Investigation of the spontaneous oscillatory in-vitro chiral conversion of l-(+)-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 process commences in solution before chromatography, catalyzed by the solid-liquid interface of the TLC plate, or commences immediately on dissolution of the acid and occurs continuously without interruption, both outside and inside the chromatographic system. It is easy to believe that the highly active surface of the silica gel adsorbent acts as a catalyst of chiral conversion. From long-term polarimetric studies it was concluded that l-(+)-lactic acid also undergoes oscillatory chiral conversion in the bulk liquid phase at ambient temperature, i.e. outside the TLC system, although the rate is much lower than in TLC and the racemic mixture is obtained after approximately one month only. In TLC racemization is practically complete within one chromatographic run.