Acid-base Concentrations Research Articles

Photophysics of 2-(6′-hydroxy-3′-pyridyl)benzimidazole: effect of solvents and acid–base concentrations

Photophysics of 2-(6′-hydroxy-3′-pyridyl)benzimidazole (6-H3PyBI) has been studied with the help of absorption, fluorescence excitation and fluorescence spectroscopy, as well as, using time dependent spectrofluorimetry. Electronic structure calculations on various isomers of neutral 6-H3PyBI and different ionic species were carried out using AM1 semi-empirical calculations and density functional theory (DFT) B3LYP with 6-31G** basis set and using Gaussian 98 program. Transition absorption energies for each species have been calculated using CNDO/S-CI method, single point calculations using AM1 method and time-dependent (TD) DFT B3LYP with 6-31G** basis set. These results have confirmed the presence of three isomers/rotamers of neutral 6-H3PyBI (enol, keto-1 and keto-2), two kinds of monocations (MCE-1 and MCK-1), one kind of dication, monoanion and dianion each in S 0 and S 1 states. p K a and p K a ∗ for the various equilibriums have been determined using absorption and fluorescence data and discussed.

Ground and excited state prototropism in 2-(2′-methoxyphenyl)-1H-imidazo[4,5-c]pyridine

Absorption, fluorescence and fluorescence excitation spectra of 2-(2′-methoxyphenyl)-1H-imidazole[4,5-c]pyridine (2-MPIP-c) have been studied in different solvents and at different acid-base concentrations. Single photon counting fluorescence spectroscopic measurements were carried out to find out the excited state lifetimes under the above environments. AM1 semi-empirical quantum mechanical, as well as, electronic structure calculations using the Hartree–Fock (HF), and density functional theory (DFT) B3LYP with 6-31G** basis set using gaussian 98 program were carried out to supplement the experimental data. Spectral data in different solvents and theoretical calculations suggest the presence of only one species (a-1) of 2-MPIP-c in all the solvents. Out of five different possible monocations (MC's), only one MC (formed by protonating the benzimidazole N3 atom) is present in the aqueous solution, as well as, in different non-aqueous solvents. Nature of first excited singlet state in neutral 2-MPIP-c and its MC are π π ∗ and charge transfer, respectively. Only one dication (formed by protonating both the basic centers, benzimidazole N3 and pyridyl N5) and one kind of monoanion (formed by deprotonating >N–H) are present in the solution. pK a and pK a ∗ for various equilibriums were determined and discussed.

Synthesis, stability, and protonation studies of a self‐complementary dodecamer containing the modified nucleoside 2′‐deoxyzebularine

The nucleoside 2'-deoxyzebularine (K) was incorporated into the self-complementary dodecamer 5'-CGTACGKGTACG-3' by solid-phase 2-cyanoethylphosphoramidite chemistry using dimethoxytrityl (DMT) as the 5'-hydroxyl protecting group. Standard synthesis cycles using trichloroacetic acid and short ammonia treatment (50 degrees C for 30 min) were found to be the optimal conditions to obtain the desired dodecamer with minimum acid and basic degradation of the acid- and base-sensitive 2-pyrimidinone residue. The protonation equilibria of the K nucleoside and of the dodecamer at 37 degrees C were studied by means of spectroscopically monitored titrations. For the K nucleoside, a pK(a) value of 3.13 +/- 0.09 was obtained. For the dodecamer, four acid-base species were found in the pH range 2-12, with pK(a) values of 9.60 +/- 0.07, 4.46 +/- 0.16, and 2.87 +/- 0.19. Melting experiments were carried out to confirm the proposed acid-base concentration profiles. Finally, kinetic experiments were also carried out at several pH values to evaluate the stability of the K nucleoside and of the dodecamer. An increased stability was shown by the K nucleoside when incorporated into the dodecamer. Multivariate methods based on both hard- and soft-modeling were applied for the analysis of spectroscopic data, allowing the estimation of concentration profiles and pure spectra.

