Enhancing Enantiomeric Excess by Achiral Zeolites: A Study of Enantiomer Adsorption in Zeolites by Solid State NMR, X-Ray, and TGA
Authors:Jessica Cardenas , Andrea Martinez
Mentor:Deniz Cizmeciyan, Professor of Physical Science & Mathematics , Mount St. Mary's College
Chemistry of life crucially depends on asymmetric molecules, and favors one enantiomer over the other. How the enantiomeric purity was achieved in a prebiotic world is a profound question still awaiting an answer. Enantiomeric purity is also of great practical importance in the pharmaceutical industry, since the presence of the wrong enantiomer can cause death or irreversible damage to health. As an achiral structure, a zeolite does not favor one enantiomer over the other. However, removing enantiomers at a 1:1 ratio will result in an enantiomeric enrichment of a solution with a preexisting enantiomeric imbalance. This enrichment is achieved when the D- and L-enantiomers are adsorbed as a heterodimer. Whereas pure NaY shows 34 % water adsorption, only 4% of the amino acid sample can be adsorbed onto the zeolite as shown by preliminary TGA results. The microenvironment of the adsorbed solutes was studied using solid-state 13C NMR. The spectrum of the racemic mixture is significantly different from that of the D- or L-enantiomer of N-acetyl-Alanine, implying that the two enantiomers form a racemic compound where the two enantiomers crystallize together rather than forming a mechanical mixture of D- and L-crystals. This should favor the adsorption of the enantiomers as heterodimers, which is necessary for removing equal amounts of each enantiomer from solution. Spectra of pure enantiomers of N-acetyl-Alanine change significantly upon adsorption while their racemate’s one carbonyl peak at 175 ppm is replaced with three when adsorbed, as is the -Carbon peak at 51 ppm. X-ray powder diffraction was used in order to determine the spatial orientation of the amino acid within the zeolite.