Enthalpies Of Protonation Research Articles

Protonation equilibria of the tryptophan metabolite 8-hydroxyquinoline-2-carboxylic acid (8-HQA) and its precursors: A potentiometric and calorimetric comparative study

The protonation constants of quinaldic acid (QA), 8-hydroxyquinoline (8-HQ) and 8-hydroxyquinoline-2-carboxylic acid (8-HQA) were determined potentiometrically in KCl(aq) at I = 0.2 mol dm−3 at different temperatures (288.15 ≤ T/K ≤ 318.15). Their temperature dependence was modeled by the van't Hoff equation, which allowed the calculation of other thermodynamic parameters, such as ΔH0 and ΔS0. Protonation enthalpy changes were also experimentally determined by isothermal titration calorimetry (ITC) at T = 298.15 K in the same medium and ionic strength conditions. From the obtained results, it emerged that all stepwise protonation reactions for the three ligands are exothermic, with protonation constants decreasing with increasing temperature. Then, thermodynamic protonation parameters obtained by both approaches were critically analyzed and compared, evidencing that protonation enthalpy changes obtained experimentally by direct calorimetry are more accurate than those derived by the van't Hoff equation. However, the latter approach proved useful to evidence possible variability of this thermodynamic parameter with temperature, thus allowing the eventual calculation of the corresponding ΔCp. Furthermore, on the basis of both the analysis of the obtained parameters and the results of detailed 1D and 2D 1H NMR studies, it was possible to unequivocally determine the protonation sequence of the different functional groups of 8-HQA (as well as QA and 8-HQ): from basic to acidic pH, the first group to undergo protonation is the phenolate, followed by the quinolinic nitrogen and, finally, by the carboxylate.

Guiding Principles for the Rational Design of Hybrid Materials: Use of DFT Methodology for Evaluating Non-Covalent Interactions in a Uranyl Tetrahalide Model System.

Together with the synthesis and experimental characterization of 14 hybrid materials containing [UO2 X4 ]2- (X=Cl- and Br- ) and organic cations, we report on novel methods for determining correlation trends in their formation enthalpy (ΔHf ) and observed vibrational signatures. ΔHf values were analyzed through isothermal acid calorimetry and a Density Functional Theory+Thermodynamics (DFT+T) approach with results showing good agreement between theory and experiment. Three factors (packing efficiency, cation protonation enthalpy, and hydrogen bonding energy [ ]) were assessed as descriptors for trends in ΔHf . Results demonstrated a strong correlation between and ΔHf , highlighting the importance of hydrogen bonding networks in determining the relative stability of solid-state hybrid materials. Lastly, we investigate how hydrogen bonding networks affect the vibrational characteristics of uranyl solid-state materials using experimental Raman and IR spectroscopy and theoretical bond orders and find that hydrogen bonding can red-shift U≡O stretching modes. Overall, the tightly integrated experimental and theoretical studies presented here bridge the trends in macroscopic thermodynamic energies and spectroscopic features with molecular-level details of the geometry and electronic structure. This modeling framework forms a basis for exploring 3D hydrogen bonding as a tunable design feature in the pursuit of supramolecular materials by rational design.

Guiding Principles for the Rational Design of Hybrid Materials: Use of DFT Methodology for Evaluating Non‐Covalent Interactions in a Uranyl Tetrahalide Model System

AbstractTogether with the synthesis and experimental characterization of 14 hybrid materials containing [UO2X4]2− (X=Cl− and Br−) and organic cations, we report on novel methods for determining correlation trends in their formation enthalpy (ΔHf) and observed vibrational signatures. ΔHf values were analyzed through isothermal acid calorimetry and a Density Functional Theory+Thermodynamics (DFT+T) approach with results showing good agreement between theory and experiment. Three factors (packing efficiency, cation protonation enthalpy, and hydrogen bonding energy [ ]) were assessed as descriptors for trends in ΔHf. Results demonstrated a strong correlation between and ΔHf, highlighting the importance of hydrogen bonding networks in determining the relative stability of solid‐state hybrid materials. Lastly, we investigate how hydrogen bonding networks affect the vibrational characteristics of uranyl solid‐state materials using experimental Raman and IR spectroscopy and theoretical bond orders and find that hydrogen bonding can red‐shift U≡O stretching modes. Overall, the tightly integrated experimental and theoretical studies presented here bridge the trends in macroscopic thermodynamic energies and spectroscopic features with molecular‐level details of the geometry and electronic structure. This modeling framework forms a basis for exploring 3D hydrogen bonding as a tunable design feature in the pursuit of supramolecular materials by rational design.

