Oscillatory Chiral Conversion Research Articles

Chiral thin-layer chromatography in dynamic studies: A short review

Thin-layer chromatography (TLC) is a simple, cost-effective, and robust analytical technique, very useful in various different application fields like, e.g., fast screening and quality control of pharmaceuticals, phytochemicals, and alimentary products. Its versatility to a large extent owes to the seminal works of the late Professor Edward Soczewinski and, in the first instance, to the rationale implemented to liquid chromatography by the Soczewinski‒ Wachtmeister and the Snyder‒Soczewinski equations. Among the most challenging applications of TLC, one can name the enantioresolution of the racemic and scalemic mixtures and a statement that the chiral TLC in this particular respect outperforms the instrumentally more advanced chromatographic techniques is far from being an exaggeration. In the course of the past ca. 12 years, the authors of this paper have extensively investigated — mostly by means of chiral TLC — the enantioresolution of the low molecular weight carboxylic acids, to discover by sheer luck that all of them spontaneously undergo an oscillatory process, i.e., chiral conversion. Later, they collected abundant experimental evidence that the oscillatory chiral conversion is accompanied by the oscillatory condensation. Herewith, we highlight applications of the chiral TLC to demonstrate the dynamic phenomenon of the spontaneous oscillatory chiral conversion with the low molecular weight carboxylic acids.

On Spontaneously Pulsating Proline-Phenylalanine Peptide Microfibers.

Earlier, we have collected an experimental evidence showing that low molecular weight chiral carboxylic acids (amino acids included) can spontaneously undergo an oscillatory chiral conversion and an oscillatory condensation in abiotic aqueous and non-aqueous liquid systems, stored for certain amount of time under mild external conditions. These earlier findings are summarized in the introductory part of this study. In the second part, a preliminary report is given on spontaneous pulsation of peptide microfibers in the aged proline-phenylalanine (Pro-Phe) solution in 70% aqueous acetonitrile. The experimental evidence originates from a number of advanced analytical techniques. In view of our earlier and present findings, a presumption is made that the mechanism of spontaneous pulsation (formation and decay) of Pro-Phe microfibers is directly related to the oscillatory chiral conversion and oscillatory peptidization. The entity of the discussed results pointing out to spontaneous and uncontrolled instability of peptide structures might be a bad prognostic for employing such structures in nanobiotechnology.

Investigation of Spontaneous Chiral Conversion and Oscillatory Peptidization of L-Methionine by Means of TLC and HPLC

From our earlier studies, it comes out that proteinogenic amino acids undergo spontaneous oscillatory chiral conversion and condensation. An understanding of these phenomena is crucial in view of the fact that amino acids are the building blocks of proteins and peptides present in all living organisms. Moreover, amino acids play an increasingly important role as components of drugs, dietary supplements, and cosmetics. In this study, we trace spontaneous oscillatory reactions of L-methionine (L-Met) with use of thin-layer chromatography with densitometric and mass spectrometric detection, and high-performance liquid chromatography with evaporative light scattering detection. Additional measurements were carried out with use of scanning electron microscopy. The results obtained confirm that L-Met spontaneously undergoes chiral conversion and peptidization. As an outcome of these two processes, peptides are formed of considerably different chemical structures, able to self-organize in nano- and microstructures under the mild ambient conditions without any catalysts.

Thin-layer chromatographic investigation ofl-cysteine in solution

Spontaneous oscillatory chiral conversion and condensation of low-molecular-weight chiral carboxylic acids have been investigated by our research group for almost 10 years now. However, dynamics of these oscillatory processes substantially differ from one compound to another, moreover, spontaneous chiral conversion and condensation of sulfur-containing amino acids have not been investigated so far. To this effect, we present in this paper the results of our current investigations on spontaneous oscillatory chiral conversion and condensation of l-cysteine (l-Cys), a biologically important sulfur-containing semiessential amino acid. In our thin-layer chromatographic experiments, we employ the Mn(II) and Zn(II) cations to facilitate the enantioseparation of l-Cys from the spontaneously formed d-Cys, to prevent chiral conversion of the L form, and to highlight rapid consumption of Cys in the course of condensation. Spontaneous peptidization of Cys is confirmed with use of thin-layer chromatography-mass spectr...

