Xiande Wang

EVALUATION OF THE MACROCYCLIC ANTIBIOTIC AVOPARCIN AS A NEW CHIRAL SELECTOR FOR HPLC

Avoparcin is a macrocyclic glycopeptide antibiotic structurally related to vancomycin, teicoplanin, and ristocetin A. When attached to 5-microns spherical silica gel, the avoparcin proved to be an effective chiral stationary phase (CSP) that could be used in the reversed-phase, normal-phase, and polar-organic modes. The avoparcin CSP was complimentary to the other macrocyclic glycopeptide CSPs in that it could resolve some racemates that the others could not, and vice versa. Some important compounds resolved on the avoparcin CSP include verapamil, thyroxine, mephenytoin, and 2-imidazolidone-4-carboxylic acid. The use of this CSP and the optimization of separations on it are discussed. Avoparcin appears to be a useful addition to this family of CSPs.

Enantiomeric composition of nornicotine, anatabine, and anabasine in tobacco

Literature reports on the optical purity of the minor alkaloids in tobacco leaf and its products often contradict one another. The enantiomeric compositions of nornicotine, anatabine, and anabasine were measured using gas chromatography/mass spectrometry (GC/MS) (with a chiral stationary phase) in three types of tobacco leaf (Burley, Turkish, and Virginia); three types of smokeless tobacco (loose-leaf, dry snuff, and moist snuff); and four types of cigarettes. Regardless of the tobacco type or product, anabasine always had the highest relative percentage of the minor (R)-(+)-enantiomeric component (between 40 and 46% vs. 54–60% of the (S)-(−)-enantiomer). Of the four common tobacco alkaloids, nicotine seems to have the highest enantiomeric excess (e.e.) while anabasine has the lowest (in the tobacco leaf and tobacco products analyzed). Nornicotine and anatabine have intermediate e.e. values. Chirality 11:82–84, 1999.

Comparison of the enantioselectivity of β-cyclodextrin vs. heptakis-2,3-O-dimethyl-β-cyclodextrin LC stationary phases

Abstract The enantioselectivity of native β-cyclodextrin and its 2,3-methylated analogue were examined in both the “polar-organic” mode and the reversed phase mode. By comparing these two chiral selectors, the function of the secondary 2,3-hydroxyl groups at the mouth of the cyclodextrin can be examined (in regard to selectivity and retention). In the “polar-organic” mode, compounds that are easily resolved on the native β-cyclodextrin chiral stationary phase (CSP) cannot be resolved on the methylated CSP. These results, as well as the retention data, lend support to the previously proposed “noninclusion” mechanism. In the reversed phase mode, many more compounds could be resolved on the native β-cyclodextrin CSP than on the methylated analogue. However, a few compounds were resolved only on the 2,3-methylated cyclodextrin CSP. In these cases, steric interactions at the mouth of the cyclodextrin cavity may be more important to chiral recognition than hydrogen bonding. Current Address: Advanced Separation ...

Separation of enantiomers by capillary electrophoresis using pentosan polysulfate

Pentosan polysulfate, a semisynthetic polysaccharide, was employed as a chiral run buffer additive in capillary electrophoresis. Twenty‐eight racemic analytes were resolved. The separations were successful only at low pH when the analytes were significantly protonated. This suggests that ionic interactions were the dominant associative interactions between the anionic pentosan polysulfate and the positively charged analytes. Compared to other linear, carbohydrate‐based chiral selectors (i.e., chondroitin sulfates, heparin and dextran sulfate) pentosan polysulfate has some characteristics common of anionic polysaccharides; yet it has several differences in its structure and properties which account for its unusual enantioselectivity. The effects of pH, concentration of phosphate buffer, concentration of pentosan polysulfate and the type and concentration of organic modifier on the enantiomeric separations were investigated. The optimization of these separations were dependent on the nature of the analytes and could be achieved by the proper choice of experimental conditions.

