Counterions in polymeric amino acid based surfactants: Effect on physical properties and enantioselectivity
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.
Publication Source (Journal or Book title)
Tarus, J., Shamsi, S., Morris, K., Agbaria, R., & Warner, I. (2003). Counterions in polymeric amino acid based surfactants: Effect on physical properties and enantioselectivity. Langmuir, 19 (18), 7173-7181. https://doi.org/10.1021/la034198n