Determination of the Dissociation Constants of Urocanic Acid Isomers in Aqueous Solutions

Summary. (E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation constants (pK _a1 and pK _a2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation (pK _a1 = 3.43 ± 0.12 and pK _a2 = 5.80 ± 0.04) and (pK _a1 = 2.7 ± 0.3 and pK _a2 = 6.65 ± 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride solutions up to the isotonic salt concentration (0.154 mol dm⁻³), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at temperature 25°C and for (E)-urocanic acid also at 37°C. The obtained pK _a2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived. The obtained pK _a1 and pK _a2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature. An erratum to this article is available at .     

Determination of the Dissociation Constants of Urocanic Acid Isomers in Aqueous Solutions

(E)- and (Z)-Urocanic acids are endogenous chemicals in the normal mammalian skin. The first and the second thermodynamic dissociation constants (pK _a1 and pK _a2) of urocanic acid isomers were determined using UV spectrophotometry in aqueous solutions. The values with standard deviation (pK _a1 = 3.43 ± 0.12 and pK _a2 = 5.80 ± 0.04) and (pK _a1 = 2.7 ± 0.3 and pK _a2 = 6.65 ± 0.04) were obtained to (E)- and (Z)-urocanic acids, respectively. The second dissociations were studied also by potentiometric titration in aqueous sodium chloride solutions up to the isotonic salt concentration (0.154 mol dm⁻³), and the second thermodynamic dissociation constants as well as activity parameters for both isomers were determined at temperature 25°C and for (E)-urocanic acid also at 37°C. The obtained pK _a2 values were close to those found by UV spectrophotometry. The equations for the calculation of the second stoichiometric dissociation constants of urocanic acid isomers on molality and molarity scale in aqueous sodium chloride solutions were derived. The obtained pK _a1 and pK _a2 values for (Z)-urocanic acid appear to be essentially lower than some previously reported values in literature.     

聚天冬氨酸酸解离常数的测定

采用pH电位滴定法，利用先进的BEST程序，针对聚羧酸在水溶液中解离及平衡常数测定的复杂性，构建了聚天冬氨酸（PASP）在水溶液中解离的H4L和H2L两种模型，分别测得了相应的各级酸解离常数，并得到了PASP在不同pH条件下的物种分布曲线。在35℃，I=0．1时，对于H4L模型，各级酸解离常数的pK,值为：2．27、3．66、4．22、5．40；对于H2L模型，相应的pK,值为2．92、4．84。研究结果表明，两种模型计算结果均能与实验结果很好吻合，但H4L模型误差更小，因此是一种更为合理的模型。     

Seasonal Variation in Urocanic Acid Isomers in Human Skin

Abstract— Urocanic acid (UCA) is a major chromophore for UV in the skin and has been suggested to act as an initiator of UV-induced immunosuppression. It converts from the naturally occurring trans-isomer to the cis-isomer on UV exposure. Isomerization is dose dependent until the photostationary state is reached, and the seasonal variation in irradiance from the sun may lead to changes in the percentage of UCA present as cis -UCA throughout the year. Thirty young healthy subjects, skin types I-IV, were followed from early summer till spring. At each of six visits (June, July, August, October, December, March), pigmentation and the concentration of UCA isomers were measured at six body sites: forehead, upper chest, upper back, outer upper arm, inner upper arm and buttock. In exposed as well as unexposed regions a variation in pigmentation was found, peak values being recorded in August. Total UCA concentration was lower in July and August than in the rest of the year, irrespective of body site. In July, the percentage of cis -UCA was close to the maximal obtainable (50-60%) at all sites except the buttock. In the three winter months the percentage of cis-UCA was below 7% in all regions except for the forehead, where the mean cis -UCA was 18% in October and March. No consistent relationship was found between UCA isomers and pigmentation or skin type.     

