Molecular recognition with micellar and micelle-like aggregates in aqueous media

Enantiomer separation by reversed-phase liquid chromatography with novel hydrophobic phases composed of chiral cationic surfactants

This paper describes enantiomer separation using four kinds of chiral stationary phases (CSPs) where quaternary ammonium surfactants containing L-valine diamide moieties into long alkyl chains were bound to silicagel supports by reversed phase liquid chromatography. Our aim was to examine hydrogen bonding association of the chiral moiety in hydrophobic phase brought about by aggregation of the micelle-forming surfactants on the surface. The following CSPs were thus derived from the vinyl-terminated chiral surfactants via hydrosilylation: CSP 1 from N-[3-(10-undecenoyl-L-valylamino)propyl]-N,N,N-trimethylammonium bromide, CSP 2 from N-[6-(10-undecenoyl-L-valylamino)hexyl]-N,N,N-trimethyl-ammonium bromide, CSP 3 from N-[3-(10-undecenoyl-L-valylamino)propyl]-N-octadecanyl-N,N-dimethyl-ammonium bromide and CSP 4 from N-[6-(10-undecenoyl-L-valylamino)hexyl]-N-octadecanyl-N,N-dimethylammonium bromide. The degree of hydrophobicity in the interfacial phase was observed by measuring pyrene fluorescence in aqueous media including an organic modifier. Retention of racemic N-acylleucine isopropyl esters was highest in CSP 4, followed by 3, 2, and 1. Largest alpha values toward enantiomer separation were observed for CSP 4 where the chiral moieties were kept through a hexamethylene unit apart from the polar head groups and to which another long alkyl chain was attached, as compared with those for CSP 4. In CSP 4, the chiral moiety to interact with enantiomeric solutes should be buried into the interfacial phase deeply in more extent than CSP 3. In a similar manner, CSP 2 has more effective for enantiomer separation than CSP 1. The interfacial phase of these CSPs was easily exposed to the bulk phase because of the affinity between the bulk phase and the polar head groups as well as their electrostatic repulsion. However, degree of the enantiomer separation can be controlled by the depth of the chiral moiety in the hydrophobic interfacial phase.

Enantiomeric separation using temperature-responsive chiral polymers composed of l-valine diamide derivatives in aqueous liquid chromatography

This paper describes enantiomer separation by aqueous liquid chromatography using chiral stationary phases (CSPs) in which temperature-responsive polymers derived from acryloyl-L-valine N-methylamide (1) and its N,N-dimethylamide analogue (2) were bound on silica gel supports. The linear polymers composed of monomer 1 and monomer 2 are temperature-responsive in solution and their aggregation and extension states related to water solubility are reversible at particular critical temperatures. During chromatography, enantioselectivity and retentivity for solute enantiomers were controlled by column temperature, which changes the aggregation and extension states of the chiral polymers depending upon their interior hydrophobic nature. Two different types of CSPs were made: a temperature-responsive linear polymer derived from 3-mercaptopropyl silica gel, and another polymer cross-linked with ethylene dimethacrylate from 3-methacryloyloxypropyl silica gel. The former CSP could separate racemic N-(3,5-dinitrobenzoyl(DNB))amino acid isopropyl esters. Retention of the amino acid derivatives was prolonged with an increase in column temperature. Enantioselectivity was also enhanced with temperature increase until the particular critical temperature. The latter, cross-linked CSP could not provide enantioselectivity for the amino acid derivatives in aqueous media, although the chiral valine diamide moieties were effective for enantiomer separation in non-aqueous media. The degree of hydrophobicity and volume of the bonded phase formed by the polymers on the support surface was determined by measuring the fluorescence of pyrene.

Employment of detergent-tag/solute interactions in capillary electrophoresis of neutral polysaccharides

Neutral and inherently immobile polysaccharides are induced to migrate in an electric field through interactions with a detergent added to the electrophoretic electrolyte buffer. Before analysis the polysaccharides are converted to fluorescent derivatives to enable detection, but choice of a tag can also be utilized for modulation of the electrophoretic mobility. Three cases are discussed and exemplified, namely detergent-solute, detergent-solute+tag, and detergent-tag interactions. Anionic as well as cationic surfactants were exploited along with different derivatization reagents. Depending on the approach chosen, different kinds of information about sample composition and distribution(s) can be obtained, including degree of substitution, distribution of molecular weight (obtained in free solution without sieving media) and polymer conformation. A shift in polymer conformation upon a change in solvent composition can be monitored.

