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Biomedical subjects

A M Stalcup

Publications and source records attributed to A M Stalcup.

33 records · Page 2Linked to original sources

Use of capillary electrophoresis as a method development tool for classical gel electrophoresis.

Capillary electrophoresis (CE) was used to optimize the buffer pH, ionic strength and sulfated cyclodextrin concentrations for enantiomeric separation of piperoxan. These enantioseparation conditions were then applied to a classical gel electrophoresis system. Binding constants of the sulfated beta-cyclodextrin-piperoxan couple were approximated using CE and the effects of organic solvents on the system were also investigated.

Buffers↗

Separation of aromatic acids, DOPA, and methyl-DOPA by capillary electrophoresis with dendrimers as buffer additives.

Polyamidoamine starburst dendrimers with terminal carboxylate groups are used as a pseudo-stationary phase in electrokinetic chromatography for separating the selected aromatic amino acids phenylalanine, phenylglycine, homophenylalanine, and tyrosine and the catecholamines 3-(3,4-dihydroxyphenyl)alanine and 3-(3,4-dihydroxyphenyl)-2-methylalanine at different pH levels. A significant difference in analyte selectivity is observed between dendrimers of different generations and at different pH levels. At pH 7.0, the utility of the dendrimers for separating these analytes is limited by relatively low selectivity and a noisy baseline. However, good separation and selectivity are obtained with low generation dendrimers (G0.5 and G1.5) at low pH. Strong association of the solute with dendrimers is observed for high generations (G2.5, G3.5, and G5.5).

Biocompatible Materials↗

Chiral capillary electrophoresis with noncyclic oligo- and polysaccharide chiral selectors.

A number of noncyclic oligo- and polysaccharides have been used as chiral additives in capillary electrophoresis (CE). This review offers a broad survey of the types of oligo- and polysaccharides which have been investigated and also some of the conditions developed for the enantioseparation of a wide range of drugs and other racemic compounds. Details of enantioseparation mechanisms are also discussed, in addition to some of the parameters required for optimization of the enantioresolution of chiral molecules.

Aminoglycosides↗

Application of sulfated cyclodextrins to chiral separations by capillary zone electrophoresis.

Mixtures of randomly substituted sulfated cyclodextrins (degree of substitution, approximately 7-10) were successfully used as chiral additives for the enantioseparation of 56 compounds of pharmaceutical interest, including anesthetics, antiarrhythmics, antidepressants, anticonvulsants, antihistamines, antihypertensives, antimalarials, relaxants, and bronchodilators. The separations were accomplished at pH 3.8, with the anode at the detector end of the column. Under these conditions, in which electroosmotic flow is directed toward the injection end of the column and the electrophoretic mobility of the negatively charged cyclodextrin is toward the detector, none of the analytes reached the detector in the absence of the sulfated cyclodextrin. Most (40) of the successfully resolved enantiomers contained basic functionality and a stereogenic carbon. However, the versatility of this sulfated cyclodextrin additive was also demonstrated by the fact that three atropisomers, 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate, 1.1'-binaphthyl-2,2'-diol, and Troger's base, and several neutral analytes were also successfully enantioresolved under these conditions. The separation mechanism seems to involve inclusion complexation.

Cyclodextrins↗

A sulfated cyclodextrin chiral stationary phase for high-performance liquid chromatography.

A new sulfated beta-cyclodextrin (degree of substitution approximately 13-14/cyclodextrin) bonded chiral stationary phase (CSP) for high-performance liquid chromatography is introduced. The novel CSP was used to resolve a number of enantiomeric pairs, including antihistamines, antidepressants and phenylhydantoins, using HPLC under a variety of mobile phase conditions. Among the 33 analytes successfully enantioresolved, all but six had some amine functionality. Generally, the best separations were obtained for analytes in which the stereogenic center was either incorporated in a ring system or positioned between two aromatic rings. Retention seemed to arise from either inclusion complexation or electrostatic interactions. Ionic strength and pH were found to influence chiral recognition.

Chromatography, High Pressure Liquid↗

Sulfated cyclodextrins for the chiral separations of catecholamines and related compounds in the reversed electrophoretic polarity mode.