UV-Vis Spectrophotometric Determination of the Dissociation Constants for Monochlorophenols in Aqueous Solution at Elevated Temperatures

The dissociation constants of monochlorophenols (2-, 3-, 4-chlorophenols) were examined using direct UV-vis spectroscopy at temperatures from 25 to 175^°C and at saturated vapor pressures in a high-temperature, high-pressure cell. The dissociation constant, K _a increased under experimental temperatures in the order: 2-chlorophenol, 3-chlorophenol, and 4-chlorophenol. The dissociation constant of 4-chlorophenol increased with increasing temperature under experimental conditions, while those of 2- and 3-chlorophenol reached maximum values at around 125^°C, and then decreased with further increases in temperature. The slope of (log K)/ (1/T) was nonconstant and positive, that is, endothermic, below 150^°C. The data on dissociation constants were analyzed by simultaneous regression to yield a five-term equation that described the Van't Hoff isobar. The magnitude of enthalpy H increased at 25^°C in the order: 3-chlorophenol, 4-chlorophenol, and 2-chlorophenol. The decrease in enthalpy at the absolute temperature was larger for 3-chlorophenol than for either 2- or 4-chlorophenol. Considering the equilibrium constant K _b for the isocoulombic reaction of monochlorophenol with OH^–, the nearly linear relationship of log K _b vs. 1/T for temperatures between 25 and 175^°C indicates that the C_p values for this isocoulombic reaction are low.     

Gas-Chromatographic Determination of Acrylic Acid in Aqueous Solutions

A procedure was developed for determining acrylic acid in aqueous solutions at its concentration within 0.5-20 MPC.     

The Effect of Chronic Treatment of Mice with Urocanic Acid Isomers

Abstract— Trans-urocanic acid (trans-UCA) accumulates in the upper layers of the epidermis and can be isomerized to cis-UCA by UV light irradiation. Cis-urocanic acid possesses immunosuppressive properties that have led to its consideration as one of the initiators of UV-induced immunosuppression. High quantities of cis-UCA persist in human skin for prolonged periods in the summer months. In the present study, mice were injected intradermaUy with trans-UCA and cis-UCA three times a week for 4 weeks in order to ascertain the long-term effects of the presence of these compounds in the skin. The weight of mice and of their spleens were unaffected by the cis- or trans-UCA treatment. A decrease in thymus weight, accompanied by an increase in lymph node weight, was detected in the cis-UCA-treated mice compared with trans-UCA-treated mice and untreated controls. A net accumulation of lymphocytes and dendritic cells (DC) in lymph nodes was evident following cis-UCA treatment but the percentage of both CD4+and CD8+lymphocytes as well as Ia+DC remained constant among the different treatment groups, indicating that there was no specific migration or proliferation of a particular subset of cells. The in vitro lymphoproliferative response of lymph node cells to the mitogen concanavalin A was significantly sup pressed by cis-UCA treatment. The density of Langerhans cells in the epidermis of the ears was not altered by the chronic cis-UCA treatment. However, chronic cis-UCA treatment did suppress the mixed skin lymphocyte reaction response utilizing epidermal cells from the ears (an uninjected area of skin), indicating a systemic suppression. Compared with trans-UCA treatment, chronic cis-UCA treatment did not cause a significant reduction in the contact hypersensitivity response to oxazolone or the delayed hypersensitivity response to herpes simplex virus. Thus, chronic treatment with cis-UCA led to the suppression of some, but not all, of the immune parameters that are affected by UVB irradiation.     

Acid Dissociation Constants and Rare Earth Stability Constants for DTPA

Diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) is an octadentate aminopolycarboxylate complexing agent whose f-element complexes find important practical applications in nuclear medicine and in advanced nuclear fuel reprocessing. This investigation focuses primarily on the latter application, specifically on characterization of lanthanide–DTPA complexes of relevance to the Trivalent Actinide–Lanthanide Separations by Phosphorus reagent Extraction and Aqueous Komplexants (TALSPEAK) process. To function acceptably, the TALSPEAK process requires the presence of moderate concentrations (0.5–2.0 mol·L^?1) of a (Na⁺/H⁺) lactate (or citrate) buffer. Competition between DTPA, lactate, and the extractant bis(2-ethylhexyl)phosphoric acid (HDEHP) for the lanthanides and trivalent actinides governs the course of the extraction process. To facilitate modeling and to support process improvements, the acid dissociation constants and stability constants for rare earth complexes with DTPA have been determined in 2.0 mol·L^?1 ionic strength (NaClO₄) media. The acid dissociation constants for DTPA and the stability constant for [Eu(DTPA)]^2? also were determined in sodium trifluoromethanesulfonate at 2.0 mol·L^?1 ionic strength to evaluate the potential impact of changing the nature of the electrolyte. The thermodynamic data are compared with earlier reports of similar data at lower ionic strength and used to complete calculations exploring the relative stability of lanthanide–DTPA and lactate complexes under TALSPEAK extraction conditions. Lanthanide–DTPA stability trends are discussed in comparison with literature data on a variety of other metal ions.     