Protein - ligand binding affinity by differential scanning calorimetry.

Differential scanning calorimetry (DSC) determines the enthalpy change upon protein unfolding and the melting temperature of the protein. Performing DSC of a protein in the presence of increasing concentrations of specifically-binding ligand yields a series of curves that can be fit to obtain the protein-ligand dissociation constant. The enthalpy of unfolding, as directly determined by DSC, helps improving the precision of the fit. If the ligand binding is linked to protonation reactions, the intrinsic binding constant can be determined by performing the affinity determination as a function of pH. Here, the intrinsic, pH-independent, affinity of acetazolamide binding to carbonic anhydrase (CA) II was determined. A series of high-affinity ligands binding to CAIX, an anticancer drug target, and CAII showed recognition and selectivity for the anticancer isozyme. Performing the DSC experiment in buffers of highly different enthalpies of protonation enabled to observe the ligand unbinding-linked protonation reactions and estimate the intrinsic enthalpy of binding. The heat capacity of combined unfolding and unbinding was determined by varying the ligand concentrations. These parameters provided a detailed thermodynamic picture of the linked ligand binding and protein unfolding process.

Disagreements Between Calorimetric and Van't Hoff Enthalpies of Adsorption II: Effect of pH and pH Buffers on Phenobarbital Adsorption to Activated Carbon

The reported inconsistencies between the van't Hoff equation and calorimetry hinder the utility of thermodynamics in biochemical and pharmaceutical research. A novel thermodynamic approach is developed herein for ligand adsorption with a focus on the interpretation of calorimetric data in the presence of concurrent proton exchange reactions. Such exchange reactions typically result in a pH-dependence of calorimetric measurements that obscures intrinsic binding enthalpies. It is shown that for the adsorption of phenobarbital to activated carbon, the measured calorimetric enthalpy is a result of three linked acid/base equilibria. A model was established to predict the intrinsic binding enthalpy using 1) the adsorbate's pKa and 2) the adsorbate's enthalpy of protonation. The observed calorimetric enthalpy of binding exhibited both pH and buffer-dependence and was between -5 and -42 kJ/mol. Meanwhile, the predicted intrinsic enthalpy (-25.1 kJ/mol) of binding was in excellent agreement with the measured intrinsic enthalpy (-25.6 kJ/mol). Corrections to the observed calorimetric enthalpies allowed comparisons with enthalpies obtained from the van't Hoff method. It is shown that the predicted intrinsic calorimetric enthalpy agrees well with the van't Hoff enthalpies in instances where observed enthalpies significantly deviated. This treatment is general and is not specific to phenobarbital or activated carbon.

Electrokinetic Insights into the Triple Ionic and Electronic Conductivity of a Novel Nanocomposite Functional Material for Protonic Ceramic Fuel Cells.