Inter- and intramolecular hydrogen bond in methyl 2-hydroxy-9H-1-carbazole carboxylate: effect of solvents and acid concentration

The photo-physics of methyl 2-hydroxy-9H-1-carbazazole carboxylate (MPCC) in different solvents and cyclohexane-trifluoroethanol (TFE) mixtures has been studied by means of absorption, fluorescence, fluorescence excitation spectra, time dependence spectrofluoremetery and AM1 semi-empirical quantum mechanical calculations. Only one small Stoke’s shifted fluorescence band is observed under all the environments, indicating that the geometry of the molecule is not changed much on excitation to the first singlet state (S 1) and excited state intramolecular proton transfer (ESIPT) is not viable both in the ground (S 0) and S 1 states at the room temperature. AM1 calculation shows that the ESIPT is still endothermic in S 1 state. Single exponential decay is observed in the fluorescence from MPCC in all the solvents except acetonitrile and methanol. This suggests that in these two solvents, at least two different conformers are present in the S 0 state, whose absorption spectra are not different from each other. Spectral characteristics of MPCC in cyclohexane as a function of TFE have shown a slight blue shift in the λ max ab, decrease in the ε max, red shift in the λ max fl and decrease in the φ fl. This suggests that intermolecular hydrogen bonding is playing a major role in the deactivation of the fluorescence intensity than the intramoleuclar hydrogen bonding (IHB). Spectral properties of MPCC were also studied as a function of acid–base concentrations. p K a values for different prototropic equilibriums were determined in S 0 and S 1 states and discussed.

The Determination of the pKa of Multiprotic, Weak Acids by Analyzing Potentiometric Acid-Base Titration Data with Difference Plots

This paper discusses the pKa determination of mono-, di-, and triprotic weak acids with the help of difference (Bjerrum) plots, and the effect of strong acid–base concentration errors, ligand weight errors, and nonlinear electrode response. Experimental examples are given for the titration of an acidic heterocycle, as well as glycine, ethylenediamine, and tris(2-aminoethyl)amine (the last two after addition of excess HCl to ensure complete protonation) with standarized NaOH. The analysis procedure makes use of Microsoft Excel spreadsheets and nonlinear least squares curve fitting of the experimental data to the theoretical Bjerrum function. In addition to providing pKa values for mono- and multiprotic acids, this approach has been found suitable for detecting small errors in parameters, such as strong acid and ligand concentration, and corrections can often become necessary to achieve the best fit. Difference plots allow the pKa values of monoprotic and multiprotic weak acids to be determined rapidly and with good precision.

Effects of substituting starch with fat on the acid-base and mineral status of female horses.

Feeding a high-fat diet may have potential advantages by maintaining pH homeostasis during intense exercise; conversely, effects including elevated PCV and increased plasma concentration of total protein may indicate effects detrimental to performance by reducing perfusion of tissues and organs. Six horses were used to determine the effects of an replacement of starch (diet '0.00') with 0.33, 0.67, 1.00 and 1.33 g soybean oil/kg bwt/day (diets '033', '0.67', '1.00' and '1.33') on venous and renal acid-base status and fractional clearance (FC) of electrolytes in a postprandial state but prior to exercise. Venous blood and urine were sampled for 3 days at the end of the 28 day periods, in each case 3 h postprandially. Throughout the study, the horses put on weight by 35 kg (P<0.05). From diet '0.00' to '0.67', blood pH decreased (P<0.05) and carbon dioxide tension increased (P<0.05). Simultaneously, the base and net acid-base concentration in the urine rose (P<0.05) but diuresis decreased (P<0.05) and, therefore, the daily excretion of bases and net acid-bases was only unsystematically affected. From diet '0.00' to '1.33', the FC of magnesium and chloride fell (P<0.05). These results suggested that the exchange of starch with up to moderate amounts of fat affects the venous and renal acid-base status of resting horses. The decrease in urine volume is most striking. Further research is needed to investigate the importance of these effects for sporting performance.