毛细管电泳法测定电离平衡常数

１ 引 言 离解常数（如ｐＫ值）是理解化学现象如反应速率、生物活性、生物吸收等的一个重要参数。离解常数的测定方法有电势滴定法、紫外可见分光光度法和毛细管电泳法（ＣＥ）等，其中ＣＥ法被认为是一种测定ｐＫ的十分方便而有效的方法。近些年，ＣＥ已经广泛地用于测定一级电离平衡常数。对于二级电离平衡常数曾有人作了尝试，但是活度系数的影响并没有给予完全考虑。本文将全面考虑活度系数影响，导出ＣＥ测定ｐＫ值的非线性拟合公式，并应用于一些物质的测定。２ 实验部分２．１ 仪器与试剂 实验仪器是 ＷａｔｅｒｓＱｕａｎｔａ ４０…     

苯氧乙酸类化合物的酸离解常数测定及其量子化学计算

苯氧乙酸;酸离解常数;键电荷密度;量子化学计算;苯氧乙酸类化合物的酸离解常数测定及其量子化学计算     

Determination of Acid Dissociation Constants of Neamine by Potentiometric and Electrospray Mass Spectral Techniques

The acid dissociation constants of neamine have been measured using potentiometric titrations. The pK_a1, pK_a2, and pK_a3 values of neamine are 6.35 ± 0.2, 7.73 ± 0.15, and 8.62 ± 0.08, respectively. Neamine is readily characterized using positive-ion electrospray ionization–mass spectrometry (ESI–MS). Various protonated species and their solvated ions are mainly observed. Nevertheless, the abundances of the observed speciation over the pH range 5.0–9.8 do not reflect the variation in the bulk solution. Hence, reaction quotients determined from the ESI–MS investigations vary significantly from the pK_a's determined from potentiometric titrations, thereby illustrating that care must be taken in determining thermodynamic properties using the former technique.     

Multiwavelength Spectrophotometric Determination of Dissociation Constants of Mercaptopyrimidines

Abstract

The dissociation constants of some 2- and 6-mercaptopyrimidine derivatives with amino or hydroxy substituents are reported. The constants have been determined at 25° C in 0.1 M KCl aqueous solution. The multiwavelength spectrophotometric data obtained have been 3mlyzed by means of the SQUAD program (Stability Quotients from Absorbance Data).     

Excess Molar Volumes of Aqueous Solutions of Butylamine Isomers

Abstract

Excess molar volumes (V^E ) and average thermal expansivities (α) of the systems, water (W) + n-butylamine (NBA), W + sec-butylamine (SBA), and W + tert-butylamine (TBA), have been calculated from the density data at temperatures ranging from 298.15–323.15 K. The V^E and α values have been plotted as functions of mole fraction of amines. The systems show large negative excess volumes, magnitude of which varies in the order, W + TBA > W + SBA > W + NBA. The curves are found to be symmetrical along the composition axis, with minima occurring at 0.5 mole fraction of butylamines. The negative excess volumes have been interpreted primarily by two effects: (i) strong chemical interaction leading to the formation of 1:1 complexes through H-bonding and (ii) hydrophobic hydration causing significant contraction of volume.     

Multiwavelength Spectrophotometric Determination of Acid Dissociation Constants. Deconvolution of Binary Mixtures of Ionizable Compounds

ABSTRACT

A spectral deconvolution method based on target factor analysis has been developed to determine pK_a values of binary mixtures of monoprotic and/or diprotic ionizable compounds. The technique makes use of the approach of Frans and Harris, which has been implemented previously for mixtures of monoprotic compounds (Anal. Chem. 1985, 57, 1718-1721), to extract the unknown pK_a values. The method has been illustrated by using the multiwavelength spectrophotometrictitation data of binary mixtures of: I. benzoic acid and phenol, 2. benzoic acid and nicotinic acid, 3. p-aminosalicylic acid and phthalic acid. It was demonstrated that the pK_a values as determined in this study are in good agreement with the literature.     

