Search

As you may have already noted, this year introduces a new set of authors for this fundamental review. As new authors, they hope that they can do as accurate a job as did their predecessor, Professor Earl Wehry, of the University of Tennessee. The format for this review follows the basic outline used by Professor Wehry, with some modifications. They have condensed several sections and eliminated the section on gas-phase chemiluminescence. The primary areas of emphasis including advances in experimental techniques, developments in instrumentation, and applications for chemical analysis, remain the same. To keep the review at a reasonable length, they have not included articles that are only peripherally related to analytical chemistry or those that represent straightforward extensions or demonstrations of previously published research. In this first issue, it is likely that they have made some errors of omission, and they request your assistance in identifying any obvious errors. They have tried to be conscientious in surveying the literature and have also surveyed individual researchers in the field. This review covers literature indexed by Chemical Abstracts from January, 1985, Vol. 102, issue 1, through October 1987, Vol. 107, issue 16. Accordingly, there will be some overlap between this reviewmore » and Professor Wehry's last review.« less

Chiral separations employing four diastereomers of poly sodium N-undecanoyl leucylvalinate (p-SULV) as chiral selectors are probed by use of MEKC, steady-state fluorescence anisotropy, and NMR. By employing diastereomers and thus altering the stereochemistry of a single amino acid in a systematic way, one may control the enantiorecognition ability of the chiral selector. As a result, one can gain a better understanding of the mechanisms of chiral recognition for the two classes of neutral or anionic chiral analytes studied. Evaluation of the chiral interactions leading to chiral separations confirmed our earlier observation of a strong relationship between the selectivity (alpha) observed using a chromatographic separation technique (MEKC) and that determined from the spectroscopic parameter, beta. A linear alpha versus beta relationship was observed for the molecular micelle p-(L)-SULV with all eight analytes included in this study. However, as we earlier predicted, different groups of analytes had different slopes, i.e., values of m, suggesting different chiral separation mechanisms. Evaluation of the data allowed a grouping of the analytes according to the primary site of chiral interaction with the leucine or valine moiety of molecular micelle chiral headgroup.

NMR spectroscopy was used to investigate the association of four chiral molecules with the molecular micelle poly(sodium N-undecanoyl-l-leucylvalinate) (poly(SULV)). Adding poly(SULV) to the background electrolyte in electrokinetic chromatography (EKC) allows enantiomeric resolution to be achieved because enantiomers interact differentially with the chiral centers on the micelle headgroups as they both move in the electric field. Pulsed field gradient diffusion experiments were used to measure molecular micelle association constants for enantiomers of each analyte. These association constants were consistent with EKC elution order for the compounds 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (BNP), 1,1'-bi-2-naphthol (BOH), and Troger's base. In addition, nuclear Overhauser enhancement spectroscopy, nuclear Overhauser effect difference, and intermolecular cross relaxation diffusion experiments were used to generate binding interaction maps for each chiral analyte. These maps showed that BNP and BOH inserted into the surfactant headgroup's major chiral groove and interacted predominately with the leucine chiral center. (+)-Troger's base was also found to insert into the major chiral groove. However, this compound instead interacted with the valine chiral atom. In diffusion experiments with long diffusion times, the linearized diffusion plots for each analyte-molecular micelle mixture showed curvature characteristic of intermolecular cross relaxation. The magnitude of this effect scaled linearly with the analytes' free energies of binding.

A cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) method was applied to the enantioseparation of three binaphthyl derivatives using neutral CDs (i.e., beta- and gamma-CD) in combination with various chiral amino acid-based polymeric surfactants (PSs). Both the D- and L-configurations of poly(sodium N-undecanoyl alaninate), poly(sodium N-undecanoyl leucinate), and poly(sodium N-undecanoyl valinate) (poly(L-SUV)) were synthesized. The retention behavior of the three binaphthyl derivatives under optimum electrophoretic conditions using a single chiral additive (PS or CD) is discussed. In addition, the effect of CD cavity size and stereochemical configuration of polymeric surfactants on selectivity (alpha) and resolution (Rs) was investigated. The enantioseparation of (+/-)1,1'-binaphthyl-2,2'-diamine gave a reversal of enantiomeric order when using beta-CD in combination with any of the three D-configuration PS. However, better enantioseparation is obtained when using the corresponding L-configuration PS with beta-CD. A reversal of migration order (RMO) for the enantiomers of (+/-)1,1'-bi-2-naphthol was observed upon the addition of 10 mM gamma-CD to poly(L-SUV). However, no RMO of (+/-)1,1'-bi-2-naphthol was seen when either beta-CD or gamma-CD was combined with D-PS. The enantiomers of (+/-)1,1'-binaphthyl-2,2'-diyl hydrogen phosphate showed little enantioselective behavior toward the PS alone. However, combined D- or L-PS and beta-CD or gamma-CD systems gave increased Rs and alpha values. The chiral recognition of binaphthyl derivatives observed resulting from the various combinations of two chiral selectors is discussed.

This study examines the role of steric hindrance near the stereogenic centers of four glutamic acid based polymeric surfactants. The single amino acid surfactants of glutamic acid, glutamic acid methyl ester, glutamic ethyl ester, and glutamic acid tert-butyl ester were investigated. The micellar electrokinetic chromatography (MEKC) separation of three binaphthyl derivatives and three benzodiazepines were used to study these steric factors. In addition, the hydrophobicity of these polymers as a function of pH was investigated by use of fluorescence measurements.

Dipeptide micelle polymers are a new class of polymeric surfactants of which the polysodium undecanoyl-L-leucylvalinate (poly-L-SULV) was found to be a broadly applicable chiral selector for micellar electrokinetic chromatography. This negatively charged dipeptide micelle polymer is a high molecular weight compound with large countercurrent mobility, zero critical micelle concentration, low aggregation number, and high solubility in water or water-organic solvents. In an extensive chiral screening program, enantioseparation of 75 racemic compounds was tested with poly-L-SULV as chiral pseudostationary phase in neutral pH and basic pH background electrolytes. A total of 58 out of 75 racemic compounds could be resolved after choosing an appropriate concentration of poly-L-SULV. Although anionic chiral analytes are difficult to resolve using poly-L-SULV, the percent success rate for chiral resolution of cationic (77%) and neutral (85%) racemates was very high. Aspects regarding electrostatic, steric, hydrophobic, and hydrogen-bonding interactions of this dipeptide micelle polymer with various classes of chiral analytes are discussed.

A colloidal graphite-coated emitter is introduced for sheathless capillary electrophoresis/nanoelectrospray ionization time-of-flight mass spectrometry (CE/ESI-TOFMS). The conductive coating can be produced by brushing the capillary tip to construct a fine layer of 2-propanol-based colloidal graphite. The fabrication involves a single step and requires less than 2 min. Full cure properties develop in approximately 2 h at room temperature and then the tip is ready for use. The coated capillary tip is applied as a sheathless electrospray emitter. The emitter has proven to bear stable electrospray and excellent performance for 50 microm i.d. x 360 microm o.d. and 20 microm i.d. x 360 microm o.d. capillaries within the flow rate of 80-500 nL/min; continuous electrospray can last for over 200 h in positive mode. Baseline separation and structure elucidation of two clinically interesting basic drugs, risperidone and 9-hydroxyrisperidone, are achieved by coupling pressure-assisted CE to ESI-TOFMS using the described sheathless electrospray emitter with a bare fused-silica capillary at pH 6.7. It is found that the signal intensity of m/z in sheathless CE/ESI-TOFMS at pH 6.7 is approximately 50 times higher than that at pH 9.0 for the two analytes, although the electroosmotic flow (EOF) at pH 9.0 provides sufficient flow rate (approximately 150 nL/min) to maintain electrospray.