Comparative coordination studies of chiral supramolecular propellers of dialkyltartrate trimers with alkali ions using electrospray ionization mass spectrometry

Eur. J. Mass Spectrom. 5, 289–294 (1999)
DOI: 10.1255/ejms.288

Comparative coordination studies of chiral supramolecular propellers of dialkyltartrate trimers with alkali ions using electrospray ionization mass spectrometry

Roderich Süssmuth and Günther Jung
Institut für Organische Chemie, Universität Tübingen, D-72076 Tübingen, Germany.
F. Johann Winkler^* and Ramiro Medina
Lehrstuhl für Biologische Chemie, Technische Universität München, D-85350 Freising, Germany.

ABSTRACT:

Electrospray ionization mass spectrometry with a quadrupole analyzer has been used to compare the assembly of all stereoisomers of dialkyltartrates M with alkali ions A⁺ to form supramolecular trimer complexes M₃A⁺. In a first set of experiments using pure enantiomers of M, the chiral propeller complexes M₃A⁺ showed the highest stability with K⁺ as the central ion. For the ligand M = dimethyltartrate, the relative ionophore selectivities decreased in the order of K⁺(100), Na⁺(78), Rb⁺(65), Cs⁺(28), Li⁺(6). For both the pseudo crown ether complex M₃•K⁺ and the corresponding 18-crown-6•K⁺ complex, the relative ion abundances were the same when a 15-fold excess of the formal trimer M₃, in comparison with the 18-crown-6 ligand, was present in the experiment. In a second set of experiments, we investigated the chirality effect on M₃•A⁺-propeller formation by using enantiomer-labeled racemic mixtures of (S)- and (R)-dialkyltartrates and comparing the formation of the diastereomeric clusters SSS•A⁺, SSR•A⁺, SRR•A⁺ and RRR•A⁺. The chiral discrimination ratio (CDR) in favor of the homochiral propellers SSS•A⁺ and RRR•A⁺ compared with the heterochiral clusters SSR•A⁺ and SRR•A⁺ shows unusually high values of about 18 for K⁺ and Rb⁺ with dimethyltartrate (DMT) ligands. The CDR values are not as high for complexes with other alkali ions and/or the bulkier diisopropyltartrate (DIPT) ligands. The maximum value of CDR-18 falls between comparable maximum values of CDR = 5 from chemical ionization studies and CDR = 45 from studies using Fourier transform ion cyclotron resonance mass spectrometry. Semiempirical AM1 calculations have been applied to predict plausible structures for the homochiral propeller complexes (DMT)₃•K⁺.

Keywords: dialkyltartrates, electrospray mass spectrometry, supramolecular complexes, molecular recognition, alkali ion selectivity, chiral discrimination, comparative binding, AM1 calculations

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