Chiral Recognition of 18-Crown-6-tetracarboxylic Acid toward Amino Acids and Organic Amines by Fast Atom Bombardment Mass Spectrometry. A Comparison with Capillary Electrophoresis.

Abstract

Chiral recognition of the host-guest complexations between the (RRRR)-18-crown-6-tetracarboxylic acid (18 C6TCA) host (H) and α-amino acid or their ester derivative guests (G) has been systematically determined using both FAB mass spectrometry (MS) and capillary zone electrophoresis (CE). A comparison of these two techniques for the same series of guests was then done for the first time. In the case of 18C6TCA, we found that there is no correlation between the chiral recognition obtained from FABMS (i.e., the IRIS value) and from CE (i.e., the α value in an aqueous buffer solution) covering three sets of guests including amino acids, amino acid esters, and primary aromatic amines. On the other hand, the former showed a good agreement with that from NMR (i.e., the KR/KS value; the ratio of the corresponding equilibrium constants), supporting our earlier conclusion that FABMS is a good measurement tool for predicting differences (ΔΔG° values) in the respective chiral H-G interactions in solution. We also found that when a given guest changes from an amino acid to its corresponding amino acid ester, the α value by CE dramatically changes from α > 1.0 to α < 1.0, though the IRIS value by FABMS does not show any such changes. These findings were considered to be due to the characteristic contributions of the dissociable host's COOH functions under the experimental pH conditions in CE.