Triple ionic and electronic conductivity (TIEC) in cathode materials for protonic ceramic fuel cells (PCFCs) is a desirable feature that enhances the spatial expansion of active reaction sites for electrochemical oxygen reduction reaction. The realization of optimal TIEC in single-phase materials, however, is challenging. A facile route that facilitates the optimization of TIEC in PCFC cathodes is the strategic development of multiphase cathode materials. In this study, a cubic-rhombohedral TIEC nanocomposite material with the composition Ba(CeCo)0.4 (FeZr)0.1 O3- δ (BCCFZ) is designed via self-assembly engineering. The material consists of a mixed ionic and electronic conducting phase, BaCo1-( x + y + z ) Cex Fey Zrz O3- δ (M-BCCFZ), and a dominant proton-conducting phase, BaCe1-( x + y + z ) Cox Zry Fez O3- δ (H-BCCZF). The dominant cerium-rich H-BCCFZ phase enhances the material's oxygen vacancy concentration and the proton defects formation and transport with a low enthalpy of protonation of -30 ± 9kJ mol-1 . The area-specific resistance of the BCCFZ symmetrical cell is 0.089 Ω cm2 at 650 °C in 2.5% H2 O-air. The peak power density of the anode-supported single cell based on BCCFZ cathode reaches 1054mW cm-2 at 650 °C with good operation stability spanning over 500h at 550 °C. These promote BCCFZ as a befitting cathode material geared toward PCFC commercialization.

Intrinsic affinity of protein – ligand binding by differential scanning calorimetry

Differential scanning calorimetry (DSC) determines the enthalpy change upon protein unfolding and the melting temperature of the protein. Performing DSC of a protein in the presence of increasing concentrations of specifically-binding ligand yields a series of curves that can be fit to obtain the protein–ligand dissociation constant as done in the fluorescence-based thermal shift assay (FTSA, ThermoFluor, DSF). The enthalpy of unfolding, as directly determined by DSC, helps improving the precision of the fit. If the ligand binding is linked to protonation reactions, the intrinsic binding constant can be determined by performing the affinity determination at a series of pH values. Here, the intrinsic, pH-independent, affinity of acetazolamide binding to carbonic anhydrase (CA) II was determined. A series of high-affinity ligands binding to CAIX, an anticancer drug target, and CAII showed recognition and selectivity for the anticancer isozyme. Performing the DSC experiment in buffers of highly different enthalpies of protonation enabled to observe the ligand unbinding-linked protonation reactions and estimate the intrinsic enthalpy of binding. The heat capacity of combined unfolding and unbinding was determined by varying the ligand concentrations. Taken together, these parameters provided a detailed thermodynamic picture of the linked ligand binding and protein unfolding process.

Thermodynamic and spectroscopic investigations of Shewanella putrefaciens CN32 cell envelope

A quantitative understanding of prokaryotic cell envelope reactivity is necessary to accurately describe the surface complexation reactions known to impact metal transport in the subsurface. The gram-negative bacterium Shewanella putrefaciens CN32 (CN32) has been the focus of previous cell envelope studies in large part due to its ability to directly influence iron and uranium geochemistry; however, important gaps remain in our understanding of reactions occurring on its cell envelope. Combining surface complexation modeling of potentiometric titration data with isothermal titration calorimetry provides an additional method to evaluate how well surface complexation models (SCMs) represent cell envelope reactivity. It also gives us the ability to determine site-specific enthalpies and entropies of protonation, which can aid both in site structure identification and description of conformational changes to those sites, further improving our understanding of the CN32 cell envelope. We gathered potentiometric and isothermal calorimetric titration data of CN32 over a range of ionic strength (0.02 – 0.53 M) while monitoring cell viability and performing Fourier transform infrared spectroscopy (FTIR) investigations. FTIR analysis was performed over a range of pH (4 – 9.4) to confirm identification of proton-active sites. Surface complexation modeling revealed that models involving 3 and 4 sites could adequately describe the potentiometric titration data; however, a 3-site model best represented the calorimetric data. We inferred the following site identities based on characteristic enthalpies, entropies, and equilibrium constants of protonation: carboxyl (log(K): 4.79 – 4.95), phosphoryl (6.69 – 6.97), and amine groups (9.37 – 9.64). Spectral corroboration via FTIR indicated the presence of chemical moieties consistent with these functional group assignments. Ionic strength was shown to have a significant impact on the concentration of all three sites at 95% confidence. Entropies and equilibrium constants of protonation for carboxyl and phosphoryl sites did not significantly change with ionic strength. For the carboxyl group, enthalpies of protonation varied significantly between the lowest and highest ionic strengths examined, though enthalpies at intermediate concentrations were not distinguishable. Enthalpies of protonation for the amine group were statistically different at each ionic strength. Additionally, entropies and equilibrium constants of protonation for the amine group were significantly affected by all ionic strengths tested. The entropy of protonation for the amine site changed from −21.5 ± 6.1 to +109.0 ± 4.2 J/molK−1 as ionic strength increased from 0.02 to 0.53 M, respectively. A change in the entropy of protonation of this magnitude and sign is indicative of a substantial conformational change in this functional group. Cell viability was significantly impacted over the course the titrations; however, viability did not impact SCMs results. In sum, these data fill gaps in our understanding of CN32 and provide new insights into the intricacies of its cell envelope.

