Filip Sagan

Analytical aspects of achiral and cyclodextrin-mediated capillary electrophoresis of warfarin and its two main derivatives assisted by theoretical modeling

Several distinct analytical issues have been addressed by performing capillary electrophoresis-based separations of the warfarin, 7-hydroxywarfarin and 10-hydroxywarfarin in an achiral and cyclodextrin-containing media. The measurements were conducted across a range of pH in order to find optimum conditions for achiral and chiral separations. The values of acid dissociation constant (pKa) have been determined and compared. Subsequently, after performing a series of mobility shift assays at different pH and cyclodextrin concentration, the pKa values ascribed to diastereomeric complexes with methyl-β-cyclodextrin have been estimated. The significant pKa shifts upon complexation have been noticed for warfarin - up to 1.5 pH units, and only subtle for 10-hydroxywarfarin. A new approach that allows the estimation of association percentage based on the electrophoretic mobility curves has been also demonstrated. The complex mechanism of chiral separation has been found to be responsible for the observed migration profile, relying on a combined equilibrium between complexation/partition and protonation/deprotonation phenomena. The occurrence of the pKa-related migration order reversal has been demonstrated in achiral medium between warfarin and 7-hydroxywarfarin, and in chiral medium between enantiomers, causing a drop in enantioselectivity at specific pH. In parallel, the density functional theory-based calculations have been performed in order to obtain the structures of warfarin and its derivatives as well as to rationalize the shifts in pKa values.

On the Stability of Cis‐ and Trans‐2‐Butene Isomers. An Insight Based on the FAMSEC, IQA, and ETS‐NOCV Schemes

In the present account, the real space fragment attributed molecular system energy change (FAMSEC) approach, interacting quantum atoms energy decomposition scheme as well as molecular orbitals based the extended transition state scheme coupled with natural orbitals for chemical valence (ETS‐NOCV) have been, for the first time, successfully used to delineate factors of importance for stability of the 2‐butene conformers (cis‐eq, cis‐TS, trans‐eq, trans‐TS). Our results demonstrate that atoms of the controversial H–H contact in cis‐eq (i) are involved in attractive interaction dominated by the exchange‐correlation term, (ii) are weekly stabilized, (iii) show trends in several descriptors found in other typical H‐bonds, and (iv) are part of most stabilized CH–HC fragment (loc‐FAMSEC = −3.6 kcal/mol) with most favourably changed intrafragment interactions on trans‐eq→cis‐eq. Moreover, lower stability of cis‐eq vs. trans‐eq is linked with the entire HCCH (ethylenic) fragment which destabilized cis‐eq (mol‐FAMSEC, +3.9 kcal/mol) the most. Although the H–H contact can be linked with smaller, relative to trans‐, rotational energy barrier in cis‐2‐butene, we have proven that to rationalize this phenomenon one must account for changes in interactions between various fragments that constitute the entire molecule. Importantly, we discovered a number of comparable trends in fundamental properties of equivalent molecular fragments on a methyl group rotation; for example, interaction between BP‐free H‐atoms in trans‐eq (involving CH bonds of the methyl and ethylenic units) and BP‐linked H‐atoms in cis‐eq. Clearly, rotational energy barrier cannot be entirely (i) rationalized by the properties of or (ii) attributed to the H–H contact in cis‐eq.

Enthalpy–entropy relations in the acid–base equilibrium of warfarin and 10-hydroxywarfarin; joint experimental and theoretical studies

In this work we delineate basic thermodynamic factors that govern the acid–base equilibrium of the phenolic drug warfarin, pKa = 4.99, and its metabolite 10-hydroxywarfarin, pKa = 5.95. By applying experimental and theoretical approaches we have determined the enthalpic and entropic contributions to the dissociation free energy for both molecules. We have found that formation of specific intramolecular hydrogen bonds: OH⋯O by warfarin and OH⋯OH⋯O by 10-hydroxywarfarin, respectively, may be of a great importance for the changes in enthalpy and entropy during dissociation. The Car–Parrinello molecular dynamics have shown that these bonds are present at both temperatures T = 298 K and T = 378 K. We infer that an uncommon double bridge of 10-hydroxywarfarin, OH⋯OH⋯O, imposes lower flexibility on the molecule and thereby a stronger rise of heat capacity and enthalpy upon dissociation than the single OH⋯O bond noted for warfarin. It has been proposed, in addition, that the entropic contributions derive from the two opposite effects related to: solvation of ions – it leads to a loss of entropy, and breaking of intramolecular bonds – it causes a gain of entropy. For 10-hydroxywarfarin these effects seem to be equal in magnitude, and thereby, its dissociation entropy is close to zero. Furthermore, we show that addition of a surfactant and aprotic cosolvent changes the enthalpy–entropy relations and induces the upward pKa shifts of both molecules, up to 1.5 pH unit. The changes in thermodynamics which are caused by these two additives are, however, totally different.

