-like（F-L） materials, which accounted for more than 80% of fluorescence response, were dominant in five sub-fractions of FA. Based on FRI analysis, except the response of F-L materials in FA9 and FA13, maximum changes in percent fluorescence response were less than 10% as pH was increased from 2.5 to 11.5.Contents of carboxylic and phenolic groups were compared for fluorescence peaks of FA sub-fractions based on pH-dependent fluorescence derived from DFS. Static quenching was the dominant mechanism for binding of protons by FA sub-fractions. Dissociation constants（p Ka） were calculated by use of results of DFS and the modified Stern-Volmer relationship. The p Kaof H-L, F-L, tryptophan-like and tyrosine-like materials of FA subfractions exhibited ranges of 3.17–4.06, 3.12–3.97, 4.14–4.45 and 4.25–4.76, respectively, for acidic pHs. At basic pHs, values of p Ka for corresponding materials were in ranges of 9.71–10.24, 9.62–10.99, 9.67–10.31 and 9.33–10.28, respectively. At acidic pH, protein-like（P-L）materials had greater affinities for protons than did either H-L or F-L materials. The dicarboxylic and phenolic groups were likely predominant sites of protonation for both H-L and F-L materials at both acidic and basic pHs. Amino acid groups were significant factors during proton binding to protein-like materials of FA sub-fractions at basic pH.

Preparation and Characterization of HLA-A*0201 Tetramer Loaded with IE-1_(316-324) Antigenic Peptide of Human Cytomegalovirus

Major histocompatibility complex （MHC） class I tetramer technology has become the central technique for analyzing antigen-specific CD8^＋ T cell responses and it has been widely used to explore the differentiation and formation of memory CD8^＋ T cells. Previously, a simplified and efficient procedure for preparing high quality HLA-A＊0201 tetramers has been established in our lab and the tetramers loaded with HCMV peptide pp6549s.50a has been successfully applied to investigate HCMV-specific CD8^＋ T cells in Chinese populations. Using similar procedure we reported here the construction of HLA-A＊0201 tetramer loaded with another dominant epitope derived from immediate early （IE）-1 316.324 （VLEETSVML, VLE） of HCMV （A2-VLE） and characterization of this tetramer. After A2-VLE monomer was prepared and purified, its tetramer was then formed at a yield of 83%. The optimized amount of A2-VLE tetramer for staining 100 μl whole blood was 0.5 μg with incubation at 4℃ for 1 h. Furthermore, the dissociation constant of the tetramer binding to the specific CD8^＋ T cells of one HLA-A2^＋ donor was estimated to be 32.7 nmol/L, which is markedly higher than that of MHC monomer. The construction of A2-VLE tetramer provides an alternative choice for investigating HCMV-specific CD8^＋ T cell responses and will deepen our understanding of the differentiation and formation of HCMV-specific memory CD8^＋ T cells. Cellular ＆ Molecular Immunology.

Determining PPARg-ligand binding affinity using fluorescent assay with cis-parinaric acid as a probe

Upon the study of small-molecules binding to proteins, the traditional methods for calculating dissociation constants (Kd and Ki) have shortcomings in dealing with the single bind- ing site models. In this paper, two equations have been derived to solve this problem. These two equations are independent of the total concentration or initial degree of saturation of receptor and the activity of the competitive molecule. Through nonlinear fitting against these two equations, Kd value of a probe can be obtained by binding assay, and Ki value of a ligand can be obtained by competitive assay. Moreover, only the total concentrations of receptor([R]t), ligand([L]t) and probe([P]t) are required for the data fitting. In this work, Ki values of some typical ligands of PPARγ were successfully determined by use of our equations, among which the Ki value of PPARγ-LY171883 was reported for the first time.

EffectofElectroacupunctureonPulmonaryβ－AdrenoceptorinAllergicAsthmaGuinea－Pigs

Twenty four male guinea-pigs were randomly divided into 3 groups: Group A, allergic asthmaguinea-pigs prepared by peritoneal injection and spraying inhalation of albumin; Group B, allergic asthmaguinea-pigs treated with electroacupuncture through stimulating the acupoint Feishu (UB13 ) and Group C, thecontrol group. With[3H] -DHA as a radioligand, pulmonary adrenoceptor ( AC) in the 3 groups of guinea-pigs were detrermined by radioligand binding assay technique. The results showed that: ( 1 ) The maximumbinding volume of receptor ligand ( Bmax, fmol/mg prot. ) of pulmonary AC in Group A (123. 86 42. 91 )was significantly lower than that in group C (199 . 54 37. 03 , P< 0. 05) , while the difference between groupB ( 142. 00 42. 91 ) and group C was not significant ( P > 0. 05 ) . ( 2 ) The Kd ( nM) of pulmonary AC ingroup A, B and C were 5 . 10 1 . 39 , 6. 53 2 . 41 and 8. 72 2 . 02 respectively, and the differences amongthe three groups were not significant ( P >0. 05) . The results indicated that the decrease of the content anddysfunction of pulmonary AC in allergic asthma guinea-pigs might be regulated by electroacupuncture thera-py.