Experimental Investigation of the Oscillatory Transenantiomerization of L-Tyrosine

Abstract

In our earlier studies, we demonstrated an ability of selected enantiomeric profen drugs (e.g., S-(+)-ibuprofen, S-(+)-naproxen, and S-(+)- and R-(−)-flurbiprofen) and one amino acid (i.e., L-α-phenylalanine) to undergo oscillatory transenantiomerization when dissolved in simple, low molecular weight solvents (e.g., water, ethanol, dichloromethane, acetonitrile, etc.) and stored for a longer period of time at ambient temperature or in a refrigerator. Experimental evidence of this process originates from a number of analytical techniques, with thin layer chromatography (TLC) and polarimetry among the best performing ones. There are two common structural features of all these compounds, namely: (i) they are 2-arylpropionic acids (2-APAs), and (ii) their chirality center is located on the α-carbon atom of the respective molecules. It has also been established that the basic and the amphiprotic environment catalyzes the oscillatory transenantiomerization of the investigated compounds, while the acidic environment tends to hamper this process. Moreover, it has been established that all the aforementioned compounds can organize molecules present in the solution in such a manner as to produce the density anisotropy of the liquids considered. Model explanation of the oscillatory transenantiomerization of profens and L-α -phenylalanine was also developed as a starting point, adapting an earlier established oscillator known as Templator. The new model comprises two linked Templators. The quintessence of the Templator model adapted to the demands of the oscillatory transenantiomerization of profens and amino acids was based on an assumption that the H-bonded 2-APA dimer is a template, able to generate the new dimers having the same steric configuration of their respective monomeric units. From our earlier studies, it clearly comes out that in spite of common traits of the oscillatory transenantiomerization of the selected profens and L-α-phenylalanine, the dynamics of this process can significantly differ from one compound to another, due to their differentiated molecular structure and, hence, to the different electron density distribution. Thus, in this study, we investigated the ability of L-tyrosine (another 2-APA and the amino acid regarded as essential for the humans) to undergo oscillatory transenantiomerization. Solubility of L-tyrosine in the amphiprotic binary mixture (70{%} aqueous ethanol solution) widely used in our earlier studies proved too low to use it as a solvent in the present investigation. Instead, we traced the behavior of L-tyrosine when stored for over one week in the following mixed solvents: ethanol–1M NaOH (7:3, v/v) and ethanol-1M HCl (7:3, v/v). The results of our experiments clearly confirm the ability of L-tyrosine to undergo the oscillatory transenantiomerization, similar to that of the previously studied profens and L-α-phenylalanine, although the individual dynamics of the oscillatory transenantiomerization with this particular enantiomer is also evident and discussed. It is apparent that the model of the two linked Templators applies to L-tyrosine as well, as an adequate explanation of the mechanism of its oscillatory transenantiomerization.