Acido-basic Properties Research Articles

Elucidation of factors shaping reactivity of 5'-deoxyadenosyl - a prominent organic radical in biology.

This study investigates the factors modulating the reactivity of 5'-deoxyadenosyl (5'dAdo˙) radical, a potent hydrogen atom abstractor that forms in the active sites of radical SAM enzymes and that otherwise undergoes a rapid self-decay in aqueous solution. Here, we compare hydrogen atom abstraction (HAA) reactions between native substrates of radical SAM enzymes and 5'dAdo˙ in aqueous solution and in two enzymatic microenvironments. With that we reveal that HAA efficiency of 5'dAdo˙ is due to (i) the in situ formation of 5'dAdo˙ in a pre-ordered complex with a substrate, which attenuates the unfavorable effect of substrate:5'dAdo˙ complex formation, and (ii) the prevention of the conformational changes associated with self-decay by a tight active-site cavity. The enzymatic cavity, however, does not have a strong effect on the HAA activity of 5'dAdo˙. Thus, we performed an analysis of in-water HAA performed by 5'dAdo˙ based on a three-component thermodynamic model incorporating the diagonal effect of the free energy of reaction, and the off-diagonal effect of asynchronicity and frustration. To this aim, we took advantage of the straightforward relationship between the off-diagonal thermodynamic effects and the electronic-structure descriptor - the redistribution of charge between the reactants during the reaction. It allows to access HAA-competent redox and acidobasic properties of 5'dAdo˙ that are otherwise unavailable due to its instability upon one-electron reduction and protonation. The results show that all reactions feature a favourable thermodynamic driving force and tunneling, the latter of which lowers systematically barriers by ∼2 kcal mol-1. In addition, most of the reactions experience a favourable off-diagonal thermodynamic contribution. In HAA reactions, 5'dAdo˙ acts as a weak oxidant as well as a base, also 5'dAdo˙-promoted HAA reactions proceed with a quite low degree of asynchronicity of proton and electron transfer. Finally, the study elucidates the crucial and dual role of asynchronicity. It directly lowers the barrier as a part of the off-diagonal thermodynamic contribution, but also indirectly increases the non-thermodynamic part of the barrier by presumably controlling the adiabatic coupling between proton and electron transfer. The latter signals that the reaction proceeds as a hydrogen atom transfer rather than a proton-coupled electron transfer.

Predictive Potential of Acido-Basic Properties, Solubility and Food on Bioequivalence Study Outcome: Analysis of 128 Studies.

Risk assessment related to bioequivalence study outcome is critical for effective planning from the early stage of drug product development. The objective of this research was to evaluate the associations between solubility and acido-basic parameters of an active pharmaceutical ingredient (API), study conditions and bioequivalence outcome. We retrospectively analyzed 128 bioequivalence studies of immediate-release products with 26 different APIs. Bioequivalence study conditions and acido-basic/solubility characteristics of APIs were collected and their predictive potential on the study outcome was assessed using a set of univariate statistical analyses. There was no difference in bioequivalence rate between fasting and fed conditions. The highest proportion of non-bioequivalent studies was for weak acids (10/19 cases, 53%) and neutral APIs (23/95 cases, 24%). Lower non-bioequivalence occurrence was observed for weak bases (1/15 cases, 7%) and amphoteric APIs (0/16 cases, 0%). The median dose numbers at pH1.2 and pH3 were higher and the most basic acid dissociation constant (pKa) was lower in the non-bioequivalent group of studies. Additionally, APIs with low calculated effective permeability (cPeff) or low calculated lipophilicity (clogP) had lower non-bioequivalence occurrence. Results of the subgroup analysis of studies under fasting conditions were similar as for the whole dataset. Our results indicate that acido-basic properties of API should be considered in bioequivalence risk assessment and reveal which physico-chemical parameters are most relevant for the development of bioequivalence risk assessment tools for immediate-release products.

