Dorota Kronenbach

Experimental and Model Investigation of the Oscillatory Transenantiomerization of L-α-Phenylalanine

Abstract In an earlier study, we obtained experimental evidence of the oscillatory transenantiomerization of selected profen drugs (e.g., S-(+)-ibuprofen, S-(+)-naproxen, and S-(+) and R-(−)-flurbiprofen) dissolved in aqueous, aqueous-organic, and purely organic liquid media. This process was apparently catalyzed by basic or amphiprotic environments and involved keto-enol tautomerism, and the self-organization of molecules in the solution via association of the carboxylic functional group of profens through hydrogen bonding to form mixed H-bonded associates with the remaining constituents of the solution. A model of the oscillatory transenantiomerization of profens was also developed by adapting an earlier oscillatory model, the Templator. Our new model comprises two linked Templators. The essence of the Templator model adapted to the oscillatory transenantiomerization of profens is the assumption that the H-bonded profen homodimer acts as a template, able to generate new dimers having the same steric configuration as their respective monomeric units. As profens belong to the class of 2-arylpropionic acids (2-APAs), we concluded that the phenomenon of oscillatory transenantiomerization may occur in other 2-APAs as well, among them those amino acids whose molecular structure can formally be derived from propionic acid. Thus, in this study, we focus our attention on L-α-phenylalanine (LPA; one of the nine amino acids essential for humans). Using thin layer chromatography (TLC) and polarimetry, we demonstrate the ability of LPA to undergo oscillatory transenantiomerization analogous to that observed with profens. The self-organization of molecules in a 70% ethanol solution of LPA is confirmed with photographs taken in UV light (λ = 254 nm). Finally, we propose a skeleton molecular mechanism for the transenantiomerization of LPA and simulate the oscillatory interconversion of its L and D forms with two linked Templators.

TLC in a search for structural limitations of spontaneous oscillatory in-vitro chiral conversion. α-hydroxybutyric and mandelic acids

As a result of our earlier studies, we were the first research group to report the spontaneous oscillatory in-vitro chiral conversion of a considerable number of α-substituted propionic acids (i.e., selected profens, amino acids, and l-lactic acid). It is noteworthy that TLC proved highly instrumental in collecting relevant confirmatory evidence on this issue. Besides, in two papers we introduced a theoretical model and with its aid, we could simulate basic physical and physicochemical features of the discussed process. Reflecting on the oscillatory chiral conversion, we realized that it seems to be a rather general phenomenon and we started contemplating its scope and limitations. It became our primary concern to check whether it is confined to a-substituted propionic acids only or can occur with other classes of chiral compounds also. To this effect, in this study we present empirical (thin-layer chromatographic and polarimetric) evidence on an ability of R and S-α-hydroxybutyric acids, and R and S-mandelic acids to undergo spontaneous oscillatory in-vitro chiral conversion when dissolved in 70% aqueous ethanol. From the obtained results, general conclusion can be drawn that the process of interest occurs not only with chiral carboxylic acids having three carbon atoms per molecule, but also with those having two and four carbon atoms.

TLC and polarimetric investigation of the oscillatory in-vitro chiral inversion of l -alanine

In recent TLC and polarimetric studies we showed, for the first time, the tendency of profen drugs to undergo spontaneous oscillatory chiral inversion in vitro. Because profen drugs are chiral propionic acid derivatives, we sought other chiral compounds with similar chemical structures and hence a similar tendency to spontaneously change their steric configuration. We have previously demonstrated that L-α-phenylalanine and L-tyrosine also undergo oscillatory in-vitro chiral inversion. We also demonstrated the effect of temperature and mixing on the process, and proposed a detailed model of the oscillator, providing the basis for mechanistic understanding of the oscillatory chiral inversion of these compounds. In this study we focused our attention on L-alanine, another amino acid with a chemical structure formally derived from propionic acid, and, again, by use of TLC and polarimetry, investigated the tendency of L-alanine to undergo oscillatory in-vitro chiral inversion when dissolved in neutral, acidic, and basic solvents. We also studied the effect of temperature and sample mixing. It was confirmed that Lalanine also undergoes chiral inversion. The dynamics of the process are specific to each compound.

