Critical Evaluation of the Use of Surfactants in Capillary Electrophoresis

Micelle formation by the amino acid‐based surfactant undecylenyl l‐leucine was investigated as a function of solution pH with NMR, dynamic light scattering, and fluorescence spectroscopy. NMR and dynamic light scattering showed that 50 mM undecylenyl l‐leucine and 50 mM NaHCO3 solutions contained micelles approximately 20 Å in diameter and that micelle radius and the mole fraction of surfactant molecules associated with micelles changed very little with solution pH. The binding of the amino acids arginine and lysine to the anionic micelles was also investigated from pH 7.0 to 11.5. Below pH 9.0, the mole fraction of arginine cations bound to the micelles was approximately 0.4. Above pH 9.0, the arginine counterions became zwitterionic, and the mole fraction of bound arginine molecules decreased steadily to less than 0.1 at pH 11. When arginine dissociated from the micelles, their radii decreased from 14 to 10 Å. Similar behavior was observed with lysine; however, when lysine dissociated from the micelle surface, little change in micelle radius was observed. Two‐dimensional NMR experiments suggested that below pH 9.0, l‐arginine bound perpendicular to the micelle surface primarily though its side chain amine while l‐lysine bound parallel to the surface through both of its amine functional groups. Finally, the rate at which the amide protons on the surfactant headgoup exchanged with solvent was investigated with NMR spectroscopy. The exchange reaction was faster in solutions containing only surfactant monomers and slower when the surfactants were in micellar form and the headgoup amide protons were less exposed to solvent.

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Effect of pH on the Binding of Sodium, Lysine, and Arginine Counterions to l‐Undecyl Leucinate Micelles

Lewis

Hughes

Vasquez

et al. 2016

J Surfact & Detergents

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Critical Evaluation of the Use of Surfactants in Capillary Electrophoresis

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Effect of pH on the Binding of Sodium, Lysine, and Arginine Counterions to l‐Undecyl Leucinate Micelles

Effect of pH on the Binding of Sodium, Lysine, and Arginine Counterions to l‐Undecyl Leucinate Micelles

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