Thermodynamics of acid-base dissociation of several cathinones and 1‐phenylethylamine, studied by an accurate capillary electrophoresis method free from the Joule heating impact

Cyclodextrin-Enabled Enantioselective Complexation Study of Cathinone Analogs

The characteristic alkaloid component of the leaves of the catnip shrub (Catha edulis) is cathinone, and its synthetic analogs form a major group of recreational drugs. Cathinone derivatives are chiral compounds. In the literature, several chiral methods using cyclodextrins (CDs) have been achieved so far for diverse sets of analogs; however, a comprehensive investigation of the stability of their CD complexes has not been performed yet. To characterize the enantioselective complex formation, a systematic experimental design was developed in which a total number of 40 neutral, positively, and negatively charged CD derivatives were screened by affinity capillary electrophoresis and compared according to their cavity size, substituent type, and location. The functional groups responsible for the favorable interactions were identified in the case of para-substituted cathinone analog mephedrone, flephedrone, and 4-methylethcathinone (4-MEC) and in the case of 3,4-methylendioxy derivative butylone and methylenedioxypyrovalerone (MDPV). The succinylated-β-CD and subetadex exhibited the highest complex stabilities among the studied drugs. The complex stoichiometry was determined using the Job's plot method, and the complex structures were further studied using ROESY NMR measurements. The results of our enantioselective complex formation study can facilitate chiral method development and may lead to evaluate potential CD-based antidotes for cathinone analogs.

The Acid-Base/Deprotonation Equilibrium Can Be Studied with a MicroScale Thermophoresis (MST)

MicroScale thermophoresis (MST) is a rapidly developing bioanalytical technique used routinely for the examination of ligand-target affinity. It has never been used so far for the analysis of acid-base dissociation and the determination of pK_a constant. This work is the-proof-of-concept of this new idea. It demonstrates that the pK_a values obtained from the thermophoretic data are consistent with the reference methods. As a result, the analytical potential and utility of the MST technology can become even greater, especially if the new detection system of thermophoretic movement will be developed in the future. Even now, taking into account the necessity to use fluorescence, the proposed method may be useful in many respects.

Determination of the aqueous pKa of very insoluble drugs by capillary electrophoresis: Internal standards for methanol-water extrapolation

A fast determination of acidity constants (pK_a) of very insoluble drugs has become a necessity in drug discovery process because it often produces molecules that are highly lipophilic and sparingly soluble in water. In this work the high throughput internal standard capillary electrophoresis (IS-CE) method has been adapted to the determination of pK_a of water insoluble compounds by measurement in methanol/aqueous buffer mixtures. For this purpose, the reference pK_a values for a set of 46 acid-base compounds of varied structure (internal standards) have been established in methanol-water mixtures at several solvent composition levels (with a maximum of 40% methanol). The IS-CE method has been successfully applied to seven test drugs of different chemical nature with intrinsic solubilities lower than 10^-6 M. pK_a values have been determined at different methanol/aqueous buffer compositions and afterwards Yasuda-Shedlovsky extrapolation method has been applied to obtain the aqueous pK_a. The obtained results have successfully been compared to literature ones obtained by other methods. It is concluded that the IS-CE method allows the determination of aqueous pK_a values using low proportions of methanol, becoming then more accurate in the extrapolation procedure than other reference methods.

Differentiation of isomeric metabolites of carbamazepine based on acid-base properties; Experimental vs theoretical approach

Metabolism of carbamazepine is complex and leads to the three isomeric derivatives whose occurrence is dependent on the type of sample material. Their unambiguous differentiation is overall important. In this work, the qualitative analysis of 2-hydroxycarbamazepine, 3-hydroxycrbamazepine and carbamazepine-10,11-epoxide was attempted for the first time using capillary zone electrophoresis, based on the models linking electrophoretic mobility with pK_a value determining the acidity of the hydroxyl groups. For this purpose, pK_a values ​​were determined using electrophoretic and theoretical methods, and then the compliance of the obtained mobility models with the measured values ​​was analyzed. Despite the slight difference in acidity ​​(0.3-0.4 pH unit), the obtained results prove that the correct identification of the metabolites under consideration, and reliable prediction of the selectivity of their separation, are possible on the basis of experimentally determined pK_a values, even with highly simplified methods assuming the lack of certain data. However, it is important to choose the optimal pH value, which should be close to pK_a. On the other hand, worse results were obtained for the theoretically determined mobilities, which differed significantly from the experimental values. An attempt was also made to explain the acidity of hydroxycarbamazepines and the associated thermodynamic parameters - deprotonation enthalpy and entropy, with respect to their structure. The lack of intramolecular hydrogen bonds and the significant contribution of entropic effects stabilizing the protonated form seems to be significant. The higher pK_a value for CBZ-2-OH probably results from the stronger effect of the energetically unfavorable organization of solvent dipoles due to ionization.

Determination of acidity constants at 37 °C through the internal standard capillary electrophoresis (IS-CE) method: internal standards and application to polyprotic drugs

This work provides the pKa at the biorelevant temperature of 37 °C for a set of compounds proposed as internal standards for the internal standard capillary electrophoresis (IS-CE) method. This is a high throughput method that allows the determination of the acidity constants of compounds in a short time, avoiding the exact measurement of the pH of the buffers used. pH electrode calibration at 37 °C can be avoided too. In order to anchor the pKa values obtained through the IS-CE method in the pH scale, the acidity constant at 37 °C of some of the standards has been determined also by the reference potentiometric method. In general, a decrease in the pKa value is observed when changing the temperature from 25 to 37 °C, and the magnitude of the change depends on the nature of the compounds. Once the pKa values at 37 °C of the internal standards have been established, the method is applied to the determination of the acidity constants of seven polyprotic (5 diprotic and 2 triprotic) drugs. The obtained mobility-pH profiles show well-defined curves, and the fits provide precise pKa values. Due to the lack of reference data at 37 °C only the pKa values of labetalol can be compared to values from the literature, and a very good agreement is observed.

