Abstract

NMR spectroscopy was used to study the amino acid-base surfactant L-undecyl phenylalanine (und-Phe). Amino acid-based surfactants like und-Phe are used in the pharmaceutical, food, and cosmetic industries because they are biodegradable, biocompatible, have a low toxicity, and can be produced using renewable materials. They are also used as pseudostationary phases in capillary electrophoresis separations. NMR measurements of the surfactant’s critical micelle concentration (CMC) showed that und-Phe micelles began to form at approximately 6.0 mM in solutions containing the anionic surfactant and sodium counterions. However, micelles formed in the 3.5-4.0 mM range in solutions containing either arginine or lysine counterions. In addition, NMR diffusion experiments showed that arginine and lysine counterions bound to und-Phe micelles below pH 9.0, but dissociated from the micelle surface at high pH. Finally, two-dimensional NMR experiments suggested that the mechanism of arginine and lysine binding to und-Phe micelles was different and that when the und-Phe micelles formed, their aromatic rings rotated toward the non-polar micelle core.