Condensation Dynamics of the L-Pro-L-Phe and L-Hyp-L-Phe Binary Mixtures in Solution

We employ the achiral liquid chromatography with diode array, evaporative light scattering and mass spectrometric detection (HPLC-DAD, HPLC-ELSD and LC-MS) to assess structural instability (understood as spontaneous oscillatory chiral conversion and spontaneous oscillatory condensation) of the two pairs of amino acids, L-proline-L-phenylalanine (L-Pro-L-Phe) and L-hydroxyproline-L-phenylalanine (L-Hyp-L-Phe), in aqueous acetonitrile. In our earlier studies, we managed to demonstrate that single amino acids in aqueous and non-aqueous solutions undergo spontaneous oscillatory chiral conversion and oscillatory condensation. We also investigated condensation in the binary L-Pro-L-Hyp mixture in aqueous solution, and proposed a theoretical model to explain the specific dynamics of this process, which involves mutual catalytic effects of the two amino acids. In this study, we demonstrate oscillatory instability with the other two amino acid pairs in the organic-aqueous solution and reflect on the dynamics of condensation in the investigated cases. The choice of L-Pro and L-Hyp is due to their important role as building blocks of collagen, which is omnipresent in the connective tissues of mammals, and largely responsible for tissue architecture and strength. L-Phe is one of the 20 exogenous amino acids and is a building block of the majority of naturally occurring proteins.

HPLC-DAD Evidence of the Oscillatory Chiral Conversion of Phenylglycine

Previous studies by the authors have used thin-layer chromatography to demonstrate the ability of chiral low molecular weight carboxylic acids to undergo spontaneous oscillatory chiral conversion in abiotic (aqueous and nonaqueous) solutions. General mechanisms of this conversion were proposed, taking into account the enol/enolate anion intermediary species. This study provides the first experimental evidence of the spontaneous oscillatory chiral conversion of phenylglycine (Phg) with use of the chiral high-performance liquid chromatography with diode array detection. Moreover, this study provides the results of an auxiliary non-chromatographic experiment that might serve as evidence of the accumulation and relatively long lifetime of the enol/enolate anion species derived from Phg.

On ups and downs by analysis of profen drugs. A mini review

Summary Drug analysis is among the most sensitive areas of chemical analysis, simply because of its relevance to human health. Profens make the most important group of nonsteroidal anti-inflammatory drugs (NSAIDs), and in many countries some of them are freely sold over the counter. The in vitro liquid chromatographic analysis of profen drugs faces certain not commonly recognized yet acute and often impassable problems, due to the specific physicochemical properties of these compounds. In this mini review, we share our experience on the practical failures with the enantioseparation of profen drugs both by means of chiral planar and column chromatography. By use of such analytical techniques as polarimetry, 1H and 13C NMR spectroscopy, and mass spectrometry, we point out objectively the reasons that make liquid chromatographic enantioseparation of profens an almost impossible task. These compounds have an inherent ability (both in aqueous and nonaqueous solutions) to undergo oscillatory chiral conversion and oscillatory condensation, combined with a highly pronounced gelating ability. In our view, the oscillatory chiral conversion of profens questions the credibility of widespread claims on the superiority of the curative potential with the (+)-profens over that of the respective (−)-antimers (although the rightfulness of such claims cannot be dismissed, as the in vivo and in vitro behavior of many drugs are known to sometimes fundamentally differ). Moreover, it seems highly probable that, due to their highly pronounced gelating property, profen drugs can easily clog the blood vessels and thereby jeopardize living organisms.

Condensation oscillations in the condensation of mandelic acid

Summary In our earlier studies, we were the first to discover a spontaneous chiral conversion of the low-molecular-weight carboxylic acids dissolved in aqueous media, running in vitro. The investigated chiral carboxylic acids belong to the classes of profen drugs, amino acids, and hydroxy acids. Then, the spontaneous chiral conversion running in vitro and accompanied by the spontaneous condensation of the discussed compounds was discovered. From the literature, we learnt that spontaneous condensation of certain chiral compounds sometimes can be oscillatory in nature. Thus, we considered it noteworthy to check if spontaneous condensation of the chiral low-molecular-weight carboxylic acids follows a linear or a nonlinear dynamic pattern. In this paper, we present the results of our studies on the dynamics of condensation of S-, R-, and rac-mandelic acid, carried out with the aid of the high-performance liquid chromatography with the diode-array detection (HPLC-DAD), and with the aid of mass spectrometry (MS). The obtained data furnish reliable evidence that condensation of mandelic acid is oscillatory in nature. Finally, a theoretical model is recalled, which jointly describes the oscillatory chiral conversion and the oscillatory condensation with S-, R-, and rac-mandelic acid.