Force-Field-Based Computational Study of the Thermodynamics of a Large Set of Aqueous Alkanolamine Solvents for Post-Combustion CO2 Capture.

The ability to predict the thermodynamic properties of amine species in CO2-loaded aqueous solutions, including their deprotonation (pKa) and carbamate to bicarbonate reversion (pKc) equilibrium constants and their corresponding standard reaction enthalpies, is of critical importance for the design of improved carbon capture solvents. In this study, we used isocoulombic forms of both reactions to determine these quantities for a large set of aqueous alkanolamine solvent systems. Our hybrid approach involves using classical molecular dynamics simulations with the general amber force field (GAFF) and semi-empirical AM1-BCC charges (GAFF/AM1-BCC) in the solution phase, combined with high-level composite quantum chemical ideal-gas calculations. We first determined a new force field (FF) for the hydronium ion (H3O+) by matching to the single experimental pKa data point for the well-known monoethanolamine system at 298.15 K. We then used this FF to predict the pKa values for 76 other amines at 298.15 K and for all 77 amines at elevated temperatures. Additionally, we indirectly relate the H3O+ hydration free energy to that of H+ and provide expressions for intrinsic hydration free energy and enthalpy of the proton. Using the derived H3O+ FF, we predicted the pKa values of a diverse set of alkanolamines with an overall average absolute deviation of less than 0.72 pKa units. Furthermore, the derived H3O+ FF is able to predict the protonation enthalpy of these amines when used with the GAFF. We also predicted the carbamate reversion constants of the primary and secondary amine species in the data set and their corresponding standard heats of reaction, which we compared with the scarcely available experimental data, which are often subject to significant uncertainty. Finally, we also described the influence of electronic and steric effects of different molecular fragments/groups on the stabilities of the carbamates.

Protonation of Planar and Nonplanar Porphyrins: A Calorimetric and Computational Study.

Herein, we report the first calorimetric study of the protonation of planar and nonplanar free-base porphyrins: H2OETPP (strongly saddled by its substituents), H2T(tBu)P (strongly ruffled by its substituents), and the nominally planar porphyrins (npPs) H2OEP, H2TPP, H2T(nPe)P, and H2T(iPr)P. The observed enthalpies of protonation in solution (ΔHprotsoln) for formation of the dications in 1,1,2,2-tetrachloroethane with 2% trifluoroacetic acid are -45 ± 1 kcal mol-1 for the npPs, -52.0 kcal mol-1 for H2T(tBu)P, and -70.9 kcal mol-1 for H2OETPP. The corresponding enthalpies of protonation (ΔHDFT) obtained from DFT calculations (-27 ± 5, -42, and -63 kcal mol-1, respectively) reproduce this trend. The much more negative enthalpy of protonation seen for H2OETPP is consistent with this molecule being pre-deformed into the saddle structure favored by porphyrin dications. Except for OETPP, the calculated enthalpies of the first protonations (ΔH1) are significantly more positive than the enthalpies of the second protonations (ΔH2). In addition, the structural strain energies for the first protonations (ΔEst(1)) are also significantly more positive than ΔEst(2). According to the calculations, the monocations thus have higher proton affinities than the corresponding free-base porphyrins due to a structural strain effect, which is consistent with the generally elusive nature of the porphyrin monocation. The recent observations of monocations for free-base porphyrins with a high degree of saddling can be rationalized in terms of ΔH1 and ΔH2 being similar; so, the monocation is no longer an unstable intermediate.

