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  • Author or Editor: Teresa Kowalska x
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This paper reviews our findings on the susceptibility of the organic chemically bonded ligands to hydrolytic cleavage from the silica matrix for the most common TLC-type stationary phases, i.e. RP-18, RP-8, CN, and Diol. In our experiments this process was induced by repeated (i.e. one-, two-, four-, and eightfold) development of these phases with mixed mobile phases comprising methanol, water, and buffers of different pH.Quantification of the extent of hydrolytic cleavage of octadecyl, octyl, 3-cyanopropyl, and diol ligands from the silica surface was basically achieved by means of Raman spectroscopy, although other supplementary analytical techniques were also employed-namely elemental (combustion) analysis, thermogravimetry (TG), and differential scanning calorimetry (DSC).

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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 Soczewiński and, in the first instance, to the rationale implemented to liquid chromatography by the Soczewiński—Wachtmeister and the Snyder—Soczewiński 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.

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Chiral separations by gas and liquid chromatography (thin-layer chromatography included) remain an analytical challenge, hence even moderate success in this field is very likely to be regarded as a valuable separation achievement. There has been only one literature report of chiral separation of S -(+)- and R -(−)-ibuprofen by TLC. The original procedure was performed with laboratory-coated glass plates and resolution of the two enantiomers in one-dimensional mode was incomplete. In an attempt to enhance the resolution the authors made use of less convenient and considerably more time-consuming two-dimensional TLC and the final result was not very impressive (Δ R F = 0.03). These chromatograms were visualized by exposure of the developed plates to iodine vapor and no direct confirmation of the identity of the two chromatographic bands was produced.The goals of this study were: (i) to adapt the experimental conditions used for this separation to commercial chromatographic glass plates; (ii) to enhance the resolution of the two antipodes; and (iii) to produce up-to-date UV spectroscopic evidence of successful separation.

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In our earlier papers we provided indirect experimental evidence strongly in support of our long-established hypothesis that aliphatic ligands on chemically bonded TLC-type stationary phases (i.e. RP-18, RP-8, CN, and Diol) are partially aromatized relatively easily at considerably elevated temperatures (i.e. much higher than 100°C). The first indication of such a possibility was apparent from Raman spectra obtained from these phases by use of a high-power laser-light source emitting in the NIR range. Irradiation of samples of the adsorbents with the NIR light caused their evident heating and the consequent appearance of a predominant Raman band in the so-called aromatic region of the spectra. Later this unexpected, and in some ways disturbing, phenomenon was utilized positively in a new method for Raman spectroscopic evaluation of the density of coverage of the silica matrix with octadecyl ligands.In this paper we report results from an experiment designed to provide more convincing and direct evidence in support of our hypothesis about this partial aromatization. To collect such evidence we had to re-design the procedure used to heat the samples of stationary phase, and to find the most efficient means of extracting and separating the aromatization products. It became obvious that to obtain readily measurable quantities of such products the samples should be heated in a closed system (e.g. in a sealed glass ampoule or in the stainless steel tube which forms part of the apparatus for accelerated solvent extraction, ASE) which prevents the compounds from escaping from the reaction system at high temperatures. We then used two advanced methods of solvent extraction — ultrasonic extraction and ASE — to separate the expected aromatization products from the solid matrix of the adsorbents investigated. Finally, analysis of the extracts obtained was performed by HPLC with diode-array detection. Our results furnish convincing evidence of an efficient aromatization process occurring equally with TLC-type octadecyl and octyl ligands heated at 170°C.

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JPC - Journal of Planar Chromatography - Modern TLC
Authors: Agnieszka Godziek, Anna Maciejowska, Ewa Talik, Mieczysław Sajewicz and Teresa Kowalska

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 spectrometry (TLC-MS). Additionally, we emphasize the oscillatory nature of the investigated process with use of high-performance liquid chromatography-evaporative light scattering detection (HPLC-ELSD) and provide a complementary insight in the chemical structure of the spontaneously formed Cys-derived oligopeptides with use of HPLC-MS.

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Recently, a growing interest has been observed in preventive and “anti-ageing” medicine, accompanied by a dynamic growth of the cosmetic industry worldwide. Different companies compete in introducing the best performing antioxidants to cosmetic preparations and to diet supplements, and in the course of recent years, great attention has been drawn by trans-resveratrol. For economic reasons, the main sources of trans-resveratrol for the cosmetic industry are plants and plant extracts, and the most popular plant is common grape vine (Vitis vinifera L.). The novelty of this study was the development of a simple thin-layer chromatographic method in the reversed-phase (RP) mode with densitometric detection in the fluorescence mode (the irradiation wavelength λ = 340 nm), enabling quantification of trans-resveratrol in selected cosmetic raw materials of natural origin. Cosmetic raw materials considered were red, white, and fruit wines (employed in “wine spas”), commercial extracts of red wines and other fruits, and commercial extracts of grape skins and seeds, making twenty different samples altogether. Prior to thin-layer chromatographic analysis, trans-resveratrol was isolated from the investigated raw materials by means of solid-phase extraction (SPE). The polyphenols fraction adsorbed on SPE cartridges was eluted, and the eluates containing trans-resveratrol (along with other stilbene derivatives) were developed by means of TLC. The chromatograms were densitometrically scanned and trans-resveratrol extracted from raw materials was quantified, based on the calibration curve obtained for the trans-resveratrol standard. The limit of detection (LOD) and limit of quantification (LOQ) values of the developed method were determined as equal to 1.88 and 5.69 ng, respectively. The presence of trans-resveratrol in the analyzed samples was additionally confirmed by visualizing the chromatograms with anisaldehyde as a selective visualizing reagent.

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JPC - Journal of Planar Chromatography - Modern TLC
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.

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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).

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We have previously discussed in detail thin-layer chromatographic studies of the retention of the 2-arylpropionic acids (2-APA) S -(+)-and S , R -(±)-ibuprofen, S -(+)-naproxen, and S , R -(±)-2-phenylpropionic acid. Chiral TLC was performed on commercial silica gel layers impregnated with l -arginine in the cationic form as chiral ion-pairing reagent. Retention and separation of the species was checked by densitometry.In this paper we report the striking skewed migration of these chiral APA in these TLC systems. Scanning of the chromatographic plates at 1-mm intervals revealed that the tracks of the S -(+) and the R -(−) APA can deviate markedly from the vertical in mutually opposite directions. This striking effect in the two-dimensional separation of the enantiomers is most probably caused by stereospecific intermolecular interactions which occur during ion-pair formation between l -arginine in the cationic form, deposited on the silica gel layer as chiral selector, and each individual 2-APA in the anionic form.

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