1. Performance of the Chiral TLC in Physicochemical Studies An Episode
Mieczysław SAJEWICZ & Teresa KOWALSKA
Institute of Chemistry, University of Silesia, KATOWICE, POLAND
An Episode

2. Chromatography in analytical mode
Primary purposes of chromatography:
Separation
Identification
Quantification
Secondary purpose of chromatography:
Extraction of physicochemical information from purely analytical data (if only possible)
It should be regarded as an added value to this primarily analytical chromatography mode.

3. Physicochemical information encoded in the TLC (& HPLC) data
(in service of pharmacy and the other life sciences)
Logarithmic relationship: the Snyder-Soczewiński equation
Semi-logarithmic relationship: the Soczewiński-Wachtmeister equation
ln k = a φ + b
ln k = a ln φ + b
where k: the retention factor, φ: the volume fraction of a modifier, and a and b: the equation constants.
Practical consequences:
The eluotropic solvent series (εo)
The polarity index and the selectivity parameters of solvents (P’, xe, xd, xn)
Snyder’s solvent selectivity „triangle”

4. Chiral TLC is an excellently well performing enantioresolution technique
Amino Acids · Chromatographic Separation and Enantioresolution
Ravi Bushan & Jürgen Martens
HBN Publishing, New York, 2010
Chiral TLC offers more possibilities for the development of chiral stationary phases than HPLC by self-impregnation of the commercial TLC layers with chirally pure compounds.
In this sense, TLC outperforms HPLC and GC.

5. I. Self-impregnation of a commercial adsorbent with a chiral selector
There is a vast number of practical possibilities. E.g., impregnation of a silica gel layer with a chiral compound having an ionic structure results in the formation of two diastereoisomeric salts.
L-Arg+ + S-(+)-profen- ↔ L-Arg+·S-(+)-profen-; K1
L-Arg+ + R-(-)-profen- ↔ L-Arg+·R-(-)-profen-; K2
K1  K2
II. Addition of a chiral selector (CS) to mobile phase
There is also a vast number of possible additives (e.g., cyclodextrins, crown ethers, blood albumins, transition metal cations, etc.).
CS(m) + L-Ala(s) ↔ CS···L-Ala(m); K1
CS(m) + D-Ala(s) ↔ CS···L-Ala(m); K2
K1  K2

6. Non-linear (oscillatory) processes
Owing to the chiral TLC, we discovered the oscillatory chiral conversion of low-molecular weight chiral carboxylic acids
A comparison of the temporary concentration changes:
with the substrate [S] and product [P] in a typical linear chemical reaction;
with the intermediates [X] and [Y] in an oscillatory process.
(a)
(b)
Linear reactions
Non-linear (oscillatory) processes
vs.

7. Dynamic Stereochemistry of Chiral Compounds: Principles and Applications
Author: Christian Wolf
RSC Publishing, Cambridge, UK, 2008
NOT spontaneous;
NOT with use of (HP)TLC;
NOT to the OSCILLATORY processes
This monograph presents physico-chemical applications of chromatographic techniques to monitoring chiral conversion, yet:

8. Discovery of a new type oscillatory reactions: Spontaneous oscillatory chiral conversion
with low molecular weight chiral carboxylic acids (profens, hydroxy acids and α-amino acids)
Schematic representation of oscillatory changes of the RF position of low molecular weight chiral carboxylic acids on the planar chromatogram in the function of storage time in 70% aqueous ethanol, valid for the periodic chiral conversion from (+) to (−) and back
M. Sajewicz, R. Piętka, A. Pieniak, T. Kowalska, Acta Chromatogr., 15, (2005)