Harnessing aromaticity and intramolecular hydrogen bonding to tailor organosuperbases by using 2,4,6-cycloheptatriene-1-imine scaffold

The density functional theory (DFT) calculations at B3LYP/6-31 + G(d,p) level were performed for a series of new 2,4,6-cycloheptatriene-1-imine scaffolds which have potential to introduce as organosuperbases. In the present study, we have exploited the 2,4,6-cycloheptatriene-1-imines which strategically substituted at 2 and 7 positions by two potential anchoring arms having lone pair possessing atoms. The designed organic superbases were found to have the higher basicity than the benchmarked molecule, 1,8-bis(dimethylamino)-naphthalene. The proposed superbase B14 bearing guanidine and aniline groups possesses the highest proton affinity value as 1143.8 kJ mol−1. The protonation enthalpy, Gibbs free energy, harmonic oscillator model of aromaticity (HOMA), and nucleus-independent chemical shift (NICS) were used for the basicity comparison. After the protonation of the designed molecules, the positive charge on the conjugate acid is expected to be delocalized in the cycloheptatriene ring and led to its aromatization. The results indicate that the HOMA and NICS values of the ring in the conjugate acid are strongly elevated. On the other hand, very strong intramolecular hydrogen bonding is formed after protonation. These two factors enhance the stability of the conjugate acids and thereby increase the basicity of the designed molecules.

Thermodynamic study on levulinic acid in NaCl, (C2H5)4NI and mixed MgCl2/NaCl and CaCl2/NaCl aqueous solutions at T = 298.15 K

Some thermodynamic properties of levulinc acid (LA) were studied under different experimental conditions. The partition coefficient between 2-methyl-1-propanol/pure water (or NaCl aqueous solutions up to I = 1.00 mol dm−3) was measured at T = 298.15 K by means of potentiometric titrations. The protonation constant of levulinic acid was also determined potentiometrically at different ionic strengths in NaCl(aq), (C2H5)4NI(aq) as well as in mixed MgCl2/NaCl and CaCl2/NaCl aqueous solutions.The protonation enthalpy was determined by calorimetric titrations in the same conditions of temperature and ionic strength.The linear fitting of the distribution ratio (log KD) determined at different salt concentrations vs. the salt concentration allowed for the determination of the distribution coefficient at infinite dilution (logKD00 = −0.14 ± 0.02), the Setschenow (km = 0.35 ± 0.03) and the activity coefficient of the neutral species (log γHL = km·msalt). The activity coefficient of the charged L− species was also computed by means of the Specific Interaction Theory (SIT) approach by keeping constant the value of the activity coefficient for the neutral HL species.The medium effect on the protonation constant was also studied using the weak interaction model, namely by comparing the protonation constants obtained in NaCl(aq), (C2H5)4NI(aq) as well as in mixed aqueous MgCl2/NaCl and CaCl2/NaCl mixtures by considering the formation of a NaL(aq), CaL+(aq), and MgL+(aq) species.

CO2 Capture via Crystalline Hydrogen-Bonded Bicarbonate Dimers

Summary Limiting global temperature rises is increasingly dependent on the development of energy-efficient carbon-capture methods. Here, we report a simple CO2-separation cycle using an aqueous bis(iminoguanidine) (BIG) sorbent that reacts with CO2 and crystallizes into an insoluble bicarbonate salt. X-ray diffraction analysis of the bicarbonate crystals revealed “anti-electrostatic” hydrogen-bonded (HCO3−)2 dimers, stabilized by guanidinium cations and water. Mild heating of the crystals releases the CO2 and regenerates the BIG sorbent quantitatively, thereby closing the CO2-separation cycle. Experimental and computational investigations support a CO2-release mechanism consisting of surface-initiated low-barrier proton transfer from guanidinium groups to bicarbonate anions with the formation of carbonic acid dimers, followed by CO2 and H2O release in the rate-limiting step, with a measured activation energy of 102 ± 12 kJ/mol. The minimum energy required for sorbent regeneration is 151.5 kJ/mol CO2, which is 24% lower than the regeneration energy of monoethanolamine, a benchmark industrial sorbent.