From Saturated BN Compounds to Isoelectronic BN/CC Counterparts: An Insight from Computational Perspective

In the present study, the inorganic analogues of alkanes as well as their isoelectronic BN/CC counterparts that bridge the gap between organic and inorganic chemistry are comparatively studied on the grounds of static DFT and Car-Parrinello molecular dynamics simulations. The BN/CC butanes CH3 CH2 BH2 NH3 , BH3 CH2 NH2 CH3 , and NH3 CH2 BH2 CH3 were considered and compared with their isoelectronic counterparts NH3 BH2 NH2 BH3 and CH3 CH2 CH2 CH3 . In addition, systematical replacement of the NH2 BH2 fragment by the isoelectronic CH2 CH2 moiety is studied in the molecules H3 N(NH2 BH2 )3-m (CH2 CH2 )m BH3 (for m=0, 1, 2, or 3) and H3 N(NH2 BH2 )2-m (CH2 CH2 )m BH3 (for m=0, 1, or 2). The DFT and Car-Parrinello simulations show that the isosteres of the BN/CC butanes CH3 CH2 BH2 NH3 , BH3 CH2 NH2 CH3 , and NH3 CH2 BH2 CH3 and of larger oligomers of the type (BN)k (CC)l where k≥l are stable compounds. The BN/CC butane H3 NCH2 CH2 BH3 spontaneously produces molecular hydrogen at room temperature. The reaction, prompted by very strong dihydrogen bonding NH⋅⋅⋅HB, undergoes through the neutral, hypervalent, pentacoordinated boron dihydrogen complex RBH2 (H2 ) [R=(CH2 CH2 )n NH2 ]. The calculations suggest that such intermediate and the other BN/CC butanes CH3 CH2 BH2 NH3 , BH3 CH2 NH2 CH3 , and NH3 CH2 BH2 CH3 as well as larger BN/CC oligomers are viable experimentally. A simple recipe for the synthesis of CH3 CH2 BH2 NH3 is proposed. The strength of the dihydrogen bonding appeared to be crucial for the overall stability of the saturated BN/CC derivatives.

Quasi-aromatic Möbius Metal Chelates

We report the design as well as structural and spectroscopic characterizations of two new coordination compounds obtained from Cd(NO3)2·4H2O and polydentate ligands, benzilbis(pyridin-2-yl)methylidenehydrazone (LI) and benzilbis(acetylpyridin-2-yl)methylidenehydrazone (LII), in a mixture with two equivalents of NH4NCS in MeOH, namely [Cd(SCN)(NCS)(LI)(MeOH)] (1) and [Cd(NCS)2(LII)(MeOH)] (2). Both LI and LII are bound via two pyridyl-imine units yielding a tetradentate coordination mode giving rise to the 12 π electron chelate ring. It has been determined for the first time (qualitatively and quantitatively), using the EDDB electron population-based method, the HOMA index, and the ETS-NOCV charge and energy decomposition scheme, that the chelate ring containing CdII can be classified as a quasi-aromatic Möbius motif. Notably, using the methyl-containing ligand LII controls the exclusive presence of the NCS- connected with the CdII atom (structure 2), while applying LI allows us to simultaneously coordinate NCS- and SCN- ligands (structure 1). Both systems are stabilized mostly by hydrogen bonding, C-H···π interactions, aromatic π···π stacking, and dihydrogen C-H···H-C bonds. The optical properties have been investigated by diffused reflectance spectroscopy as well as molecular and periodic DFT/TD-DFT calculations. The DFT-based ETS-NOCV analysis as well as periodic calculations led us to conclude that the monomers which constitute the obtained chelates are extremely strongly bonded to each other, and the calculated interaction energies are found to be in the regime of strong covalent connections. Intramolecular van der Waals dispersion forces, due to the large size of LI and LII, appeared to significantly stabilize these systems as well as amplify the aromaticity phenomenon.

Origin of Remarkably Different Acidity of Hydroxycoumarins—Joint Experimental and Theoretical Studies

In the present work the origin of highly varied acidity of hydroxycoumarins (pKa values) has been for the first time investigated by joint experimental and computational studies. The structurally simple regio-isomers differing in the location of hydroxyl group, 3-hydroxycoumarin (3-HC), 4-hydroxycoumarin (4-HC), 6-hydroxycoumarin (6-HC), 7-hydroxycoumarin (7-HC), as well as 4,7-dihydroxycoumarin (4,7-HC) and the larger 4-hydroxycoumarin-based derivatives: warfarin (WAR), 7-hydroxywarfarin (W7), coumatetralyl (CT), and 10-hydroxywarfarin (W10), have been compared in terms of enthalpy-entropy relationships accounting for the observed pKa values. We have revealed that in the case of large molecules the acidic proton is stabilized by the following noncovalent interactions OH···O (WAR and W7), OH···π (CT), and OH···OH···O (W10), this effect leads to a compensatory enthalpy-entropy relation and yields a moderate pKa increase. On the other hand, different location of the hydroxyl group in the regio-isomers (3-HC, 4-HC, 6-HC, and 7-HC) leads to the massive changes in acidity due to a lack of enthalpy-entropy compensation. Our results suggest that the solvent-solute interactions and electron delocalization degree in anions contribute to the observed behaviors. Such knowledge can be useful in the future to design novel systems exhibiting desired acid-base properties, and to elucidate enthalpy-entropy compensation phenomena.

