Amplification of enantiomeric excess of aminoacids by co-adsorption into zeolites: An investigation, using Solid-State NMR, Thermogravimetry and Differential Scanning Calorimetry.
Authors:Jessica Cardenas , Andrea Martinez
Mentor:Deniz Cizmeciyan, Professor of Chemistry , Mount St. Mary's College
Chemistry of life depends on molecular asymmetry (chirality), whose origination is a fascinating and fundamental question. We study Zeolites as a possible medium for enriching the enantiomeric excess of solutions. Achiral Zeolite lattices do not prefer one enantiomer over the other, and adsorb the enantiomers equally well. However, if the D- and L- enantiomers adsorb together as a heterodimer, the enantiomeric excess of the solution they leave behind is augmented. We used NMR, TGA, DSC to explore the adsorption behaviors of D-, L-, and DL- N-acetyl Leucine, Alanine and Methionine into Zeolite NaY. The solid state NMR spectra of the pure D- and L- acetyl Leucine showed the same results as the racemic mixture of N-acetyl-DL-Leucine, indicating a preference to form microcrystals of pure D and L forms. The racemic mixture also displays the same spectrum when adsorbed in Zeolite NaY indicating that they are adsorbed as homodimers. In contrast, the 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 α-Carbon peak at 51 ppm. Similarly, the 13C spectrum of D- and L-N-Acetyl methionine exhibit peaks at 170, 55 and 30 ppm for the carbonyl, α and β carbons respectively, while the racemic mixture shows a second peak for the carbonyl group at 165 ppm, implying that the latter crystallizes as a racemic compound or mixed crystal, giving reason to speculate an increased tendency for heterodimer adsorption onto the zeolite. Preliminary TGA data for all compounds exhibits roughly 12-15% adsorption of the amino acids under air. Pure NaY shows 34 % water adsorption; which decreases to 4% after drying at 4250C for 4.5 hours. Exothermic decomposition of the samples is observed with DSC, and TGA performed under Argon.