Enthalpies of Glycine Protonation in Water–Acetone and Water–Ethanol Solvents

Heat effects of the reaction of glycine protonation at 298 K in water–ethanol (µ = 0.1 (NaClO4)) and water–acetone (µ = 0.3 (NaClO4)) solvents of various compositions are determined via calorimetry. The enthalpies of the resolvation of glycinium ions in aqueous solutions of ethanol and acetone are calculated using the literature data. It is established that a minor change in the heat effect of the reaction of glycine protonation in water–ethanol and water–acetone solvents is associated with the mutual compensation of the enthalpies of proton, glycinium-ion, and glycine resolvation in these media. It is shown that the contribution from the enthalpy component to the change in the Gibbs energy of this reaction is insignificant.

Thermodynamic Analysis of Nickel(II) and Zinc(II) Adsorption to Biochar

While numerous studies have investigated metal uptake from solution by biochar, few of these have developed a mechanistic understanding of the adsorption reactions that occur at the biochar surface. In this study, we explore a combined modeling and spectroscopic approach for the first time to describe the molecular level adsorption of Ni(II) and Zn(II) to five types of biochar. Following thorough characterization, potentiometric titrations were carried out to measure the proton (H+) reactivity of each biochar, and the data was used to develop protonation models. Surface complexation modeling (SCM) supported by synchrotron-based extended X-ray absorption fine structure (EXAFS) was then used to gain insights into the molecular scale metal-biochar surface reactions. The SCM approach was combined with isothermal titration calorimetry (ITC) data to determine the thermodynamic driving forces of metal adsorption. Our results show that the reactivity of biochar toward Ni(II) and Zn(II) directly relates to the site densities of biochar. EXAFS along with FT-IR analyses, suggest that Ni(II) and Zn(II) adsorption occurred primarily through proton-active carboxyl (-COOH) and hydroxyl (-OH) functional groups on the biochar surface. SCM-ITC analyses revealed that the enthalpies of protonation are exothermic and Ni(II) and Zn(II) complexes with biochar surface are slightly exothermic to slightly endothermic. The results obtained from these combined approaches contribute to the better understanding of molecular scale metal adsorption onto the biochar surface, and will facilitate the further development of thermodynamics-based, predictive approaches to biochar removal of metals from contaminated water.

Protonation States of Glufosinate in Aqueous Solution

Glufosinate, an analogue of glutamic acid (also known as phosphinothricin), is an important herbicide. The protonation constants of glufosinate were determined by means of potentiometric titrations at variable temperatures with ionic strength 0.5 mol·L−1 (NaCl solution). The heat effects of the protonation reactions of glufosinate were measured by direct calorimetry. NMR spectroscopies have demonstrated that the first protonation site occurs on the nitrogen atom in the amino group, followed by one of the oxygen atoms in the phosphono group, and finally the carboxyl oxygen atom. This trend is in good agreement with the enthalpy of protonation and crystal structure results. These data will help to predict the speciation of glufosinate in physiological systems.

Heat Effect of the Protonation of Glycine and the Enthalpies of Resolvation of Participating Chemical Species in Water–Dimethylsulfoxide Solvent Mixtures

Enthalpies of the protonation of glycine in water‒dimethylsulfoxide (DMSO) mixed solvents are determined calorimetrically in the range of DMSO mole fractions of 0.0 to 0.9, at T = 298.15 K and an ionic strength μ = 0.3 (NaClO4). It is established that the protonation of glycine becomes more exothermic with an increasing mole fraction of DMSO, and the enthalpies of resolvation of glycine and glycinium ions in water‒DMSO solvent mixtures are calculated. It is shown that the small changes in the enthalpy of protonation observed at low mole fractions of DMSO are caused by the contributions from the solvation of proton and protonated glycine cancelling each other out. The enthalpy term of the Gibbs energy of the reaction leading to the formation of glycinium ion is estimated along with the enthalpy of resolvation of the reacting species in the water‒DMSO mixed solvent.

