Gas chromatographic enantiomer separation on a chiral self-associating selector

Tripodal trispeptide selector as a model for establishing the importance of hydrogen-bonding in enantiomer-separation

“…1). 22 We here extended the already performed 1 H-NMR-experiments over a 100°C temperature range (between −50°C and +50°C). Retention of the NMR-pattern for the diastereotopic protons over this large temperature range established that a single, hydrogen-bonded conformation is present.…”

“…22 NMR-spectra were recorded on a Bruker WH-270 or AMX-400 NMR spectrometer. The assignment of the NMR signals to the various protons in the trispeptide, described in reference, 22 was done according to the well-known rules of integration, multiplett structure, correlation with other tripodal compounds, and intermediate products, and was confirmed by COSY measurements.…”

“…Particularly so, as the non-equivalence of the diastereotopic protons (⌬␦ values) represent semiquantitative measures for the extent of these hydrogen bonds' presence. 22 1 H NMR titration experiments of the tripodal selector with L-and D-N-TFA-alanine-isopropyl, respectively (Fig. 2), showed identical behavior for both en antiomers; a collapse of the CH 2 CO-system to a triplet (indicating cleavage of the intra-strand hydrogen bonds), and a small increase in ⌬␦AB of the C quart -(CH 2 O) 3 -protons, indicating preservation, and even strengthening of the inter-strand hydrogen bonds.…”

“…This selector adopts a propeller-like arrangement by virtue of distinct inter-and intra-strand hydrogen bonds. 22 We recently linked the above selector via Pt-catalyzed hydrosilylation of the double bond to a polymethylhydrosiloxane in order to obtain a chiral phase, and examined its separation capabilities Although only N-TFA-amino acid isopropyl esters and ␣-hydroxy acid ethyl esters have been examined so far, the consistent elution order of the enantiomers within these two families of compounds [N-TFA-amino acid isopropyl esters, D before L; ␣-hydroxy acid ethyl esters, (S) before (R)], provides an informative test case. …”

“…1). 22 Here we elucidate possible modes of interactions between peptide selectors and solutes by identifying the above trispeptide's detailed conformation, and the nature of its interactions with chiral substrates. We investigate the selector's conformation and helical sense by 1D-NOEdifference measurements, and by molecular modeling, using Lifson's empirical force field (EFF) and the commercial CFF91 force field from Biosym, Inc. We examine the mode of trispeptide-solute interactions by 1 H NMR titration experiments of the selector with representative solutes, and by molecular modeling.…”