Experimental Investigation of the Oscillatory Transenantiomerization of L-Tyrosine

Abstract In our earlier studies, we demonstrated an ability of selected enantiomeric profen drugs (e.g., S-(+)-ibuprofen, S-(+)-naproxen, and S-(+)- and R-(−)-flurbiprofen) and one amino acid (i.e., L-α-phenylalanine) to undergo oscillatory transenantiomerization when dissolved in simple, low molecular weight solvents (e.g., water, ethanol, dichloromethane, acetonitrile, etc.) and stored for a longer period of time at ambient temperature or in a refrigerator. Experimental evidence of this process originates from a number of analytical techniques, with thin layer chromatography (TLC) and polarimetry among the best performing ones. There are two common structural features of all these compounds, namely: (i) they are 2-arylpropionic acids (2-APAs), and (ii) their chirality center is located on the α-carbon atom of the respective molecules. It has also been established that the basic and the amphiprotic environment catalyzes the oscillatory transenantiomerization of the investigated compounds, while the acidic environment tends to hamper this process. Moreover, it has been established that all the aforementioned compounds can organize molecules present in the solution in such a manner as to produce the density anisotropy of the liquids considered. Model explanation of the oscillatory transenantiomerization of profens and L-α -phenylalanine was also developed as a starting point, adapting an earlier established oscillator known as Templator. The new model comprises two linked Templators. The quintessence of the Templator model adapted to the demands of the oscillatory transenantiomerization of profens and amino acids was based on an assumption that the H-bonded 2-APA dimer is a template, able to generate the new dimers having the same steric configuration of their respective monomeric units. From our earlier studies, it clearly comes out that in spite of common traits of the oscillatory transenantiomerization of the selected profens and L-α-phenylalanine, the dynamics of this process can significantly differ from one compound to another, due to their differentiated molecular structure and, hence, to the different electron density distribution. Thus, in this study, we investigated the ability of L-tyrosine (another 2-APA and the amino acid regarded as essential for the humans) to undergo oscillatory transenantiomerization. Solubility of L-tyrosine in the amphiprotic binary mixture (70{%} aqueous ethanol solution) widely used in our earlier studies proved too low to use it as a solvent in the present investigation. Instead, we traced the behavior of L-tyrosine when stored for over one week in the following mixed solvents: ethanol–1M NaOH (7:3, v/v) and ethanol-1M HCl (7:3, v/v). The results of our experiments clearly confirm the ability of L-tyrosine to undergo the oscillatory transenantiomerization, similar to that of the previously studied profens and L-α-phenylalanine, although the individual dynamics of the oscillatory transenantiomerization with this particular enantiomer is also evident and discussed. It is apparent that the model of the two linked Templators applies to L-tyrosine as well, as an adequate explanation of the mechanism of its oscillatory transenantiomerization.

Condensation oscillations in the peptidization of phenylglycine

In earlier studies, we showed that certain low-molecular-weight carboxylic acids (profens, amino acids, hydroxy acids) can undergo spontaneous in vitro chiral conversion accompanied by condensation to from oligomers, and we proposed two simple models to describe these processes. Here, we present the results of investigations using non-chiral high-performance liquid chromatography with diode array detector (HPLC-DAD) and mass spectrometry (MS) on the dynamics of peptidization of S-, R-, and rac-phenylglycine dissolved in 70% aqueous ethanol and stored for times up to one year. The experimental results demonstrate that peptidization of phenylglycine can occur in an oscillatory fashion. We also describe, and carry out simulations with, three models that capture key aspects of the oscillatory condensation and chiral conversion processes.

In vitro Chiral Conversion, Phase Separation, and Wave Propagation in Aged Profen Solutions

Abstract We have shown, earlier, that profens, i.e., 2-arylpropionic acids can undergo oscillatory in vitro chiral conversion, exhibiting oscillatory changes in the chromatographic retention parameter (R F) and the specific rotation ([α]D) for periods of up to two weeks after preparation of the solutions in aqueous ethanol. Here, we examine the oscillatory chiral conversion of S-(+)-ibuprofen and S,R-(±)-ketoprofen dissolved in 70% aqueous ethanol and stored in tightly closed colorless glass vials for one year. During that time, each initially homogenous, transparent profen solution separates into two immiscible layers, one clear and the other turbid. Employing chiral thin layer chromatography, high performance liquid chromatography with diode-array detection, gas chromatography, and Raman spectroscopy, we find that both samples undergo chiral conversion to yield a strong predominance of the R-(−) antimer. Additionally, microscopy reveals characteristic spatial patterns in both layers of the demixed ibuprofen and ketoprofen solutions. We use a model proposed earlier for the homogeneous oscillatory interconversion of the S and R profens involving two linked templators to simulate microscopic spatial patterns analogous to those observed in the solutions investigated here.

