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

Abstract

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.

Keywords:

• chiral TLC
• enantioseparation of β-hydroxybutyric acid
• oscillatory chiral conversion
• polarimetry

ACKNOWLEDGMENT

The work of two of the authors (D.K. and M.G.) was partially supported by PhD scholarships granted to them in 2008 within the framework of the ‘University as a Partner of the Economy Based on Science’ (UPGOW) project, subsidized by the European Social Fund (EFS) of the European Union.