How to Suppress the Spontaneous Oscillatory In-Vitro Chiral Conversion of alpha-Substituted Propionic Acids? A Thin-Layer Chromatographic, Polarimetric, and Circular Dichroism Study of Complexation of the Cu(II) Cation with L-Lactic Acid

摘要

In our earlier investigations we showed, for the first time, with numerous practical examples that alpha-substituted propionic acids dissolved in the low-molecular-weight solvents are able to undergo spontaneous oscillatory in-vitro chiral conversion. In this study, we focused on attempting to suppress the spontaneous oscillatory in-vitro chiral conversion of alpha-substituted propionic acids using, as example, L-lactic acid dissolved in water in the presence of copper(II) cations. Our intention was to check whether the coordinate covalent bonds between copper(II) and L-lactic acid ligands prevented the latter species from oscillatory chiral conversion. To do this we stored aqueous solutions of copper(II) acetate and lactic acid in the molar ratios 1:1, 1:2, and 1:3 for a long period of time. Scrutiny of possible chiral conversion of L-lactic acid was carried out by thin-layer chromatography (TLC), polarimetry, and circular dichroism (CD) spectroscopy. Seemingly contradictory results were obtained from our investigations. From the TLC data it was apparent that chelating of copper(II) cations with L-lactic acid molecules did not result in suppression of the spontaneous oscillatory in-vitro chiral conversion of the acid. It was also established that different molar proportions of copper(II) cation and L-lactic acid molecules had somewhat different effects on the dynamics of conversion. In contrast, from polarimetric and circular dichroism studies it was apparent that when L-lactic acid is dissolved in water in the presence of copper(II) cations almost no chiral conversion is observed. Hence a final conclusion was drawn that chelating of copper(II) cations with L-lactic acid stabilizes the chiral structure of the acid in solution. Intermolecular interactions between the copper(II)-L-lactic acid complex and the silica gel stationary phase evidently affects the structure of the complex, however, most probably resulting in partial "liberation" of L-lactic acid ligands. Thus the chiral structure-stabilizing effect of copper(II) cations is apparently weakened by the TLC system and the freed L-lactic acid molecules can undergo chiral conversion.