Potentiometric Determination of the Dissociation Constants of Glycylglycine in Various Aquo-Organic Media

The dissociation constant values of glycylglycine were determined at 25 ± 0.1 °C (I = 0.1 mol L^?1 KNO₃) with Potentiometric pH titration in pure water and various mixture of water and organic solvents. The organic solvents used were methanol, ethanol, N, N-dimethylformamide, dimethylsulfoxide, acetonitrile and dioxane. Initial estimates of the dissociation constants of glycylglycine were refined with ESAP2M computer program. The results obtained are discussed in terms of average macroscopic properties of the mixed solvents and the possible variation in microheterogeneity of the solvation shells around the solute.     

Determination of Aqueous Nickel-Carbonate and Nickel-Oxalate Complexation Constants

An ion-exchange method was used to determine complexation constants for the Ni-oxalate and Ni-carbonate systems in a NaClO₄ background electrolyte. The Ni-oxalate data were interpreted in terms of a single Niox(aq) complex having log K ₁ values for Ni²⁺ + ox^2– Niox(aq) of 3.9 ± 0.1 (I.S. = 0.5 mol-L^–1 p[H] = 7.1) and 4.4 ± 0.1 (I.S. = 0.1 mol-L^–1 p[H] = 8.6) at 22 ± 1C. Specific ion-interaction theory (SIT) was used to obtain log K ₁ = 5.17 ± 0.05 (95% confidence level and = –0.23 ± 0.15) at I.S. = 0. The Ni-carbonate studies were carried out at p[H] values of 7.5, 8.5, and 9.6 in 0.5 mol-L^–1 NaClO₄/NaHCO₃ solutions. The NiCO₃(aq) species was the dominant complex in the [CO₃ ^2–] concentration ranges studied at all three p[H] values. A log K ₁ value for Ni²⁺ + CO₃ ^2– NiCO₃(aq) of 2.9 ± 0.3 was deduced at I.S. = 0.5 mol-L^–1. Extrapolating this value to zero ionic strength using the SIT approach yielded log K ₁ = 4.2 ± 0.3 (95% confidence level and = –0.26 ± 0.04). The data allowed upper bound values for the complexation constants for NiHCO₃ ⁺ and Ni(CO₃)₂ ^2– to be estimated, i.e., log K < 1.4 for Ni²⁺ + HCO₃ ^– NiHCO₃ ⁺, and log K ₂ < 2 for NiCO₃(aq) + CO₃ ^2– Ni(CO₃)₂ ^2–, respectively.     

Universal Trends between Acid Dissociation Constants in Protic and Aprotic Solvents

pKa values in non-aqueous solvents are of critical importance in many areas of chemistry. Our knowledge is, despite their relevance, still limited to the most fundamental properties and few pKa values in the most common solvents. Taking advantage of a recently introduced computationally efficient procedure we computed the pKa values of 182 compounds in 21 solvents. This data set is used to establish for the first time universal trends between all solvents. Our computations indicate, that the total charge of the molecule and the charge of the acidic group combined with the Kamlet-Taft solvatochromic parameters are sufficient to predict pKa values with at least semi- quantitative accuracy. We find, that neutral acids such as alcohols are strongly affected by the solvent properties. This is contrasted by cationic acids like ammonium ions whose pKa is often almost completely independent from the choice of solvent.     

Chromatographic Determination of Dissociation Constants of Pramipexole and its Impurities

The main goal in this investigation was monocratic HPLC determination of dissociation constant values (pK _a) of pramipexole and its impurities, BI-II 546 CL, BI-II 751 xx and 2-aminobenzothiazole. The chromatographic method is advantageous as a small amount of substance is needed and the degree of substance purity is less important. Analysis was carried out using stationary phases stable in a wide pH range. Triethylamonium phosphoric buffer was selected as appropriate pH controlling solution because it can cover a wide pH range. Detection was done on two different wavelengths, 262 nm for pramipexole, BI-II 751 xx and 2-aminobenzothiazole and at 326 nm for BI-II 546 CL. The constants were calculated from the typical sigmoidal curves of analyte obtained as retention factor versus the pH of the mobile phase.