9. Striking enantioseparation results
Sequence of densitometric concentration profiles of R,S-2-phenylpropionic acid after: (a) 0 h (racemic mixture); (b) 22.5 h (S(+) form); (c) 27.5 h (racemic mixture); (d) 46.5 h (R() form; (e) 51.5 h (shift from R() form to racemic mixture); (f) 70.5 h (racemic mixture).
Stationary phase: silica gel impregnated with L-arginine
Mobile phase: ACN + MeOH + H2O, 5:1:0.75 (v/v/v) plus 0.5 mL glacial CH3COOH
M. Sajewicz et al., J. Phys. Org. Chem., 23, (2010)
Striking enantioseparation results
Changes of:
concentration profile
retardation factor, RF

10. Striking enantioseparation results (continued)
Densitograms and chromatograms of L-Cys solution with equimolar addition of Mn(II) acetate. (a) Fresh prepared L-Cys solution; (b) Cys solution after 60 days ageing; (c) fresh prepared DL-Cys solution. 1: Densitometric scans of the whole chromatograms (a)–(c); 2: photographs of the whole chromatograms (a)–(c); 3: enlarged densitometric scans of the Cys bands (a)–(c). Circles and arrows indicate the Cys bands.
Striking enantioseparation results (continued)
Chiral inversion confirmed (selected example)
A. Godziek, A. Maciejowska, E. Talik, M. Sajewicz, T. Kowalska, J. Planar Chromatogr. – Modern TLC, 28, (2015)

11. Striking enantioseparation results (continued)
Chiral inversion confirmed (selected example)
A. Godziek, A. Maciejowska, E. Talik, M. Sajewicz, T. Kowalska, J. Planar Chromatogr. – Modern TLC, 28, (2015)
Densitograms and chromatograms of L-Cys solution with equimolar addition of Zn(II) nitrate. (a) Fresh prepared L-Cys solution; (b) Cys solution after 1 h ageing; (c) Cys solution after 2 h ageing; (d) Cys solution after 3 h ageing; (e) Cys solution after 4 h ageing; (f) L-Cys solution after 5 h ageing. 1: Densitometric scans of the whole chromatograms (a)–(f); 2: photographs of the whole chromatograms (a)–(f); 3: enlarged densitometric scans of the Cys bands (a)–(f). Red circles and arrows indicate the Cys bands.

12. Mechanism of chiral conversion
Molecular mechanism of chiral inversion for Phe. The intermediary structures of an „enolic” character, marked with the black ovals, demonstrate an absence of the asymmetric carbon atom. At this stage, chiral conversion is possible.
A. Maciejowska, A. Godziek, M. Sajewicz, T. Kowalska, Reac. Kinet. Mech. Cat., 120, (2017)

13. Application of the TLC-MS interface to tracing condensation (=peptidization) process
(b)
Thin-layer chromatograms, signals of the chromatographic spots representing an oligopeptide fraction (red framed on the respective chro-matograms), directly eluted from the chromatographic plates, and the respective mass spectra recorded for the (a) fresh L-Cys sample and (b) aged L-Cys sample.
A. Godziek, A. Maciejowska, E. Talik, M. Sajewicz, T. Kowalska, J. Planar Chromatogr. – Modern TLC, 28, (2015)

14. Chiral conversion and condensation running in the parallel
Molecular mechanism of condensation (=peptidization) for Phe
Chiral conversion and condensation running in the parallel

15. Conclusions
Discovery of an oscillatory chiral inversion with the low molecular weight chiral carboxylic acids (of an indisputable physiological importance) WAS ONLY POSSIBLE WITH AID OF THE CHIRAL TLC.
This discovery was made exclusively by sheer good luck.
We are aware that oscillatory reactions make an essence of our biological life, yet there are very low rates in discovering such reactions, basically due to general insufficiency of available analytical tools.
IN THIS CONTEXT, PERFOMANCE OF THE CHIRAL TLC IS EXTRAORDINARY AND PRAISEWORTHY.
We believe that many more contributions to the physical organic chemistry are still possible by applying (HP)TLC.
We are convinced that our discovery could have the far reaching future repercussions for life sciences (and more specifically, for molecular biology, proteomics, genetics etc.)

16. Thank you for your kind attention!
(…patterns in Nature and perhaps Life itself are due to oscillatory reactions…)