Physicochemical Properties Govern the Activity of Potent Antiviral Flavones

Ladanein (i.e., 5,6,7-trihydroxylated flavone) was demonstrated to act as a powerful virucidal agent toward a broad range of enveloped virus particles. Fe(III) coordination and pH are indeed among the key parameters that might favor both bioactivation of the flavone and consequent host cell entry inhibition. In this present work, the impact of fluorinated groups on the physicochemical and antiviral properties of the flavone was investigated, thus allowing a deeper understanding of the antiviral mode of action. The improved synthesis of ladanein allowed accessing a broad range of analogues, some of them being significantly more active than the former ladanein lead compound. We first determined the acido-basic properties of this homogenous series of compounds and then investigated their electrochemical behavior. Fe(III) coordination properties (stability, spectral behavior, and kinetics) of ladanein and its analogues were then examined (quasiphysiological conditions) and provided key information of their stability and reactivity. Using the determined physicochemical parameters, the critical impact of the iron complexation and medium acidity was confirmed on hepatitis C virus (HCV) particles (pre)treated with ladanein. Finally, a preliminary structure–HCV entry inhibition relationship study evidenced the superior antiviral activity of the ladanein analogues bearing an electron-withdrawing group in para position (FCF3 > FOCF3 > FFCF3 > FF > FOMe) on the B cycle in comparison with the parent ladanein itself.

How the acido-basic properties of Mg silicates and clays govern the catalytic mechanism of transesterification reactions

Choosing acido-basic properties of magnesium silicates to tune the transesterification mechanism in the liquid phase.

Beyond the classical thermodynamic contributions to hydrogen atom abstraction reactivity

Hydrogen atom abstraction (HAA) reactions are cornerstones of chemistry. Various (metallo)enzymes performing the HAA catalysis evolved in nature and inspired the rational development of multiple synthetic catalysts. Still, the factors determining their catalytic efficiency are not fully understood. Herein, we define the simple thermodynamic factor η by employing two thermodynamic cycles: one for an oxidant (catalyst), along with its reduced, protonated, and hydrogenated form; and one for the substrate, along with its oxidized, deprotonated, and dehydrogenated form. It is demonstrated that η reflects the propensity of the substrate and catalyst for (a)synchronicity in concerted H+/e- transfers. As such, it significantly contributes to the activation energies of the HAA reactions, in addition to a classical thermodynamic (Bell-Evans-Polanyi) effect. In an attempt to understand the physicochemical interpretation of η, we discovered an elegant link between η and reorganization energy λ from Marcus theory. We discovered computationally that for a homologous set of HAA reactions, λ reaches its maximum for the lowest |η|, which then corresponds to the most synchronous HAA mechanism. This immediately implies that among HAA processes with the same reaction free energy, ΔG0, the highest barrier (≡ΔG≠) is expected for the most synchronous proton-coupled electron (i.e., hydrogen) transfer. As proof of concept, redox and acidobasic properties of nonheme FeIVO complexes are correlated with activation free energies for HAA from C-H and O-H bonds. We believe that the reported findings may represent a powerful concept in designing new HAA catalysts.

Physico-Chemical Properties of MgGa Mixed Oxides and Reconstructed Layered Double Hydroxides and Their Performance in Aldol Condensation of Furfural and Acetone.

MgGa layered double hydroxides (Mg/Ga = 2–4) were synthesized and used for the preparation of MgGa mixed oxides and reconstructed hydrotalcites. The properties of the prepared materials were examined by physico-chemical methods (XRD, TGA, NH3-TPD, CO2-TPD, SEM, and DRIFT) and tested in aldol condensation of furfural and acetone. The as-prepared phase-pure MgGa samples possessed hydrotalcite structure, and their calcination resulted in mixed oxides with MgO structure with a small admixture phase characterized by a reflection at 2θ ≈ 36.0°. The interaction of MgGa mixed oxides with pure water resulted in reconstruction of the HTC structure already after 15 s of the rehydration with maximum crystallinity achieved after 60 s. TGA-MS experiments proved a substantial decrease in carbonates in all rehydrated samples compared with their as-prepared counterparts. This allowed suggesting presence of interlayer hydroxyls in the samples. Acido-basic properties of MgGa mixed oxides determined by TPD technique did not correlate with Mg/Ga ratio which was explained by the specific distribution of Ga atoms on the external surface of the samples. CO2-TPD method was also used to evaluate the basic properties of the reconstructed MgGa samples. In these experiments, an intensive peak at T = 450°C on CO2-TPD curve was attributed to the decomposition of carbonates newly formed by CO2 interaction with interlayer carbonates rather than to CO2 desorption from basic sites. Accordingly, CO2-TPD method quantitatively characterized the interlayer hydroxyls only indirectly. Furfural conversion on reconstructed MgGa materials was much larger compared with MgGa mixed oxides confirming that Brønsted basic sites in MgGa catalysts, like MgAl catalysts, were active in the reaction. Mg/Ga ratio in mixed oxides influenced product selectivity which was explained by the difference in textural properties of the samples. In contrast, Mg/Ga ratio in reconstructed catalysts had practically no effect on the composition of reaction products suggesting that the basic sites in these catalysts acted similarly in aldol condensation of acetone with furfural. It was concluded that the properties of MgGa samples resembled in a great extent those of MgAl hydrotalcite-based materials and demonstrated their potential as catalysts for base-catalyzed reactions.

