Daniel W. Armstrong

Facile liquid chromatographic enantioresolution of native amino acids and peptides using a teicoplanin chiral stationary phase

The glycopeptide antibiotic teicoplanin is shown to be a highly effective stationary phase chiral selector for the resolution of underivatized amino-acid and imino-acid enantiomers. Fifty four of these compounds (including all chiral protein amino acids) as well as a number of dipeptides were resolved. Hydro-organic mobile phases are used and no buffers or added salts are needed in most cases. Hence the purified analytes are easily isolated in pure form, if needed, by evaporating of the solvent. The effect of pH, organic modifier type and amount are discussed. The enantioselective separation mechanism is examined using both molecular modeling and retention data. The strongest stereoselective interaction is for carboxy-terminated D-amino-acids. In case of peptides, it is not necessary for these to be a D-, D-, terminal sequence for strong interactions. In some cases, including Ala-Ala, the L-, D-, terminal sequence showed greater interaction with the teicoplanin chiral stationary phase.

Examination of the origin, variation, and proper use of expressions for the estimation of association constants by capillary electrophoresis

Over the last several decades a variety of techniques have been developed to determine apparent equilibrium constants for molecular association in solution (e.g., to micelles, proteins, cyclodextrins, antibiotics, etc.). The relationships describing binding isotherms appear in several forms and have been given several different names. It is well known that most of these expressions are closely related and that some may be more advantageous than others for experimental or statistical reasons. In the case of electrophoresis, association constants are calculated from the relationship between ligand concentration and the measured electrophoretic mobility of the solute. This relationship has appeared in many forms that have been used numerous times at least since 1951. Recently they have reappeared in identical or slightly rearranged versions in several capillary electrophoresis (CE) studies. Some of these methods require the measurement of the electrophoretic mobility of the solute-ligand complex, a value that often cannot be accurately measured. Some systems require correction or normalization procedures in order to negate any changes in solute mobility that are not due to binding. The relationship between the various expressions that can be used to calculate binding constants with CE is shown. The advantages, limitations and proper use of the various approaches are discussed. Examples are given for both achiral and chiral analytes.

Use of an Aqueous Micellar Mobile Phase for Separation of Phenols and Polynuclear Aromatic Hydrocarbons via HPLC

Abstract An aqueous solution of sodium dodecyl sulfate (SDS) micelles is shown to be a novel, highly effective mobile phase in high performance liquid chromatography. Using a reverse phase column, nine phenols and two polynuclear aromatic hydrocarbons were easily separated. The possible advantages and disadvantages of aqueous micellar solutions over traditional organic and mixed solvent mobile phases is discussed.

Methods for the determination of binding constants by capillary electrophoresis

Apparent equilibrium constants for molecular association (e.g., association constants, binding constants, dissociation constants, partition coefficients) can be determined with a variety of different capillary electrophoresis (CE) approaches. In many cases, the investigated association behavior is between a smaller molecule or ion (i.e., the solute, drug, or analyte of interest) and a larger entity (e.g., proteins, micelles, polymers, chiral selectors such as cyclodextrins, etc.). Each experimental approach has advantages and disadvantages. Frequently, it is the nature of the system being evaluated that determines the optimal experimental approach. Six different CE‐based techniques for evaluating binding constants are reviewed. Examples of each method, and recent references on its use are given.

Chiral Stationary Phases for High Performance Liquid Chromatographic Separation of Enantiomers: A Mini-Review

Abstract There has been a proliferation of papers on the use of chiral stationary phases (CSPs) to separate optical isomers in high performance liquid chromatography. The chemistry, mechanism and stability of these CSPs can vary widely. Furthermore, the applicability, availability and cost of a CSP can mean the difference between its being of passing academic interest as opposed to a technique that could have a significant impact on science and technology. Six different classes of chiral stationary phases are examined and discussed including the new chiral cyclodextrin bonded phases. The separation mechanism, strengths and limitations of the CSPs are also considered whenever such information is available.

Improved Cyclodextrin Chiral Phases: A Comparison and Review

Abstract Cyclodextrin chiral phases have been shown to be widely applicable for the separation of enantiomers, diastereomers, structural isomers and routine compounds. Two innovations have recently occured in this field. First, the efficiency and selectivity of the β-cyclodextrin column has been improved. Second, compatable TLC plates which produce separations analogous to the columns have been developed. These results are discussed subsequent to a brief review of published work. In addition, the mechanism of separation on cyclodextrin bonded media, solvent effects, temperature effects and structural effects on chiral separations are considered.

(S)-2-Hydroxyprophyl-β-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.

Highly enantioselective HPLC separations using the covalently bonded macrocyclic antibiotic, ristocetin A, chiral stationary phase

The macrocyclic glycopeptide, ristocetin A, was covalently bonded to a silica gel support and evaluated as a liquid chromatographic (LC) chiral stationary phase (CSP). Over 230 racemates were resolved in either the reversed-phase mode, the normal-phase mode, or the polar-organic mode. The retention behavior and selectivity of this CSP were examined in each mode. Optimization of separations on this column is discussed. The ristocetin A CSP appeared to be complimentary to other glycopeptide CSPs (i.e., vancomycin and teicoplanin). Column stability was excellent. The CSP was not irreversibly altered when going from one mobile phase to another.

Characterization of phosphonium ionic liquids through a linear solvation energy relationship and their use as GLC stationary phases

In recent years, room temperature ionic liquids (RTILs) have proven to be of great interest to analytical chemists. One important development is the use of RTILs as highly thermally stable GLC stationary phases. To date, nearly all of the RTIL stationary phases have been nitrogen-based (ammonium, pyrrolidinium, imidazolium, etc.). In this work, eight new monocationic and three new dicationic phosphonium-based RTILs are used as gas–liquid chromatography (GLC) stationary phases. Inverse gas chromatography (GC) analyses are used to study the solvation properties of the phosphonium RTILs through a linear solvation energy model. This model describes the multiple solvation interactions that the phosphonium RTILs can undergo and is useful in understanding their properties. In addition, the phosphonium-based stationary phases are used to separate complex analyte mixtures by GLC. Results show that the small differences in the solvent properties of the phosphonium ILs compared with ammonium-based ILs will allow for different and unique separation selectivities. Also, the phosphonium-based stationary phases tend to be more thermally stable than nitrogen-based ILs, which is an advantage in many GC applications.

A New Approach for the Direct Resolution of Racemic Beta Adrenergic Blocking Agents by HPLC

Abstract Although beta-adrenergic blocking agents are a relatively new group of drugs, they have proven to be very useful in medical pharmacology. Since the successful introduction of propranolol (Inderal) a variety of analogous compounds have been developed. Most beta-blockers are racemic modifications and it is known that their enantiomers have different potencies and pharmacological effects. Hence, there has been considerable impetus for the production and study of the pure enantiomers. The chromatographic separation of these compounds has been dominated by the protein-based chiral stationary phases. Although selective, protein columns have limited capacity and stability. An efficient, alternative method has been found that does not suffer from the limitations of the protein phases. By using an unusual mobile phase consisting of a mixture of polar organic solvents in conjunction with the original native cyclodextrin bonded phase, the following compounds were resolved: propranolol, metoprolol, timolol, ...