On simple experimental evidence confirmed by HPLC data of supramolecular aggregation and aggregate mobility withS(+)-naproxen in aqueous ethanol solution

Summary In our earlier studies performed with the use of thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), polarimetry, and several other instrumental techniques, we have provided abundant experimental evidence for the oscillatory chiral conversion and the oscillatory condensation of the low-molecular-weight carboxylic acids from the groups of profen drugs, amino acids, and hydroxy acids. We have also proposed several kinetic-diffusive models aimed at elucidation of these striking phenomena, which in the diffusive term assumed contribution from the density inhomogeneity of the investigated liquid systems. Moreover, each model laid special emphasis on the importance of cyclic H-bonded carboxylic acid homodimers of the SS and RR type. In this paper, we introduce a simple experimental approach enabling convenient registration of the density inhomogeneity in the S(+)-naproxen solution prepared in 70% aqueous ethanol. To this effect, we use a zooming scanner that is able to scan this solution in a Petri dish placed in UV light (λ = 254 nm). Scanning in the selected time intervals allows tracing the dynamics of supramolecular aggregation of the investigated profen drugs. The mobility of these aggregates can probably result from the energy released in the process of chemical polycondensation. The obtained results seem to correlate well with those originating from HPLC, and the relevant conclusions are drawn. It seems possible that the zooming scanner can prove helpful in monitoring some other interesting chemical processes as well, thus contributing to the studies on the mechanism and kinetics of the nonlinear organic reactions.

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.

Liquid Chromatographic Investigation of Spontaneous Oscillatory In Vitro Chiral Conversion and Spontaneous Oscillatory Condensation of Simple Carboxylic Acids in Aqueous and Nonaqueous Media

Oscillatory reactions are a narrow reaction type among the entity of chemical reactions and those involving purely organic compounds make a small contribution to an overall number of all known oscillatory reactions. The most abundant type is purely inorganic and mixed inorganic-organic oxidation-reduction reactions, basically because monitoring them is relatively easy (e.g., with use of potentiometric measurements). Investigation of the organic reactions can be more demanding, and then chromatography is an analytical technique of choice. In this paper, we provide an overview of chromatographic evidence with oscillatory reactions discovered in our laboratory in the course of the last several years that involve the low-molecular-weight carboxylic acids (profen drugs, amino acids, and hydroxy acids). The investigated processes comprise the oscillatory chiral conversion and the oscillatory condensation, spontaneously running in the aqueous and nonaqueous abiotic media, and they were traced with use of TLC and HPLC coupled with different detector types.

Spontaneous oscillatory in vitro chiral conversion of simple carboxylic acids and its possible mechanism

AbstractIn earlier studies, we have collected experimental evidence (mostly from thin‐layer chromatography and polarimetry) on the spontaneous oscillatory in vitro chiral conversion of simple carboxylic acids dissolved in 70% aqueous ethanol. To elucidate this phenomenon, we developed a simple theoretical model of two linked Templators. Recently, we have obtained additional experimental evidence of the spontaneous condensation of chiral carboxylic acids, based on the biuret test (amino acids), high performance liquid chromatography, and 13C NMR spectroscopy (profens and hydroxy acids). We briefly describe our experimental results in the context of the existing literature and outline an improved theoretical model for these phenomena. Our system resembles in some respects the reported oscillatory condensation of organic silanols. Here, the key reaction is the formation of carboxylic acid‐derived enols. Finally, we discuss the importance of the oscillatory chiral conversion of simple carboxylic acids for biochemistry, pharmacology, and related life sciences. Copyright © 2010 John Wiley & Sons, Ltd.

Condensation oscillations in the peptidization of phenylglycine

Abstract 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.

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.

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 α-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 -m...

On the spontaneous condensation of selected hydroxy acids

Summary In a previous study we provided thin-layer chromatographic, polarimetric, and other experimental evidence that phenylglycine can undergo easy spontaneous peptidization in abiotic aqueous media. From our unpublished results it is apparent that this behaviour is also characteristic of some other amino acids (e.g., alanine and phenylalanine). It seems highly probable that this abiotic peptidization of amino acids dissolved in aqueous media is directly linked to their ability to undergo spontaneous oscillatory chiral conversion. In our earlier research it was also shown that spontaneous oscillatory chiral conversion was characteristic not only of amino acids but also of several other classes of carboxylic acid, including profen drugs and hydroxy acids. We therefore decided to check whether selected chiral hydroxy acids — lactic acid and mandelic acid — previously recognized for their ability to undergo spontaneous oscillatory chiral conversion, could also furnish the respective polyacids. Condensation of hydroxy acids can be viewed as a reaction fully analogous with peptidization of amino acids and, hence, it seemed to us highly probable that it also can be triggered by oscillatory chiral conversion. In our study, we used thin-layer chromatography and 13C NMR spectroscopy to check whether formation of polylactic acid and polymandelic acid occurred in stored solutions of lactic and mandelic acids. By means of polarimetry with continuous recording we provided experimental evidence that all three hydroxy acids investigated (i.e. l-(+)-lactic acid, S-(+)-mandelic acid, and R-(−)-mandelic acid) undergo continuous chiral conversion. From the thin-layer chromatographic results obtained it was apparent that — similar to the spontaneous and instantaneous peptidization of amino acids — the hydroxy acids investigated also undergo easy condensation to form the respective polyacids. 13C NMR spectroscopy provided additional experimental confirmation of this.