Aldol Condensation of Benzaldehyde and Heptanal Over Zinc Modified Mixed Mg/Al Oxides

Several types of zinc modified Mg–Al layered double hydroxides were prepared. Zinc was incorporated into catalyst structure using different methods—coprecipitation, kneading or impregnation. Characterization of all solid catalysts was performed using different techniques. Acido–basic properties of prepared materials were investigated using ammonia (or carbon dioxide) temperature programmed desorption. The activity of the prepared samples was compared with pure Mg–Al oxide with Mg:Al ratio 3:1 in aldol condensation of benzaldehyde and heptanal. The influence of temperature (80–120 °C) on the reaction course was monitored. Heptanal conversion and selectivity to two main products, i.e. 2-pentylcinnamylaldehyde (jasmine aldehyde) and 2 pentylnon-2-enal, were evaluated. Zinc modified catalysts exhibited under the same conditions 15–20% higher yields of the desired jasmine aldehyde.

Tuning Y-zeolite based catalyst with copper for enhanced activity and selectivity in vapor phase hydrogenolysis of glycerol to 1,2-propanediol

Highly dispersed copper oxide species supported on a Y-zeolite with different Si/Al ratios (5.1, 12, 30, and 60) were synthesized by an incipient wet impregnation method. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption (BET), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffused reflectance spectroscopy (UV-DRS), N2O chemisorption, Pyridine-adsorption Fourier transform Infrared Spectroscopy (Py-FTIR), NH3 temperature programmed desorption (NH3-TPD) and temperature programmed reduction (TPR). Copper dispersion and metallic area were measured by the N2O decomposition method. The amount of Bronsted and Lewis acidic sites was analyzed by Pyridine adsorption followed by FT-IR. Performance of the prepared catalysts in vapor phase hydrogenolysis of glycerol to 1,2-propanediol was evaluated by using a fixed-bed stainless-steel reactor at 0.2MPa and 210°C. The studies revealed that the 3wt.% CuO/Y-zeolite (Si/Al=5.1) catalyst was highly active in glycerol hydrogenolysis showing 92% conversion of glycerol and 83% selectivity to 1,2-propanediol. This seems to originate from high dispersion of Cu, smaller Cu crystallite size, and the presence of a high amount of surface beneficial acido-basic properties.

Comparing the Spectroscopic and Molecular Characteristics of Different Dissolved Organic Matter Fractions Isolated by Hydrophobic and Anionic Exchange Resins Using Fluorescence Spectroscopy and FT-ICR-MS