Design of Exceptionally Strong Organic Superbases Based on Aromatic Pnictogen Oxides: Computational DFT Analysis of the Oxygen Basicity in the Gas Phase and Acetonitrile Solution.

DFT B3LYP calculations convincingly showed that aromatic pnictogen oxides offer scaffolds suitable for tailoring powerful organic superbases exhibiting exceptional oxygen basicity in both the gas phase and polar aprotic acetonitrile solution. With their protonation enthalpies and pKa values, they surpass the basicity of classical proton sponges and related nitrogen bases. The most potent system is provided with two arsenic oxide moieties on the phenanthrene framework assisted by the two phosphazeno groups in the para-position to both basic centers. With its proton affinity PA = 300.5 kcal mol-1, the latter system breaks the gas-phase hyperbasicity threshold of 300 kcal mol-1, while its pKa = 54.8 promotes it as an unprecedented superbase in acetonitrile. The origin of such a dramatic basicity enhancement is traced to a fine interplay between (a) steric repulsions of the two negatively charged oxygens destabilizing a neutral base, (b) favorable intramolecular [O-H···O]- hydrogen bonding in conjugate acids, and (c) efficient cationic resonance upon protonation supported by the electron-donating substituents. Given the growing interest in highly basic compounds together with related basic catalysts and metal complexing agents, we hope that the results presented here will open a new avenue of research in these fields and direct attention toward utilizing aromatic pnictogen oxides in designing improved organic materials.

Modeling solubility and acid-base properties of some polar side chain amino acids in NaCl and (CH3)4NCl aqueous solutions at different ionic strengths and temperatures

The aim of the paper is to perform a critical analysis of the thermodynamic properties of five L-α-amino acids, namely arginine, aspartic acid, glutamic acid, glutamine and histidine. New potentiometric measurements to determine protonation constants and solubility (not for arginine) in NaCl and (CH3)4NCl aqueous solutions are reported, in order to investigate the medium effect. A preliminary analysis of the state of art evidenced the considerable amount of data in the ionic strength range 0 < I/mol kg−1 ≤ 0.1 and a lack of data (especially for arginine and glutamine) above 0.1 mol kg−1. For this reason, this paper deals with an analysis of both the literature and the new experimental data, assigning a weight to each datum as a function of its reliability. The ionic strength dependence of the protonation constants was interpreted using the EDH (Extended Debye-Hückel), the SIT (Specific ion Interaction Theory) and the Pitzer approaches. Simultaneous analysis of solubility data and protonation constants allowed us to obtain the Setschenow and the activity coefficients of all the species involved in the equilibria. The differences between the two media were also interpreted considering the formation of weak complexes between the ions of the supporting electrolytes and the ligands according to the complex formation model.Finally, recommended, tentative or provisional data for protonation constants and protonation enthalpy changes of the five amino acids at different temperatures and ionic strengths are provided.

Determination of Protonation Constants of O-Phospho-l-serine in Aqueous Solution: Potentiometry, Microcalorimetry, NMR Spectroscopy and Quantum Chemical Calculations

O-Phospho-l-serine is one of the naturally occurring phosphorylated amino acids, having important pharmacological activity and bioactivity. The protonation constants of O-phospho-l-serine were determined by means of potentiometric titrations at 25 °C and ionic strength of 0.5 mol·L−1 (NaCl). The heat effects of the protonation reaction of the O-phospho-l-serine were measured by direct calorimetry. NMR spectroscopy has demonstrated that the first protonation site occurs at the nitrogen atom in the amino group, followed by one of the oxygen atoms in the phosphono group, and finally the carboxyl oxygen atom. This trend is in good agreement with the enthalpy of protonation and quantum chemical calculations. These data will help to predict the speciation of O-phospho-l-serine in physiological systems.