TLC AND POLARIMETRIC INVESTIGATION OF THE OSCILLATORY IN VITRO CHIRAL CONVERSION OF R-β-HYDROXYBUTYRIC ACID

Earlier we have shown that chiral aliphatic hydroxy acids, i.e., L-lactic acid, R-α-hydroxybutyric acid, and S-α-hydroxybutyric acid, can undergo oscillatory in vitro chiral conversion, exhibiting oscillatory changes in the chromatographic retention parameter (R F ) and the specific rotation ([α] D ) for the periods of two or even many more weeks after preparation of the solutions in aqueous ethanol. As the phenomenon of spontaneous oscillatory chiral conversion with low molecular weight carboxylic acids dissolved in the abiotic aqueous media seems to be of a relatively general nature, it made us curious to examine steric limitations of this process. One essential question that we posed in this study regards the impact of the distance between the carboxylic group of the acid and the asymmetry center in the chiral molecule on its ability to undergo chiral conversion. With the earlier examined lactic acid and two α-hydroxybutyric acids, the hydroxyl group was placed in position α, hence the asymmetry center was directly neighboring the carboxylic group. It was the aim of this study, to examine one more low molecular weight hydroxy acid with the hydroxyl group placed in position β, hence with the asymmetry center separated by one methylene unit from carboxyl functionality acting as spacer. To this effect, we selected R-β-hydroxybutyric acid and investigated its ability to undergo a spontaneous oscillatory in vitro chiral conversion in the abiotic aqueous medium, for this purpose using thin layer chromatography and polarimetry as the best suiting analytical techniques. It was experimentally established that R-β-hydroxybutyric acid – similar to the earlier scrutinized α-hydroxy acids – can undergo the oscillatory chiral conversion and the methylene spacer between the carboxyl and the hydroxyl functionality cannot prevent this particular compound from undergoing steric conversion.

Oligomerization oscillations of L-lactic acid in solution.

We employ high-performance liquid chromatography with diode array, evaporative light scattering, and mass spectrometric detection to monitor the oligomerization of L-lactic acid in pure acetonitrile and in 70% aqueous ethanol. The production of higher oligomers appears to proceed in an oscillatory fashion. A model is presented that involves the formation of aggregates (micelles), which catalyze the oligomerization.

Thin-Layer Chromatographic and Polarimetric Investigation of the Oscillatory In-Vitro Chiral Inversion of S-(+)-Ketoprofen

Our earlier thin-layer chromatographic and polarimetric investigations enabled discovery of the spontaneous in-vitro oscillatory chiral inversion of the profen drugs S-(+)-ibuprofen, S-(+)-naproxen, and S-(+) and R-(–)-flurbiprofen, etc., and then of the α-amino acids l-phenylalanine, l-alanine, and l-tyrosine. In those investigations, thin-layer chromatography convincingly demonstrated its potential as a flexible and handy tool in the service of physical organic chemistry in general and investigation of organic reaction mechanisms in particular. Later — largely on the basis of thin-layer chromatographic evidence — we proposed a reaction-diffusion model that may provide the core of a mechanistic understanding of the spontaneous oscillatory in-vitro chiral inversion of profens and α-amino acids. In this study, we present thin-layer chromatographic and polarimetric evidence of the analogous process of the oscillatory chiral in-vitro inversion of S-(+)-ketoprofen, which is meant to expand an already existing database, mostly originating from our laboratory and documenting the universal nature of this process with α-substituted chiral propionic acid derivatives (in the first instance, profen drugs and α-amino acids).

An HPLC–DAD and LC–MS Study of Condensation Oscillations with S(+)-Ketoprofen Dissolved in Acetonitrile

In our earlier studies, a spontaneous chiral conversion of the selected low-molecular-weight carboxylic acids (i.e., amino acids, hydroxy acids, and profen drugs) dissolved in aqueous ethanol medium, running in vitro was described. Then it became clear that this spontaneous chiral conversion is accompanied by the spontaneous condensation of the discussed compounds. With several acids, it was established that this condensation is also oscillatory in nature. The theoretical models were developed aiming to give a rough explanation of the observed non-linear processes. In this paper, the results of these studies on the dynamics of condensation with S(+)-ketoprofen, a very popular profen drug, when stored for certain amount of time dissolved in a non-aqueous medium (i.e., acetonitrile) is presented. These investigations were carried out with the aid of two independent high-performance liquid chromatographic systems with the diode array detection and of a third high-performance liquid chromatographic system equipped with mass spectrometric detection. In one cycle of chromatographic measurements, it was possible to monitor condensation of S(+)-ketoprofen in 25-min intervals for 30 h, thus obtaining kinetic information on the progress of this process. Mass spectrometric detection confirmed the presence of new species in the stored solution with molecular weights much higher than that of S(+)-ketoprofen, which can be attributed to the condensation products. The obtained data show that condensation of S(+)-ketoprofen dissolved in acetonitrile progresses in a rapid manner, and that the observed oscillatory concentration changes with S(+)-ketoprofen and with the main condensation product characterize with an irregularity and shallow amplitudes. A theoretical model was referenced that jointly describes the oscillatory chiral conversion and the oscillatory condensation with the low-molecular-weight chiral carboxylic acids.