Despite the environmental significance of dissolved organic matter (DOM), characterizing DOM is still challenging due to its structural complexity and heterogeneity. In this study, three different chemical fractions, including hydrophobic acid (HPOA), transphilic acid (TPIA), and hydrophilic neutral and base (HPIN/B) fractions, were separated from bulk aquatic DOM samples, and their spectral features and the chemical composition at the molecular level were compared using both fluorescence excitation emission matrix-parallel factor analysis (EEM-PARAFAC) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The HPIN/B fraction was distinguished from the two acidic fractions (i.e., HPOA and TPIA) by the EEM-PARAFAC, while the TPIA fraction was discriminated by using the molecular parameters derived from the FT-ICR MS analyses. Statistical comparison suggests that the spectral dissimilarity among the three chemical fractions might result from the acido-basic properties of DOM samples, while the differences in molecular composition were more likely to be affected by the hydrophobicity of the DOM fractions. The non-metric multidimensional scaling map further revealed that the HPOA was the most heterogeneous among the three fractions. The number of overlapping formulas among the three chemical fractions constituted only <5% of all identified formulas, and those between two different fractions ranged from 2.0% to 24.1%, implying relatively homogeneous properties of the individual chemical fractions with respect to molecular composition. Although employing chemical fractionation achieved a lowering of the DOM heterogeneity, prevalent signatures of either acido-basic property or the hydrophobic nature of DOM on the characteristics of three chemical isolated fractions were not found for this study.

Surface Properties of Hydrotalcite-Based Zn(Mg)Al Oxides and Their Catalytic Activity in Aldol Condensation of Furfural with Acetone

Basic mixed oxides MgAl, ZnMgAl, and ZnAl were successfully prepared from hydrotalcite precursors synthesized by urea method. Materials with the same molar ratio (M2+/Al3+) = 2 were studied to describe the influence of Mg/Zn ratio on their physicochemical properties. Materials were tested as catalysts of the aldol condensation of furfural with acetone. For samples with similar particle sizes and surface BET areas, the varying catalytic activity was related to the different acidobasic properties. Higher furfural conversion and selectivity to longer carbon chain F2Ac product was observed for samples with higher total amount of basic sites. More specifically, it correlated with the population of Me2+–O2– pairs that represented dominant type of basic sites in all studied catalysts. At the same Al loading, Mg2Al mixed oxide exhibited higher specific surface area, higher total amount of basic sites and higher amount of acid sites than Zn2Al oxide.

Quercetin and its analogues: optical and acido-basic properties.

This study is focused on eight structurally analogous natural flavonoids that exhibit a wide range of biological activities, which are of interest in pharmacy, cosmetics and the food industry. Using both experimental and theoretical approaches, we relate their fundamental physico-chemical properties to the structural motifs, with particular focus on UV/Vis absorption properties and pH dependence. We highlight the role of the C2-C3 double bond, whose presence or absence is responsible for the switch between absorption bands in the UVB and UVA regions, which is rationalized by strong modification of the involved molecular orbitals. After deprotonation in an alkaline environment, a typical switch in intensity at the maximum absorption wavelength (λmax) is observed enabling the calculation of pKa values for compounds with a C2-C3 single bond, whereas a bathochromic shift of λmaxvs. pH is observed for the C2-C3 double bond containing compounds. These behaviors are also rationalized and understood by MO analysis. Interestingly, high pH (above 11 for ampelopsin and above 9 for myricetin) induces the formation of a long-wavelength peak arising from double and/or triple deprotonation. Substitution at position C3 by the OH group has almost no effect on λmax for taxifolin and eriodictyol, whereas the effect is larger for quercetin and luteolin. An additional sugar moiety at C3 has a stabilizing effect and induces only minor changes in spectral behavior.

Influence of acid–base properties of Mg-based catalysts on transesterification: role of magnesium silicate hydrate formation

Catalysts with the strongest basic properties are not always the most efficient ones for transesterification, a series of magnesium-based materials, exhibiting a large range of acido–basic properties, was investigated.

Reactivity of 5-nitro-2,1-benzothiazole-3-diazonium hydrogensulfate towards OH and CH acids: structure, spectral properties, tautomerism

5-Nitro-2,1-benzothiazole-3-diazonium ion reacts with various substrates (phenol, 2-naphthol, mesitylene, acetone, acetylacetone, ethyl acetoacetate) to give corresponding azo or hydrazo compounds. Characterization of the prepared compounds by means of multinuclear magnetic resonance, X-ray diffraction, and HRMS together with the study of acidobasic properties is presented, as well as significant solvatochromism.

