Kinetic Evidence for Transiently Shifted Acidity Constant of Histidine Linked to Paramagnetic Tyrosine Probed by Intramolecular Electron Transfer in Oxidized Peptides

Kinetics of electron transfer (ET) from tyrosine (Tyr) to short-lived histidine (His) radicals in peptides of different structures was monitored using time-resolved chemically induced dynamic nuclear polarization (CIDNP) to follow...

Metals speciation in Arctic lake under pollution impact (1980-2019): in-situ measurements, experimental estimates, model calculations

<p>An important fundamental block in the geochemical studies is the evaluation of the equilibrium distribution of metals in water and the influence of environmental conditions on the spetiation. It is important to understand the difference between the behavior of nanoparticles, dissolved particles, colloid particles, and suspended particles. The research deals with study and assess of geochemical processes of metal speciation in Arctic lake in the zone of metallurgical waste and other areas, where natural processes prevail. Consecutive and parallel membrane filtration methods were used to compare of the results of water analysis in the Imandra lake. The membrane pore sizes were: 8 µm, 1.2 µm, 0.45 µm, 0.2 µm. The following filterates characteristics were used: (microfiltration-based) mechanical suspension and oxidized contaminants (>8 μm, 1.2 μm, 0.45 μm, 0.2 μm, 0.1 μm); and (ultrafiltration-based) colloid, bacteria, viruses, etc (less than 0.1 μm).</p><p>Industrial effluents lead to the formation of higher concentrations of elements (Ni, Cu, Pb) in their labile forms as were found. In the wastewater-mixing zone, the concentrations of most elements are evenly distributed in depth. In areas that are more distant, there was a significant increase in the concentration of elements in the near-bottom horizon in comparison with the surface waters (Fe by more than 3 times). The obtained results showed that numerous elements had diverse distribution by speciation in the point located closer to the source of wastewaters. This indicates a significant influence of adsorption process on the system balance by such elements as Fe, Cu, and rare earth elements.</p><p>The impact of the regional geochemical and anthropogenic speciation and the possible influence of the climatic factor on the metals speciation were showed.   The authors did not have data on the metals speciation in the chosen points for the whole period of monitoring from 1980 until present. However, ElementPhasMigration (certificate 2017662509, Dinu M.I.) software was used to calculate the shares of labile and non-labile metal speciation during the years of the highest pollution (beginning of the 1990s) and during the current period of the ecosystem restoration.    </p><p>The software used mathematic modeling of chemical reactions happening in the natural waters and was based on the main laws of analytical and physical chemistry: material balance equation, equilibrium constant, equations of electrical neutrality, equations of proton balance, and competing reactions.</p><p>On the one hand, the initial data comprised a significant number of physicochemical parameters of the environment (more than 10 metal ions, рН, content of organic and non-organic anions, etc.). On the other hand, it included diverse mathematical tools for consecutive calculation of acidity constant of organic acids, conditional constants of complexes stability, the share of strong and weak acids in the system, etc. The software solved the tasks on the evaluation of the metal speciation depending on the physical and chemical parameters of the environment and provided the data on the balance speciation of a wide spectrum of elements in the system. The final stage of the calculations included the results verification with the field data.      Financing RFS 18-77-00018</p>

Acidity constant and DFT-based modelling of pH-responsive alendronate loading and releasing on propylamine modified silica surface

A computational methodology that couples the acidity (Ka) and density functional theory (DFT) calculations has been developed to explain the pH-dependent drug loading and releasing by mesoporous silica nanoparticles. The...