Depth of penetration of binaphthyl derivatives into the micellar core of sodium undecenoyl leucyl-leucinate surfactants

Two different diastereomeric forms of sodium N-undecanoyl leucyl-leucinate (SULL) (both L,L and L,D) are used to examine the role of depth of penetration of chiral analytes into the micellar core of polymeric and monomeric surfactants on enantioselectivity. In this study, chiral separation of three binaphthyl derivatives, i.e. (+/-)-1,1'-bi-naphthyl-2,2'-diamine (BNA), (+/-)-1,1'-bi-2-naphthol (BOH), and (+/-)-1,1'-binaphthyl-2,2'-dihydrogen phosphate (BNP), are studied. Chromatographic results suggest that BNP interacts approximately the same with both the C- and N-terminal amino acid of poly SULL, while the preferential site of interaction of this analyte with the monomeric form of SULL (mono SULL) is at the C-terminal amino acid. This indicates that BNP enantiomers penetrate deeper into the micellar core of the poly SULL than that of the mono SULL. Varying the temperature resulted in a change in the depth of penetration of BNP into the micellar core of the poly SULL. However, the enantiomers of BNA and BOH always interact preferentially with the N-terminal amino acid of SULL surfactants (both polymer and monomer), independent of the temperatures studied.

Comparison of monomeric and polymeric amino acid based surfactants for chiral separations

To better understand chiral recognition with polymeric amino acid based surfactants, the chromatographic performance of 18 monomeric and polymeric surfactants were compared for chiral analytes with various charge states and hydrophobicities. In this study, four amino acids (glycine, L-alanine, L-valine, and L-leucine) were chosen, and all possible combinations of the chiral single amino acid and dipeptide surfactants were synthesized. The results indicate that polymeric surfactants usually provide better chiral resolution for enantiomers of lorazepam, temazepam, 1,1'-bi-2-naphthol, and propranolol as compared to monomeric surfactants. In contrast, monomers perform better for chiral recognition of the 1,1'-bi-2-naphthyl-2,2'-diyl hydrogenphosphate enantiomers.

Molecular recognition by chiral cationic micellar and micelle-like aggregates in electrokinetic capillary chromatography

We examined the enantiomer separation with micelles and a micelle-like polymer made with trimethylammonium-terminated surfactants all of whose hydrocarbon chains contain hydrogen bonding valinediamide moieties in electrokinetic chromatography (EKC). The surfactants used were 3-(N-dodecanoyl-L-valylamino)-propyltrimethylammonium bromide (surfactant 1) and 6-(N-nonanoyl-L-valylamino)hexyl-trimethylammonium bromide (surfactant 2); the micelle-like polymer was derived from 3-(N-10-undecenoyl-L-valyl)aminopropyltrimethylammonium bromide (surfactant 3). N-Acylamino acids and their isopropyl esters were separated with enantiomers with the same configuration as the chiral surfactant and which were retained to a greater extent than the counterparts in micelles. The micellar hydrophobic environment, in which amides function as hydrogen bonding sites with solutes, and ceased micellar kinetic association-dissociation with polymerization are discussed.

Chiral separations using polymeric dipeptide surfactants: effect of number of chiral centers and steric factors

Two polymeric dipeptide chiral surfactants (PDCSs), poly sodium N-undecanoyl isoleucyl-valinate (SUILV) with three chiral centers and poly sodium N-undecanoyl leucyl-valinate (SULV) with two chiral centers, have been evaluated and compared as chiral pseudo-stationary phases in electrokinetic capillary chromatography. The performance of these surfactants, in terms of enantioselectivity was examined using anionic, cationic and neutral analytes. Analyses of the data suggest that the enantiomeric resolutions of the analytes with these two PDCSs are dependent upon steric factors rather than number of stereogenic centers.

Chiral separation of dansyl-DL-amino acids with micellar systems containing copper (II) ion and N-n-dodecyl-L-proline in electrokinetic capillary chromatography

Enantiomers of dansylated DL-amino acids were resolved by chiral copper (II)-N-n-dodecyl-L-proline (1) complexes incorporated in micelles of sodium dodecyl sulfate (SDS) in electrokinetic capillary chromatography (EKC). This resolution is caused by formation of diastereomeric ternary complexes consisting of chiral ligand 1, central copper (II) ion and enantiomeric amino acid derivatives in micellar phase. However, the resolution was not observed when SDS with an anionic polar head grop was replaced with dodecyl trimethylammonium brode (DTMAB) with a cationic polar head group. The ratio between copper (II) ion and 1 in the complex in either SDS or DTMAB was measured by UV-visible spectra, which respond to the d-d transition of copper (II). Mechanism of separation should be discussed in terms of effect of surfactant structures on constitution of copper (II) ion and 1 in the micellar phase and that of arene substituent structures linked to sulfonamide units in amino acid derivatives to be separated.