Authors:Mieczysław Sajewicz, Dorota Kronenbach, Monika Gontarska, and Teresa Kowalska
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 l
-alanine also undergoes chiral inversion. The dynamics of the process are specific to each compound.
Authors:Agnieszka Pieniak, Mieczysław Sajewicz, Krzysztof Kaczmarski, and Teresa Kowalska
The aim of the work discussed in this paper was to re-investigate the initial stage of the retention process in TLC, namely the moment of first contact between the analyte deposited at the origin and the mobile phase employed. It was our intention to assess the impact of dissolution of the analyte by the mobile phase on the overall mobility of the analyte in the chromatographic system. To accomplish this goal parallel experiments were performed with analyte samples applied to the stationary phase and chromatographed with and without drying (i.e. with and without evaporation of the solvent used to dissolve the analyte). As test analytes we selected two compounds (5-phenylpentanol and 2-phenylbutyric acid) able to participate in lateral interactions by hydrogen bonding and another four compounds (anthracene, phenanthrene, perylene, and tetralin) totally unable to self-associate. The results obtained are presented in two different ways, i.e. as the dependence of densitometrically measured concentration profiles and of retardation factors (
) on the amount of analyte and on the mode of application (dry or wet) to the stationary phase layer. It was discovered that for both types of analyte (i.e. participating and not participating in lateral interactions) the process of dissolution at the origin is time-consuming and thus contributes to the overall retardation of migration of the analytes.
Authors:Mieczysław Sajewicz, Monika Gontarska, Dorota Kronenbach, and Teresa Kowalska
Our earlier thin-layer chromatographic and polarimetric investigations enabled discovery of the spontaneous in-vitro oscillatory chiral inversion of the profen drugs
-(−)-flurbiprofen, etc., and then of the α-amino acids 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
-(+)-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).
Authors:Jaroslaw Polanski, Mieczysław Sajewicz, Magdalena Knas, Monika Gontarska, and Teresa Kowalska
In our earlier studies on the retention fundamentals in thin-layer chromatography (TLC), we have focused our attention on the important issue which was the deviation from linearity (i.e., handedness) of the chiral analytes’ migration tracks. Presently, this issue is revisited and referred to as lateral relocation of the analytes. We provide new experimental evidence of this striking phenomenon, and as a test mixture, we use (±)-2-phenylpropionic acid. Moreover, we employ three different chromatographic systems, i.e., those based on the bare silica gel layers, and the silica gel layers impregnated with l-arginine and dl-arginine. The observed lateral relocation of the 2-phenylpropionic acid enantiomers is interpreted as a non-linear motion, resembling that of the propeller-like chiral rotors. More general comments are made on lateral relocation and skewed retention of the structurally fit analytes in certain chromatographic systems.
We have previously described unexpected two-dimensionality in the thin-layer chromatographic separation of pairs of enantiomers of chiral 2-arylpropionic acids (2-APA) by one-dimensional development on a chiral stationary phase prepared by impregnating silica gel layers with l
-arginine. The two-dimensionality of this separation was because in planar chromatography all analytes undergo two-dimensional effective diffusion and thus — theoretically at least — can migrate in two mutually perpendicular directions, i.e. in the direction of the mobile phase flow and in a direction perpendicular to this. Demonstration (and quantification) of this striking behavior was possible by use of densitometric detection. Migration of analytes in a direction perpendicular to that of the mobile-phase certainly is not commonplace in thin-layer chromatography and, as far as we are aware, the example described in our previous paper was the first of its kind reported. It was apparently induced by the chirality of the analytes and the stationary phase.In the work discussed in this paper we demonstrated another example of deviation from the vertical of the migration tracks of the same analytes, optically pure
-(+)-naproxen, when chromatographed on unmodified commercial precoated silica gel plates. Keeping in mind that (i) such deviations can occur only with a chiral analyte developed in an asymmetric chromatographic system, and that (ii) silicon dioxide (chemically identical with chromatographic silica gel) can crystallize as quartz (i.e. as rock crystal) in two asymmetric — left-handed and right-handed — forms, we assumed the working hypothesis that chromatographic silica gel layers can also seem to have an asymmetric microcrystalline structure responsible for deviation from the vertical of the migration tracks of chiral analytes. To verify our hypothesis we acquired circular dichroism (CD) spectra and UV absorption spectra of samples of the silica gel used for coating commercial TLC plates. The results obtained from these measurements provide convincing evidence of the correctness of our hypothesis and it seems most likely that the commercial silica gel layers are characterized by the crystalline asymmetry of the silicon dioxide particles.
Authors:Mieczysław Sajewicz, Łukasz Wojtal, Michał Hajnos, Monika Waksmundzka-Hajnos, and Teresa Kowalska
In a previous paper we discussed the possibility of fractionating the essential oils of different sage species by low-temperature preparative layer chromatography (PLC), followed by preparative isolation of the contents of each fraction and further analysis by GC-MS. In that way we attempted to emphasize the practical usefulness of lowtemperature planar chromatography for investigation of volatile compounds. In this study, we explore a possibility of fractionating essential oils contained in the different sage species by low-temperature analytical TLC followed by direct mass spectrometric analysis of the separated fractions. This objective can be achieved by TLC-MS with on-line transfer of the eluted fractions. The densitograms obtained from five different sage species (i.e.,
S. lavandulifolia, S. staminea, S. hians, S. triloba
) are compared. Each densitogram is accompanied by mass spectra recorded for each peak. Videoscans of the chromatograms are also presented. In this way multiple fingerprints of the analyzed plant material, each comprising a densitogram and a selection of mass spectra, were obtained. Advanced chemometric treatment of these multiple fingerprints can be used to reveal statistically significant differences between the plant species. Analytical and chemotaxonomic advantages and further aspects for this kind of approach are discussed.
Authors:Mieczysław Sajewicz, Dorota Kronenbach, Monika Gontarska, Magdalena Wróbel, Robert Piętka, and Teresa Kowalska
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
-α-hydroxybutyric acids, and
-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.