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

Capillary electrophoresis is often used to the determination of the acid-base dissociation/deprotonation constant (pKa), and the more advanced thermodynamic quantities describing this process (ΔH°, -TΔS°). Remarkably, it is commonly overlooked that due to insufficient dissipation of Joule heating the accuracy of parameters determined using a standard approach may be questionable. In this work we show an effective method allowing to enhance reliability of these parameters, and to estimate the magnitude of errors. It relies on finding a relationship between electrophoretic mobility and actual temperature, and performing pKa determination with the corrected mobility values. It has been employed to accurately examine the thermodynamics of acid-base dissociation of several amine compounds - known for their strong dependency of pKa on temperature: six cathinones (2-methylmethcathinone, 3-methylmethcathinone, 4-methylmethcathinone, α-pyrrolidinovalerophenone, methylenedioxypyrovalerone, and ephedrone); and structurally similar 1-phenylethylamine. The average pKa error caused by Joule heating noted at 25 °C was relatively small - 0.04-0.05 pH unit, however, a more significant inaccuracy was observed in the enthalpic and, in particular, entropic terms. An alternative correction method has also been proposed, simpler and faster, but not such effective in correcting ΔH°/-TΔS° terms. The corrected thermodynamic data have been interpreted with the aid of theoretical calculations, on a ground of the enthalpy-entropy relationships and the most probable structural effects accounting for them. Finally, we have demonstrated that the thermal dependencies of electrophoretic mobility, modelled during the correction procedure, may be directly used to find optimal temperature providing a maximal separation efficiency.

Cyclodextrin-induced acidity modification of substituted cathinones studied by capillary electrophoresis supported by density functional theory calculations

This paper shows that the acidity of substituted cathinones can change in a diversified and poorly predictable manner upon supramolecular interaction with cyclodextrins used as buffer additives in capillary electrophoresis. The direction and range of pKa shifts may be noticeably different for the particular cyclodextrins and cathinones, suggesting a strict correlation with structure. The most interesting results were observed for 2-hydroxyethyl-β-cyclodextrin, which is capable for inducing the large negative and enantioselective apparent pKa shifts for α-pyrrolidinovalerophenone and methylenedioxypyrovalerone, even much above -1.0 pH unit. A thermodynamic analysis was performed, to identify the role of enthalpy and entropy in the formation and deprotonation of the respective diastereomeric complexes. The former process turned out to be driven by an energetically favorable increase in entropy, related probably to a hydrophobic effect. Deprotonation enthalpy in the complexed state, in turn, occurred to be more favorable than in the free molecule state, entailing the large drop in pKa after complexation. The DFT calculations allowed us to identify some structural effects that most likely contribute to these phenomena. At last, we have demonstrated that at low cyclodextrin concentration and pH ensuring partial ionization, pKa shifts contribute to chiral separation of the abovementioned cathinones. This analytically important effect may be helpful in anticipating the most efficient chiral separation mechanism of other systems.

Complexes and salts of the nitrogen-rich triazole-tetrazole hybrid ligand with alkali and alkaline earth metal cations: experimental and theoretical findings

Reaction of the nitrogen-rich triazole–tetrazole hybrid ligand 5-(4H-1,2,4-triazol-yl)-2H-tetrazole (trz–tetH) with Li+, Na+, K+ and Ba2+ in water leads to the coordination polymers [Li(trz–tet)H2O]n, [Na(trz–tet)(H2O)2]n, [K(trz–tetH)(trz–tet)(H2O)2]n and [Ba2(trz–tet)4(H2O)9]n, exhibiting different topologies and coordination modes. Salt-like structures with anionic triazole–tetrazole building blocks [Mg(H2O)6](trz–tet)2 and [Ca(H2O)8](trz–tet)2 were also obtained. All structures were simplified by topological analysis, which showed that the structures of [Mg(H2O)6](trz–tet)2 and [Ca(H2O)8](trz–tet)2 can be considered as underlying networks, constructed from the hydrogen bonded [Mg(H2O)6]2+ or [Ca(H2O)8]2+ cations and trz–tet ligands, with the rare binodal 5,10-connected topology alb-5,10,P21/c-1 and a unique binodal 5,12-connected topology, respectively. According to TGA/DTA analysis, all compounds show endothermic mass losses below 200 °C due to dehydration processes. The dehydrated residues are stable up to 300 °C. It was also established that the anionic form of the coordinated ligand trz–tet decomposes with an abrupt mass loss accompanied by a sharp and intense exothermic effect, while the non-coordinated trz–tet, as in the structures of the Mg- and Ca-based compounds, decomposes with a more gradual mass loss and significantly broad exothermic effect. Static DFT, ab initio molecular dynamics simulations as well as charge and energy decomposition (ETS-NOCV) based studies are performed in order to shed light on the stability of the newly obtained crystals.