USING POTENTIOMETRIC ACID-BASE TITRATION TO DETERMINE PKA FROM MANGOSTEEN PERICARPS EXTRACT

One of the uses of acid-base indicators is to show the end point of the titration, so the accurate determination of acidity constant and pH range of indicators needs to be done. This study aims to determine the acidity constant (pKa) of mangosteen pericarp extract and its accuracy as an indicator of acid-base titration. Determination of pKa was done by a simple potentiometric titration method. The titration data were plotted in three graphs, i.e., pH, ΔpH/ΔV (the (the first derivative), and Δ2pH/ΔV2 (the second derivative) versus titrant volume to determine the equivalence point of the titration. The accuracy test was carried out by comparing the volume of oxalic acid used to titrate NaOH solution using the indicator of mangosteen pericarp extract and phenolphthalein indicator. The result showed that the equivalence point was found on the titrant volume of 8.6 mL and a measured pH of 9.84. so the pKa value of mangosteen pericarp extract was 7.20, and the pH range was 6.20 to 8.20. the average volume of oxalic acid used to titrate 5 mL of NaOH using phenolphthalein as the indicator was 5.2 mL while the titration used mangosteen pericarp extract was 5.23 mL. The accuracy of mangosteen pericarp extract was 99.42%. By the result, it can be concluded that potentiometric titration can be used as a simple way to determine the acidity constant of mangosteen pericarp extract. Moreover, the mangosteen pericarp extract can be used as an alternative acid-base titration indicator to substitute the common acid-base titration in the laboratory.

Structure and Properties of 1,3-Phenylenediboronic Acid: Combined Experimental and Theoretical Investigations

The structure and properties of 1,3-phenylenediboronic acid are reported. Molecular and crystal structures were determined by single crystal as well as by powder X-ray diffraction methods. Acidity constant, thermal behavior, and NMR characterization of the title compound were also investigated. In addition to the experimental data, calculations of rotational barrier and intermolecular interaction energies were performed. The compound reveals a two-step acid–base equilibrium with different pKa values. TGA and DSC measurements show a typical dehydration reaction with formation of boroxine. In crystals, hydrogen-bonded dimers with syn-anti conformation of hydroxyl groups form large numbers of ribbon motifs. The 2D potential energy surface scan of rotation of two boronic groups with respect to phenyl ring reveals that the rotation barrier is close to 37 kJ⋅mol−1, which is higher than the double value for the rotation of the boronic group in phenylboronic acid. This effect was ascribed to intermolecular interaction with C–H hydrogen atom located between boronic groups. Furthermore, the molecules in the crystal lattice adopt a less stable molecular conformation most likely resulting from intermolecular forces. These were further investigated by periodic DFT calculations supported by an estimation of dimer interaction energy, and also by topological analysis of electron density in the framework of AIM theory.

Molecular Topology and Other Promiscuity Determinants as Predictors of Therapeutic Class - A Theoretical Framework to Guide Drug Repositioning?

Much interest has been paid in the last decade on molecular predictors of promiscuity, including molecular weight, log P, molecular complexity, acidity constant and molecular topology, with correlations between promiscuity and those descriptors seemingly being context-dependent. It has been observed that certain therapeutic categories (e.g. mood disorders therapies) display a tendency to include multi-target agents (i.e. selective non-selectivity). Numerous QSAR models based on topological descriptors suggest that the topology of a given drug could be used to infer its therapeutic applications. Here, we have used descriptive statistics to explore the distribution of molecular topology descriptors and other promiscuity predictors across different therapeutic categories. Working with the publicly available ChEMBL database and 14 molecular descriptors, both hierarchical and non-hierchical clustering methods were applied to the descriptors mean values of the therapeutic categories after the refinement of the database (770 drugs grouped into 34 therapeutic categories). On the other hand, another publicly available database (repoDB) was used to retrieve cases of clinically-approved drug repositioning examples that could be classified into the therapeutic categories considered by the aforementioned clusters (111 cases), and the correspondence between the two studies was evaluated. Interestingly, a 3- cluster hierarchical clustering scheme based on only 14 molecular descriptors linked to promiscuity seem to explain up to 82.9% of approved cases of drug repurposing retrieved of repoDB. Therapeutic categories seem to display distinctive molecular patterns, which could be used as a basis for drug screening and drug design campaigns, and to unveil drug repurposing opportunities between particular therapeutic categories.