Pyrochemical reprocessing of molten salt fast reactor fuel: focus on the reductive extraction step

Abstract The nuclear fuel reprocessing is a prerequisite for nuclear energy to be a clean and sustainable energy. In the case of the molten salt reactor containing a liquid fuel, pyrometallurgical way is an obvious way. The method for treatment of the liquid fuel is divided into two parts. In-situ injection of helium gas into the fuel leads to extract the gaseous fission products and a part of the noble metals. The second part of the reprocessing is performed by ‘batch’. It aims to recover the fissile material and to separate the minor actinides from fission products. The reprocessing involves several chemical steps based on redox and acido-basic properties of the various elements contained in the fuel salt. One challenge is to perform a selective extraction of actinides and lanthanides in spent liquid fuel. Extraction of actinides and lanthanides are successively performed by a reductive extraction in liquid bismuth pool containing metallic lithium as a reductive reagent. The objective of this paper is to give a description of the several steps of the reprocessing retained for the molten salt fast reactor (MSFR) concept and to present the initial results obtained for the reductive extraction experiments realized in static conditions by contacting LiF-ThF4-UF4-NdF3 with a lab-made Bi-Li pool and for which extraction efficiencies of 0.7% for neodymium and 14.0% for uranium were measured. It was concluded that in static conditions, the extraction is governed by a kinetic limitation and not by the thermodynamic equilibrium.

Conversion of isopropyl alcohol over Ru and Pd loaded N-doped carbon nanotubes

Ru and Pd (2 wt%) loaded on pure and on N-doped carbon nanotubes (N-CNTs) were prepared and tested using the isopropyl alcohol decomposition reaction as probe reaction. The presence of nitrogen functionalities (pyridinic, pyrrolic, and quaternary nitrogen) on the nitrogen doped support induced a higher metal dispersion: Pd/N-CNT (1.8 nm) < Pd/CNT (4.9 nm), and Ru/N-CNT (2.4 nm) < Ru/CNT (3.0 nm). The catalytic activity of the supports was determined first. Isopropyl alcohol conversion produces acetone on CNTs while on N-CNTs it led to both dehydration and dehydrogenation products. At 210 °C and in the presence of air, the isopropyl alcohol conversion was higher on the N-CNTs (25%) than on the CNTs (11%). The Pd loaded catalysts were more active and more selective than the Ru ones. At 115 °C, the Pd catalysts were 100% selective towards acetone for a conversion of 100%, whereas the Ru catalysts led to dehydration and dehydrogenation products. The nitrogen doping induced the appearance of redox properties when oxygen is present in the reaction mixture. The introduction of nitrogen in the carbon framework changes the acido-basic properties of the material. The Pd catalysts on nitrogen-doped nanotubes allows reaching turnover frequencies of 600 s −1 with 100% selectivity towards acetone formation.

Gas-phase structures and thermochemistry of neutral histidine and its conjugated acid and base

Extensive exploration of the conformational space of neutral, protonated and deprotonated histidine has been conducted at the G4MP2 level. Theoretical protonation and deprotonation thermochemistry as well as heats of formation of gaseous histidine and its ionized forms have been calculated at the G4 level considering either the most stable conformers or an equilibrium population of conformers at 298 K. These theoretical results were compared to evaluated experimental determinations. Recommended proton affinity and protonation entropy deduced from these comparisons are PA(His) = 980 kJ mol(-1) and ΔpS(His) ∼ 0 J mol(-1) K(-1), thus leading to a gas-phase basicity value of GB(His) = 947.5 kJ mol(-1). Similarly, gas phase acidity parameters are ΔacidH(o)(His) = 1373 kJ mol(-1), ΔacidS(His) ∼ 10 J mol(-1) K(-1) and ΔacidG(o)(His) = 1343 kJ mol(-1). Computed G4 heats of formation values are equal to -290, 265 and -451 kJ mol(-1) for gaseous neutral histidine and its protonated and deprotonated forms, respectively. The present computational data correct, and complete, previous thermochemical parameter estimates proposed for gas-phase histidine and its acido-basic properties.

