Fmoc-L-isoleucine

We have studied the dynamic interaction of surfactants with carbon surfaces by using a series of Fmoc- (N-(fluorenyl-9-methoxycarbonyl)) terminated amino acid derivatives (Fmoc-X, where X is glycine, tyrosine, phenylalanine, tryptophan, or histidine) as a model system. In these systems, highly conjugated fluorenyl groups and aromatic amino acid side chains interact with the carbon surface, while carboxylate groups provide an overall negative charge. Ideal carbon surfaces were selected which possessed either predominantly macroscale (graphite) or nanoscale features (multiwalled carbon nanotube (MWNT) mats). The adsorption equilibrium for the Fmoc-X solutions with the graphitic surfaces was well-described by the Freundlich model. When a library containing various Fmoc-X compounds were exposed to a target graphite surface, Fmoc-tryptophan was found to bind preferentially at the expense of the other components present, leading to a substantial difference in the observed binding behavior compared to individual adsorption experiments. This approach therefore provides a straightforward means to identify good surfactants within a library of many candidates. Finally, the fully reversible nature of Fmoc-X binding was demonstrated by switching the surface chemistry of carbon substrate through sequential exposure to surfactants with increasing binding energies.

Various chiral N-palmitoyl amino acid surfactants (AAS) derived from methionine, proline, leucine, threonine, phenylalanine and phenylglycine were prepared and converted to their sodium salt. The properties of the aggregates formed in aqueous solution were studied for both the optically-active compounds and their racemic mixture. Characterization was made by surface tensiometry, fluorimetry, dynamic light scattering, circular dichroism (CD) and transmission electron microscopy. It appeared that most of the AAS studied in this work spontaneously formed different types of aggregates, including micrometer-sized aggregates. No significant difference could be found between the critical aggregation concentration (cac) value of pure enantiomers and that of the racemic forms. CD spectra did not reveal any aggregation-induced chirality.

Three different chiroptical spectroscopic methods, namely, optical rotation, electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) have been evaluated for studying the aggregation of sodium dodecylsulfate (SDS), an achiral surfactant, using garcinia acid disodium salt (GADNa) as a chiral probe. The specific rotation and ECD of GADNa are found to be altered by the aggregation of SDS, suggesting for the first time that achiral surfactants can be characterized with chiroptical spectroscopy using appropriate chiral probes. In addition, a chiral compound, fluorenyl methyloxy carbonyl l-leucine sodium salt (FLNa) is found for the first time to behave as a surfactant in water, with 205 Å(2) surface area per molecule at the air-water interface, critical micelle concentration (CMC) of 0.18 M, and Gibbs energy of micellization of -14 kJ/mol. The specific rotation of FLNa in water is found to increase with concentration beyond CMC, suggesting the formation of chiral aggregates. Different conformations of FLNa amenable to micellization have been identified using quantum chemical conformational analysis and their specific rotations calculated. The formation of lamellar aggregates of FLNa in water is suggested to be the cause for increase in specific rotation with concentration beyond CMC.