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Abstract
Chiral stationary phases (CSPs) from N-(10-undecenoyl)-L-valine tert-butylamide (1) and N-(5-hexenoyl)-L-valine tert-butylamide (2) were each found to afford a hydrophobic interfacial phase by which hydrogen-bond association could be induced for the resolution of enantiomeric N-acylated amino acid esters in aqueous phase liquid chromatography.
This was shown using the fluorescence fine structure of pyrene sorbed on the interfacial phase in three kinds of wetting water-organic solvent mixtures: water-methanol, water-acetonitrile, and water-tetrahydrofuran (THF). The intensity ratios of pyrene vibronic emission peaks gave strong indication of changes in microenvironment polarity around pyrene as a function of overlaying solvent composition.
Increase in organic solvent concentration enhanced polarity due to solvent intercalation in the interfacial phase. A less polar organic solvent, such as THF, was caused to become distributed to a greater degree within the phase than a more polar methanol and consequently, thus giving rise to greater polarity. Enantioselective association between the chiral moiety and enantiomers was hindered by increased intercalation of the less polar organic solvent, thus lessening the degree of chiral recognition.
The relationship between the polarity of the interfacial phase and extent of enantiomer resolution in liquid chromatography is discussed in regard to the difference in the spacer length between CSP1 and 2 and that between CSP1b trirnethylsilylated to remove remaining surface silanols and CSP1 without trimethylsilylation.