Organo-apatites for lead removal from aqueous solutions: A comparison between carboxylic acid and aminophosphonate surface modification

Organo-apatites were prepared by in situ incorporation of adipic acid, citric acid and nitrilotris(methylene)triphosphonic acid. The resulting powders consist of poorly-crystalline nanosized apatites with organic content in the 2–8wt% range. All materials exhibit faster sorption rate and higher sorption capacity for Pb2+ in acidic solutions compared to the pure apatite. This is especially true for carboxylate-modified powders that exhibit the highest-to-date maximum lead sorption capacity (≈gg−1). These properties can be correlated with the ability of introduced organic moieties to modify the acido-basic properties of the material surface and to form stable complexes with Pb2+.

Anionic RR120 dye adsorption onto raw clay: Surface properties and adsorption mechanism

Clay minerals are efficient adsorbent for dyes due to their colloidal properties. A Tunisian raw clay (Fouchana) composed of different species of clay minerals (kaolinite, smectite and minor illite) has shown efficiency for removal of anionic dye Reactive Red 120 (RR120) from aqueous solutions. In order to provide a better understanding of the role played by the different clay species in Fouchana, as well as the nature of the adsorption processes which are involved, a comparative study was conducted using standard clays, namely kaolinite KGa-2, smectite SWy-1 and illite IMt-2. Batch adsorption experiments were performed at ambient temperature and pH of the dye solutions. Acido-basic properties of the solid surfaces, electrophoretic mobility of particles, cation exchange capacity, amount of exchangeable cations and specific surface were investigated. Principal component analysis was used to highlight the influence of the physicochemical characteristics of the clay minerals in the adsorption behavior. The results show that for the conditions set in this study, kaolinite in the Fouchana raw clay plays a major role in anionic dye adsorption, mainly due to its acidic surface sites, but also to exchangeable aluminum (III) cations acting as bridges between the clay mineral and the dye molecules. Smectite and illite most likely play a secondary role by adsorption on exchangeable calcium (II) and dissociated Al (III)OH groups.

Development of a screening method for the analysis of organic pollutants in water using dual stir bar sorptive extraction-thermal desorption-gas chromatography–mass spectrometry

The development of a method for screening of organic compounds with a wide range of physico-chemical properties in water, based on dual stir bar sorptive extraction coupled with thermal desorption and gas chromatography–mass spectrometry (dual SBSE-TD-GC–MS) is described. The investigated water sample is divided into two aliquots and extracted with stir bar sorptive extraction at two different conditions: using addition of methanol or sodium chloride, respectively. Following extraction, the two stir bars are inserted into the same glass thermal desorption liner and are simultaneously desorbed and analysed by GC–MS. The method optimisation was performed using 45 environmentally harmful substances with different volatilities (boiling point from 193 to 495°C), polarity (logKow from 2.17 to 8.54) and acido-basic properties. The majority of model compounds was selected from the EU list of priority substances in the field of water policy and from the US EPA method 625, respectively. Optimisation was performed for extraction parameters (sample volume, extraction time, stirring rate, addition of modifiers) as well as for the thermal desorption conditions (desorption flow, desorption time, cryofocusing temperature). Performance characteristics (recovery, repeatability, carryover, linearity, limits of detection and quantification) were determined for the optimised method. An example of analysis of a contaminated groundwater sample is presented.

Comparison of the pure TiO 2 and kaolinite/TiO 2 composite as catalyst for CO 2 photocatalytic reduction

The kaolinite/TiO 2 composite was prepared using thermal hydrolysis of kaolinite/titanyl sulphate suspension and characterized by XRFS, XRPD, SEM and N 2 physical adsorption. Its photocatalytic properties were evaluated by photocatalytic reduction of CO 2 by water and compared with commercial TiO 2 photocatalyst Degussa P25. Results showed that the yields of CO 2 photocatalytic reduction products methane and methanol were higher over a kaolinite/TiO 2 composite than over commercial TiO 2 (Degussa P25) in spite of smaller proportion of TiO 2 in the composite. Introducing of TiO 2 nanoparticles into the kaolinite structure caused a decrease of anatase crystallite size. Kaolinite can also change acidobasic properties of catalyst surface, inhibit the recombination of electron–hole pairs and prevent the formation of TiO 2 aggregates in suspension. These facts can contribute to the observed higher photocatalytic efficiency of kaolinite/TiO 2 compared to the commercial TiO 2 photocatalyst.