The utility of negatively charged sulfated cyclodextrins (SCD) as chiral additives (CA) in capillary electrophoresis (CE) was studied in the chiral resolution of several compounds of pharmaceutical interest, including catecholamines such as norepinephrine, epinephrine, DOPA and their precursors, phenylalanine, and tyrosine. Experiments were conducted using 10 mM sodium phosphate monobasic solution and 2% SCD adjusted to pH 3.2 with phosphoric acid. Chiral recognition mechanisms were explored using structurally related analytes including basic, acidic, and neutral compounds as well as 3,5-dinitrobenzoyl phenylglycine, phenylalanine, and homophenylalanine. The advantage of the reversed electrophoretic polarity mode for the enantioresolution of these compounds is also discussed.

Binding Sites↗

Comparison of 1-(1-naphthyl)ethylcarbamate derivatives of a carbohydrate bonded chiral stationary phase.

New chiral stationary bonded phases (CSPs) based on derivatized malto-oligosaccharides are reported. Chiral separations are reported for 3,5-dinitrobenzoyl-derivatized amines and amino acids as well as some 3,5-dinitrophenylcarbamoylated alcohols. These new CSPs incorporate a 1-(1-naphthyl)ethylcarbamate (NEC) moiety which introduces additional stereogenicity and provides a useful probe for investigating chiral recognition. The elution order, when known, was dependent upon the configuration of the NEC substituent; that is, the S enantiomers were retained longest on the S column and the R enantiomers were retained longest on the R column. Elution order from the RS column, when known, was the same as that observed for the S column. In most cases, retention correlated with bonded ligand concentration on the silica substrate. In general, the best enantioselectivities and resolution were obtained on the S column.

Alcohols↗

Capillary zone electrophoresis study of naphthylethylcarbamoylated beta-cyclodextrins.

Regiospecifically monosubstituted 1-(1-naphthyl)ethylcarbamoylated beta-cyclodextrins (NEC-beta-CDs) were successfully employed as chiral additives in capillary zone electrophoresis (CZE) to achieve chiral separation of N-(3,5-dinitrobenzoyl)phenylglycine (3,5-DNB-PG), phenylalanine (3,5-DNB-PA), and homophenylalanine (3,5-DNB-HPA). The enantioselectivity of the various site-substituted NEC-beta-CDs in CZE was compared with that of native beta-CD. Complexation constants of the three 3,5-DNB amino acids with beta-CD were determined from the CZE results: 3,5-DNB-L-HPA, 473 +/- 9 M-1; 3,5-DNB-D-HPA, 460 +/- 10 M-1; 3,5-DNB-L-PA, 260 +/- 4 M-1; 3,5-DNB-D-PA, 161 +/- 3 M-1; and 3,5-DNB-D, L-PG, 43 +/- 4 M-1.

Aminobutyrates↗

Heparin: a chiral mobile-phase additive for capillary zone electrophoresis.

Heparin, a naturally occuring, polydisperse, polyanionic glycosaminoglycan, was investigated as a chiral selector for capillary zone electrophoresis. Baseline separations were obtained for a variety of underivatized drugs including antimalarials and antihistamines. Analysis was carried out at a pH of 4.5 or 5 using 2% heparin (w/w) in a 10 mM phosphate buffer under an applied voltage of 15 kV. All of the solutes successfully resolved contained at least two nitrogens with one of the nitrogens incorporated in a heterocyclic aromatic ring. Further, successful enantioresolution seemed to require that the nitrogens be somewhat distant from each other in the molecule. The results suggest that, in addition to electrostatic interactions, solute size may also play a role in the heparin chiral recognition mechanism.

Antimalarials↗

Regioselective synthesis and characterization of naphthylethylcarbamoyl-beta-cyclodextrins.

Regioselective reactions of 1-(1-naphthyl)ethyl isocyanate (NEIC) with beta-cyclodextrin (beta-CD) were studied with and without NaH activation of beta-CD in N,N-dimethylformamide (DMF) and pyridine. All six possible monosubstituted CD products were separated and characterized by proton NMR. Primary substitution product predominates when the reaction was carried out under reflux condition in pyridine without NaH activation. The C-2 substitution product predominates when the reaction was carried out in DMF. Conversion of 2-O-(1-(1-naphthyl)ethylcarbamoyl)-beta-CD to 6-O-(1-(1-naphthyl)ethylcarbamoyl)-beta-CD was observed when NaH was used to activate hydroxyl groups of CD.