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

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.

How to suppress the spontaneous oscillatory in-vitro chiral conversion of α-substituted propionic acids? A thin-layer chromatographic, polarimetric, and circular dichroism study of complexation of the Cu(II) cation withl-lactic acid

Summary In our earlier investigations we showed, for the first time, with numerous practical examples that α-substituted propionic acids dissolved in the low-molecular-weight solvents are able to undergo spontaneous oscillatory in-vitro chiral conversion. In this study, we focused on attempting to suppress the spontaneous oscillatory in-vitro chiral conversion of α-substituted propionic acids using, as example, l-lactic acid dissolved in water in the presence of copper(II) cations. Our intention was to check whether the coordinate covalent bonds between copper(II) and l-lactic acid ligands prevented the latter species from oscillatory chiral conversion. To do this we stored aqueous solutions of copper(II) acetate and lactic acid in the molar ratios 1:1, 1:2, and 1:3 for a long period of time. Scrutiny of possible chiral conversion of l-lactic acid was carried out by thin-layer chromatography (TLC), polarimetry, and circular dichroism (CD) spectroscopy. Seemingly contradictory results were obtained from our investigations. From the TLC data it was apparent that chelating of copper(II) cations with l-lactic acid molecules did not result in suppression of the spontaneous oscillatory in-vitro chiral conversion of the acid. It was also established that different molar proportions of copper(II) cation and l-lactic acid molecules had somewhat different effects on the dynamics of conversion. In contrast, from polarimetric and circular dichroism studies it was apparent that when l-lactic acid is dissolved in water in the presence of copper(II) cations almost no chiral conversion is observed. Hence a final conclusion was drawn that chelating of copper(II) cations with l-lactic acid stabilizes the chiral structure of the acid in solution. Intermolecular interactions between the copper(II)-l-lactic acid complex and the silica gel stationary phase evidently affects the structure of the complex, however, most probably resulting in partial “liberation” of l-lactic acid ligands. Thus the chiral structure-stabilizing effect of copper(II) cations is apparently weakened by the TLC system and the freed l-lactic acid molecules can undergo chiral conversion.

On the spontaneous abiotic peptization of phenylglycine in an aqueous medium

Summary In this report we provide thin-layer chromatographic evidence that phenylglycine dissolved in 70% aqueous ethanol and kept at ambient temperature (22°C) undergoes spontaneous peptization, as additionally confirmed by use of the biuret test. It was also shown that an important precondition for instantaneous peptization of phenylglycine is the simultaneous presence of the R and S antimers in solution, and the most spectacular peptization effect is obtained with racemic R,S-phenylglycine. An assumption is made that polycondensation of phenylglycine results from its ability to undergo spontaneous oscillatory chiral conversion and can be regarded as a step following enolization, and competitive with chiral conversion.

Investigation of the spontaneous oscillatory in-vitro chiral conversion ofl-(+)-lactic acid

Summary In previous studies we focused our attention on the in-vitro oscillatory chiral conversion of selected profens and amino acids when dissolved in low-molecular-weight solvents. The results obtained encouraged us to check other propionic acid derivatives of biological importance and we therefore decided to investigate the tendency of l-(+)-lactic acid to undergo an analogous process of spontaneous oscillatory in-vitro chiral conversion. As far as we are aware, this type of investigation on lactic acid has not previously been conducted. In our studies, we used solutions of l-(+)-lactic acid in three different mixed solvents, ethanol-water, ethanol-glacial acetic acid, and ethanol-basic buffer (pH 9), in the volume proportion 7:3. The investigations were conducted by thinlayer chromatography (TLC) and polarimetry. The results obtained by TLC gave evidence of the spontaneous oscillatory chiral conversion of l-(+)-lactic acid solution. From TLC results alone it is hardly possible to judge whether this p...