Enantiomeric composition of monoterpenes in conifer resins

Abstract Resins from Agathis and Araucaria genera were collected to study the concentration as well as the enantiomeric compositions of selected monoterpenes, i.e. α-pinene, β-pinene, camphene and limonene. The compositions of the compounds were found to be species-specific and little variation was observed with season. The enantiomeric ratios for some compounds, i.e., α-pinene, vary tremendously between trees of different age but of the same species. The possible significance of this information to biological studies of conifers and to the study of amber are discussed. The enantiomeric compositions of the monoterpenes, α-pinene, β-pinene, camphene and limonene in resins from Agathis and Araucaria genera are reported.

Comparison of the selectivity and retention of β-cyclodextrin vs. heptakis-2,3-O-dimethyl-β-cyclodextrin lc stationary phases for structural and geometric isomers

Abstract Both β-cyclodextrin and 2,3-methylated-β-cyclodextrin bonded stationary phases effectively separate a variety of structural and geometrical isomeric compounds in the reversed phase mode. The retention of neutral structural isomeric compounds, as well as substituted phenols and anilines, was usually longer on the methylated cyclodextrin stationary phase. Conversely, the retention of all substituted carboxylic acids was greater on the native β-cyclodextrin stationary phase. The selectivity differences between the native β-cyclodextrin and its 2,3-methylated analogue were not as great as expected for the structural isomers. In general, the structural isomer that was retained longest on the native β-cyclodextrin stationary phase also was retained longest on the 2,3-methylated cyclodextrin stationary phase. Current address: Advanced Separation Technologies, Inc., Whippany, NJ.

Quantitative and stereoisomeric determination of light biomarkers in crude oil and coal samples

Abstract Indans and tetralins are considered biological markers (biomarkers). These C9–C11 hydrocarbons are present in small amounts in organic geological samples. Methyl substituted indans or tetralins may possess a stereogenic center (carbon). Thus they can exist as enantiomers and, in the case of disubstituted entities, also as diastereoisomers. The concentrations of 1-methylindan, 1,3-dimethylindan, 1-methyltetralin, and 2-methyltetralin were determined in sixteen crude oil samples of different sources and in fourteen coal samples of different sources and ranks. Deuterated homologues were synthesized as standards to spike the samples and to assure accurate quantitative analysis. A procedure using HPLC fractionation followed by GC/MS analysis allowed the determination of μg/g (ppm) amounts of these compounds in oils. The concentration of substituted indans and tetralins was 3–4 orders of magnitude less in coal than in crude oil. The select ion mass spectrometry (SIM) mode in GC/MS and the deuterated standards allowed detection of the much lower amounts (ng/g, ppb down to pg/g, ppt) of these compounds in coal samples. The stereochemistry of the biomarkers was determined, and the relationship between their relative concentrations and the location and type of the deposits was examined. Racemic mixtures of the indans and tetralins studied were found in all samples of oil and coal. It is postulated that there is an inverse relationship between the retention of stereochemical configuration and the molecular weight of hydrocarbons in crude oil. The chiral retention of configuration cut-off is thought to be between molecular weights of 146 and 208. An excess of cis-1,2-dimethylindan was found in all oil samples (average cis/trans ratio: 3/2). The 2-methyltetralin concentration was found to be about twice that of 1-methyltetralin in all oil and coal samples. Similar concentration correlations were found for the indan derivatives in oils and coals.

Analysis of Fissure-Filled Emeralds and Rubies by Diffuse Reflectance Fourier Transform Infrared Spectroscopy

ABSTRACT Although diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is mainly used for powdered solid samples, it was found to be highly effective for the analysis of certain gemstones. Small amounts of oils and resins used to fill cracks and fissures in substandard (included) emeralds and rubies could be detected. Because of their very low levels, these organic adulterants can be difficult to detect by measurement of the infrared transmittance spectrum. Rubies have a significantly larger “window-of-detection” for organic clarifying agents than do emeralds. In some cases the spectral information was sufficient to identify the oil or polymer resin. Emeralds that were treated over 20 years ago are still easily identified. The position of the emerald or ruby in the sample holder can affect the intensity of C-H and O-H stretching frequencies as well as those brands characteristic to the gemstones.