Prototropic reactions of 2(2′-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine in aqueous and organic solvents

The effect of acid–base concentration on the spectral characteristics of 2-(2′-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (2-HPIP) has been studied in both organic and aqueous solvents. Due to the poor fluorescent quantum yield of 2-HPIP in pure water, prototropic reactions were carried out in a 25% ethanol–water mixture. One kind of dication, three kinds of monocations, one kind of monoanion and one dianion are observed. In pure water, p K a values for the dication–monocation, the monocation–neutral, the neutral–monoanion and the monoanion–dianion equilibria have been determined. In protic solvents, one of the cation dissociates to yield the phototautomer on excitation. Semi-empirical quantum mechanical calculations have been carried out to assign the formation of species and site of protonation.

Effect of micelles on the spectral characteristics of the prototropic species of 2-(2′-aminophenyl)benzimidazole

The absorption and fluorescence spectra of 2-(2′-aminophenyl)benzimidazole (2-APBI) have been studied in anionic (sodium dodecylsulphate, SDS), cationic (cetyltrimethylammonium bromide, CTAB) and non-ionic micelles (Tween-80, TritonX-100) at different acid–base concentrations. Spectral characteristics of 2-APBI at various acid concentrations have established only one kind of monocation (MC) in Tween-80 and TritonX-100 (TX-100), whereas two kinds of MCs are present in SDS. Above study has further shown that there is a strong hydrogen bonding interaction between the polar polyoxyethylene groups of non-ionic micelles and the MCs of 2-APBI. This interaction is responsible for the stabilization of the MC III to MC III′ which is more planar than either MC II or MC III, which is otherwise less stable in water. This is substantiated by the lifetime data, fluorescence excitation spectra and the p K a values of the monocation–neutral (MC–N) equilibrium.

Ground and excited state prototropic reactions in 2-(2′-methoxyphenyl)-3H -imidazo[4,5-b]pyridine †

The absorption, fluorescence and fluorescence excitation spectra of 2-(2′-methoxyphenyl)-3H-imidazo[4,5-b]pyridine (2-MPIP) have been studied in five different solvents and at different acid–base concentrations. Semi-empirical quantum mechanical calculations have been carried out to supplement the experimental results. Spectral characteristics of 2-MPIP in different solvents and semi-empirical quantum mechanical calculations suggest the presence of both a-1 and b-1 isomers. The observation of a single exponential decay and one lifetime suggests an equilibrium between the isomers. Out of five monocations two kinds of monocations (MC’s) are possible, one from protonation of N1 (MC1) and the second from protonation of N4 (MC2) atoms. Spectral characteristics and semi-empirical calculations carried out on MC’s neglect the presence of isomer b-1. Only one kind of dication and one kind of monoanion are formed by protonating both the nitrogen atoms (N1 and N4) and deprotonating the >NH atom respectively. pKa and pKa* values for the various prototropic reactions are determined and discussed.

Luminescence Characteristics of 5-Aminoindazole in Cationic and Anionic Micelles at Various pH

Fluorescence characteristics of 5-aminoindazole (5-INH2) have been studied at various acid-base concentrations in sodium dodecyl sulfate and cetyltrimethylammonium bromide (CTAB). The decrease in the fluorescence quantum yields of 5-INH2 in CTAB at pH 9 is partly due to the effect of the medium and partly due to bromide ion-induced fluorescence quenching. Interaction of this amine with detergent molecules at pH 9 leads to premicellar formation. Prototropic reactions in the micellar interface in S0 and S1 states involve the protonation and deprotonation of the amino group only. Biprotonic phototautomerism of singly protonated species is not observed in the micellar interface. Apparent pKa values, which were determined for both equilibrium reactions, are discussed.