Carbamates↗

Determination of the enantiomeric purity of scopolamine isolated from plant extract using achiral/chiral coupled column chromatography.

The optical purity of scopolamine derived from Datura sanguinea was determined using coupled column chromatography. A C18 column was used to separate scopolamine from the additional alkaloids and other biological material present in the vegetal extract. The C18 column was coupled through a six-port switching valve to two beta-cyclodextrin columns in series which were used to resolve the scopolamine enantiomers. A single acetylated beta-cyclodextrin column gives equivalent results to the native cyclodextrin columns because of slightly higher enantioselectivity for scopolamine. A multistep extraction procedure is used to isolate scopolamine from the vegetal material. 4-6% of the scopolamine in the final extract was found to be the d enantiomer. Sample extracts as well as commercial scopolamine hydrobromide were treated under various conditions commonly encountered during typical commercial extraction procedures and analyzed in order to determine if the d enantiomer was present in the original material or if it was produced during the extraction process and, if so, determine which step and conditions contribute to racemization. Both the salt and the extract were found to be susceptible to racemization under basic conditions (greater than or equal to pH 9) although the extract appeared to be more susceptible than the salt. Tropic acid formed from the hydrolysis of scopolamine seemed to be completely racemized even though the remaining scopolamine was only partially racemized. Within experimental error, no d enantiomer was found in the original fresh plant material.

Acetylation↗

(S)-2-hydroxypropyl-beta-cyclodextrin, a new chiral stationary phase for reversed-phase liquid chromatography.

(S)-2- and (R,S)-2-hydroxypropyl-beta-cyclodextrin have been bonded to silica gel and evaluated as stationary phases for reversed-phase liquid chromatography. Stationary phases also were prepared on two silicas having different pore sizes and surface areas. Dissimilarities were observed in enantiomeric selectivities between these columns and also between these and the native beta-cyclodextrin columns. With the exception of compounds 5 and 10, all other racemates reported here which have been successfully resolved on the new phases are enantiomers which have not been previously reported as separated on the beta-cyclodextrin stationary phase. In some cases, there were also differences in enantioselectivities observed between the (S)- and the (R,S)-hydroxypropyl-beta-cyclodextrin phases on the same silica. The results are discussed in terms of the retention mechanism and compared to results reported earlier for beta-cyclodextrin columns.

Chemical Phenomena↗

Separation of carotenes on cyclodextrin-bonded phases.

The separation of carotenoids and retinoids on a beta-cyclodextrin-bonded stationary phase with conventional mobile phases is reported. Compounds studied include beta-carotene (all-trans), 15,15'-cis-beta-carotene, 7,8,7',8'-dihydro-beta-carotene, alpha-carotene, lycopene, lutein, zeaxanthin, retinal, retinol, retinol palmitate and retinol acetate. The best resolution of carotenes was obtained with low concentrations (less than or equal to 1%) of polar solvents (e.g., 2-propanol or ethyl acetate) in hexane or cyclohexane. Xanthophylls required much higher concentrations of polar solvents. The best solvent for the resolution of lutein and zeaxanthin was found to be dichloromethane. The resolution of cis/trans-isomers and the tentative identification of other isomers present in newly synthesized carotenoid standards is also reported. All-trans-isomers were found to be eluted before cis-isomers.

Carotenoids↗

Separation of benzodiazepines on a new carbohydrate-based chiral stationary phase for HPLC.

Several benzodiazepines were enantioresolved on a new carbohydrate chiral stationary phase based on maltooligosaccharides. The role of organic modifier, ionic strength, pH and temperature are examined and the results are discussed. In general, selectivity and retention were found to decrease with increasing organic modifier concentration. The appearance of two peaks for diazepam suggests that the reported separations are based on separation of conformers and was supported using circular dichroism detection. Counterintuitively, resolution and enantioselectivity were found to improve with increasing temperatures.

Benzodiazepines↗