Counterions in polymeric amino acid based surfactants: Effect on physical properties and enantioselectivity

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The influence of alkali metal counterions on micellar properties and chiral recognition of poly-N-undecenoyl-L-leucinate (poly-L-UL) surfactant was investigated by use of surface tensiometry, fluorescence spectroscopy, pulsed field gradient NMR (PFG-NMR), and micellar electrokinetic chromatography (MEKC). The larger counterions (Rb+ and Cs+) showed a dramatic increase in the critical micelle concentration. In addition, fluorescence and PEG-NMR experiments showed a general increase in micellar polarity and diffusion coefficients, respectively, with increasing counterion size. Fluorescence and PFG-NMR data were interpreted in terms of different micellar aggregates that form in the presence of the different counterions: Li+, Na+, K+, Rb+, and Cs+. Significant effects were observed in the presence of Rb+ and Cs+. A group of six chiral analytes were used to test the chiral selectivity (α), chiral resolutions (Rs), and retention factors (k′) using the alkali metal counterions of poly-L-UL micelles in MEKC. In general the measured, α, Rs, and k′ values depended on the counterions. For example, benzoin derivatives (±)-benzoin and (±)-benzoin methyl ether as well as binaphthyl derivatives (±)-1, 1′-binaphthyl-2, 2′-dihydrogen phosphate, (±)-1, 1′-binaphthyl-2, 2′-diamine, and (±)-1,1′-bi-2-napthol showed the highest Rs and α with poly-L-CsUL. In contrast, poly-L-KUL showed better Rs and α values for (±)-aminoglutethimide (AMGL). In poly-L-CsUL, high capacity factors (k′) were exhibited except for (±)-AMGL analyte. The highest k′ values for the latter analyte were observed in the presence of poly-L-KUL. The MEKC data collected after removal of the counterions followed by reconstruction of the sodium salts of poly-L-UL suggest that not only is the counterion size essential during polymerization of L-UL surfactant but also the presence of counterions in the running buffer is necessary to influence chiral resolution and selectivity.

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