Luminescent Determination of Enantiomeric Purity using Rare-Earth Probes
Доклады на конференциях
The present work is devoted to the luminescent probing of natural chiral α-amino acids with the aim of the determination of their enantiomeric purity. Luminescence spectroscopy is one of the most sensitive techniques and the idea of our approach is based on the well known ability of amino acids to form complexes with rare earth ions; in addition the presence of a “heavy-atom” in the molecule allows the T → S0 radiative transition. The choice of rare-earth ions allows to reveal molecular phosphorescence and characteristic f-f intraconfigurational electronic transitions.
Alanine being the simplest chiral proteinogenic α-amino acid was used as model substance. Depending on chiral purity we revealed considerable differences in the spectra of photoluminescence (PL) and photoluminescence excitation (PLE) of alanine (L, DL, D, its non-racemic mixtures), containing luminescent probes (Gd3+, Eu3+, Тb3+) in a range of concentrations (0.01 - 10% molar). It is worth mentioning that the introduction of the luminescent probes to the alanine solid matrix does not cause any detectable racemization that has been confirmed by chiral GC analysis. In the PL spectra of Gd3+ we observed well pronounced vibronic transitions that can be attributed to the alanine functional groups vibrations. Based on the PL and PLE spectra of alanine containing Gd3+, Eu3+ and Тb3+ we have determined the energy of its triplet level. The curve of the dependence of PL and PLE Gd3+ intensities on the alanine enantiomeric excess has a maximum around 25% ee, by that a considerable increase of the luminescent intensity was observed already at slight decrease of enantiomeric purity. In the case of Eu3+ probe the difference in PL relative intensities between a virtually enantiopure L alanine and its 99.3% non-racemic mixture made up more than 7 times. Thus the approach can allow determining a trace amounts of one enantiomer in another one with high sensitivity. The results of the investigations confirm the possibility to use the proposed luminescent technique for chiral analysis.
This work was supported by the Russian Foundation for Basic Research (partially by projects RFBR 16-33-00369, CNRS/RFBR 16-53-150004).