Buffer-facilitated proton transport. pH profile of bound enzymes

1. 1. In heterogeneous systems conjugate acid base pairs, besides their conventionally accepted static role as buffers, can also play a dynamic role in facilitating proton transport. When protons are generated and consumed at different locations, the conjugate base binds the proton at the “source” and the resulting acid diffuses to the “sink”. 2. 2. Both the buffering capacity of an acid base pair and its ability to facilitate proton transport are maximum when its pK A is close to the pH of the medium. The transport facilitating effect can be appreciable at much lower concentrations than those needed for buffering capacity. 3. 3. The effect of proton transport has been theoretically investigated by using a model consisting of a H +-producing enzymic surface reaction, which occur with bound enzymes both in vitro and in vivo. 4. 4. The enhancement of proton transport is quantitatively expressed by the extent of transport facilitation, ξ. The interplay of facilitated proton diffusion and reaction kinetics is quantified by the proton modulus, μ. 5. 5. At a given pK A the magnitude of transport facilitation increases with the acid-base concentration and the pH of the solution. The pK A value plays a dual role in determining both the concentration of available carriers and the affinity of the proton to the carrier. 6. 6. Due to its high proton affinity OH − can also facilitate proton transport already at pH 7, but in the presence of buffers its effect becomes significant only at higher pH values. 7. 7. When the extent of transport facilitation sharply increases with the pH of the substrate solution, at relatively low buffer concentrations bound enzymes display sigmoidal pH profiles with sharp peaks. Previously reported experimental data with membrane bound enzymes can be interpreted in view of these theoretical findings. 8. 8. It is postulated that buffers ubiquitous in biochemical and living systems can play an important role also by facilitating proton transport.

Acid–base equilibria of nitropyridines in protium oxide and in deuterium oxide

Equilibrium constants for the dissociation of 2-, 3-, and 4-NO2·C5H4NH+ in H2O have been measured as a function of temperature in the range 3–70 °C by spectrophotometric determination of acid–base concentration ratios in aqueous solution of constant ionic strength (0·02M) or in aqueous sulphuric acid. Thermodynamic functions (ΔG°, ΔH°, and ΔS°) have been calculated for these equilibria as well-behaved functions of temperature and compared with those relative to acid–base equilibrium of unsubstituted pyridine. Entropy values for the above reactions are nearly equal, so that the free-energy differences, due to the different acidities, are mainly found as differences between the corresponding enthalpies. Equilibrium constants for the dissociation of 2-, 3-, and 4-NO2·C5H4ND+ in D2O have also been measured at 298·2 K. Isotope effects seem to be mainly determined by solvation characteristics of the system.

Ultraviolet absorption and protonation equilibria of amino- and nitro-substituted pyridines

Solvent effects on the bathochromic shift in the first π–π* band of aminonitropyridines were examined. They were interpreted in terms of inter- and intra-molecular hydrogen bonding, as well as of transition polarizability which determines solvatochromism. All possible dissociation constants of conjugate acids of amino-, diamino-, nitro-, and aminonitro-pyridines have also been measured at 25°C by spectrophotometric determination of acid–base concentration ratios. The influence of amino- and nitro-groups in the pyridine ring on pKa1 and pKa2 was discussed on the basis of electrical characteristics of substituents and electron availability of reaction site.

Isotope effects on acid–base equilibria of pyridine and [2H5]pyridine in protium and in deuterium oxide

Equilibrium constants for the dissociation of C5H5NH+ and C5D5NH+ in H2O, as well as of C5H5ND+ and C5D5ND+ in D2O have been measured in the range 5–65°, by spectrophotometric determination of acid–base concentration ratios. Thermodynamic functions (ΔG°, ΔH°, ΔS°) have been calculated for these equilibria, and for exchange reactions between deuteriated and non-deuteriated acid and base, in H2O and D2O. Primary, solvent, and secondary isotope effects were evaluated as a function of temperature.

Heat of Neutralization Studies at High Acid-Base Concentration. II. Sodium Hydroxide-Hydrobromic Acid1,2

Heat of Neutralization Studies at High Acid-Base Concentration. II. Sodium Hydroxide-Hydrobromic Acid^1,2

Heat of Neutralization Studies at High Acid—Base Concentrations. I. Sodium Hydroxide—Hydrochloric Acid1

Heat of Neutralization Studies at High Acid—Base Concentrations. I. Sodium Hydroxide—Hydrochloric Acid¹