Study on the shift of ultraviolet spectra in aqueous solution with variations of the solution concentration

In this study, we mainly focused on predictable regularities of the red shift of ultraviolet spectra for β-phenylethylamine (PEA), NaCl and NaOH in aqueous solution. The absorption peaks of the UV spectra near 191 nm of NaCl, NaOH and PEA in aqueous solution moved in the direction of a red shift while the molar absorption coefficient at the peak increased regularly with the increasing solution concentration. These shifts were obtained for solutions with concentrations ranging from 3.68 to 1000 mmol/L for NaCl, from 0.762 to 2000 mmol/L for NaOH, and from 0.0515 to 8.91 mmol/L for PEA. The plots of the logarithm of the solution concentration for NaCl and PEA versus the absorbance at 191 nm and at the peak were linear, and the plots of the logarithm of the solution concentration for NaCl and PEA versus the wavelength at the peak (shifted from 191 nm) were also linear. In addition, the plots of the logarithm of the solution concentration for NaOH that ranged from 0.762 to 1.96 mmol/L versus the absorbance at 191 nm and at the peak were linear as well as the plots of the logarithm of the solution concentration for NaOH that ranged from 1.96 to 2000 mmol/L versus the wavelength at the peak. The slopes of the absorbance at 191 nm of PEA, NaCl and NaOH were somewhat similar to the absorbance at the peak separately, whereas the slopes of the wavelengths at the peak were different from them. Finally, in order to obtain the predictable regularity of the red shift of the UV spectrum for the mixture, 22 ternary mixtures were prepared. The results indicate that the inhibiting effect of hydroxide ions (OH-) caused the wavelength near 206 nm to remain unchanged when the solution concentration of NaOH in the mixture was more than 0.762 mmol/L.

Interpol review of controlled substances 2016-2019

This review paper covers the forensic-relevant literature in controlled substances from 2016 to 2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/content/download/14458/file/Interpol%20Review%20Papers%202019.pdf.

Acidity constant of pH indicators in the supramolecular systems studied by two CE-based methods compared using the RGB additive color model

Acid-base properties of methyl orange, bromocresol green, bromophenol blue, and bromothymol blue were thoroughly investigated in the past due to their application as colorimetric pH indicators. However, it is still unknown how these properties change upon the supramolecular host-guest interactions. Owing to the growing interest in using supramolecular host-guest interactions to reach expected modification of various physicochemical properties of guests, we decided to address this question in the present article. We estimated the shifts of pK_a values induced by diverse hosts (cyclodextrins, cucurbiturils, calixarenes, micelles, and serum albumin) and performed a thermodynamic analysis of the selected systems. To make a deeper insight, we confronted the aforementioned dyes with the other kinds of molecules studied by us in the past. In overall, the results obtained demonstrate a large multiplicity of possible pK_a behaviors, their poor predictability, and the existence of subtle structure-acidity relationships. In addition, we observed three thermodynamically different mechanisms of pK_a alteration. Therefore, more studies are needed to bring closer the promising perspective of a programmable acidity's tuning. Our methodology was based on capillary electrophoresis (CE) applied in two parallel variants: a classical method based on the fitting of a nonlinear function, and an alternative two-value method (TVM), which requires over twice less measurements to estimate pK_a. To identify the optimal approach for further studies, both methods were comprehensively compared and discussed based on the RGB additive color model, a user-friendly scale that integrates three primary aspects of an analytical method: analytical performance, green chemistry, and practicality.

Heat dissipation in slab gel electrophoresis: The effect of embedded TiO2 nanoparticles on the thermal profiles

Despite the fast development of novel and high-resolution electrophoresis techniques such as capillary-based methods and microfluidic devices, the slab gel electrophoresis is still a popular method for the separation of biomolecules in medicine and biology. It is a low cost and simple method and offers high throughput. However, this technique is limited to low voltages leading to slow separations. Producing the heat during the electrophoresis known as Joule heating inevitably leads to a rise in the gel temperature. For the first time, this work offers a whole gel temperature measurement by using a thermal camera which presents accurate temperature profiles in the gel with a resolution of more than 10 pixel/mm² and a precision of 0.1 °C. Titania, TiO₂, nanoparticles (NPs) were embedded into the polyacrylamide (PA) gel to improve the electrophoretic separation of proteins. By embedding 0.025% w/v TiO₂ NPs, heat dissipation increases by 16.5% at applied voltage of 200 V compared with that of PA gel with no embedded TiO₂ NPs. The thermal images showed that the composite gel was 2.5 °C in average cooler than PA gel after 15 min of electrophoresis run at 200 V. The maximum separation voltage increased by 30 V in the composite PA/TiO₂ gel compared with the pure PA gel. Moreover, the average number of theoretical plates over the 10 protein peaks, as a criterion of separation performance, increased by about 63% at 180 V when TiO₂ NPs were included into the gel.