Origin of peak asymmetry and the effect of temperature on solute retention in enantiomer separations on imprinted chiral stationary phases

C.I. Acid Black 1 transfer from dilute solution to perlite framework in organic waste management

Dyes, considered as toxic and persistent pollutants, must be removed from organic wastes prior to their composting and application in sustainable agriculture. Azo dyes, capable of altering the physicochemical properties of soil, are difficult to expel by conventional wastewater treatments. C.I. Acid Black 1 (AB 1), a sulfonated azo dye, inhibits nitrification and ammonification in the soil, lessens the nitrogen use efficacy in crop production and passes substantially unaltered through an activated sludge process. The retention of C.I. Acid Black 1 by raw and expanded perlite was investigated in order to examine the potential effectiveness of this aluminosilicate material toward organic waste cleanup. Dye adsorption proved spontaneous and endothermic in nature, increasing with temperature for both perlites. Expanded perlite having a more open structure exhibited a better performance compared to the raw material. Several of the most widely recognized two-parameter theoretical models, i.e., Langmuir, Freundlich, Temkin, Brunauer-Emmett-Teller (BET), Harkins-Jura, Halsey, Henderson, and Smith, were applied to reveal physicochemical features characterizing the adsorption. The Langmuir, Freundlich, Temkin, BET, Henderson, and Smith equations best fitted experimental data indicating that the adsorption of anionic dye on perlites is controlled by their surface, i.e., non-uniformity in structure and charge. This heterogeneity of surface is considered responsible for promoting specific dye adsorption areas creating dye "islands" with local dye supersaturations.

Sensing by Molecularly Imprinted Polymer: Evaluation of the Binding Properties with Different Techniques

The possibility of investigating the binding properties of the same molecularly imprinted polymer (MIP), most probably heterogeneous, at various concentration levels by different methods such as batch equilibration and sensing, is examined, considering two kinds of sensors, based respectively on electrochemical and surface plasmon resonance (SPR) transduction. As a proof of principle, the considered MIP was obtained by non-covalent molecular imprinting of 2-furaldehyde (2-FAL). It has been found that different concentration ranges of 2-FAL in aqueous matrices can be measured by the two sensing methods. The SPR sensor responds in a concentration range from 1 × 10⁻⁴ M down to about 1 × 10⁻⁷ M, while the electrochemical sensor from about 5 × 10⁻⁶ M up to about 9 × 10⁻³ M. The binding isotherms have been fit to the Langmuir adsorption model, in order to evaluate the association constant. Three kinds of sites with different affinity for 2-FAL have been detected. The sites at low affinity are similar to the interaction sites of the corresponding NIP since they have a similar association constant. This is near to the affinity evaluated by batch equilibration too. The same association constant has been evaluated in the same concentration range. The sensing methods have been demonstrated to be very convenient for the characterization of the binding properties of MIP in comparison to the batch equilibration, in terms of reproducibility and low amount of material required for the investigation.

Characterization of the Binding Properties of Molecularly Imprinted Polymers

The defining characteristic of the binding sites of any particular molecularly imprinted material is heterogeneity: that is, they are not all identical. Nonetheless, it is useful to study their fundamental binding properties, and to obtain average properties. In particular, it has been instructive to compare the binding properties of imprinted and non-imprinted materials. This chapter begins by considering the origins of this site heterogeneity. Next, the properties of interest of imprinted binding sites are described in brief: affinity, selectivity, and kinetics. The binding/adsorption isotherm, the graph of concentration of analyte bound to a MIP versus concentration of free analyte at equilibrium, over a range of total concentrations, is described in some detail. Following this, the techniques for studying the imprinted sites are described (batch-binding assays, radioligand binding assays, zonal chromatography, frontal chromatography, calorimetry, and others). Thereafter, the parameters that influence affinity, selectivity and kinetics are discussed (solvent, modifiers of organic solvents, pH of aqueous solvents, temperature). Finally, mathematical attempts to fit the adsorption isotherms for imprinted materials, so as to obtain information about the range of binding affinities characterizing the imprinted sites, are summarized.

Preparation and evaluation of monolithic molecularly imprinted stationary phase for S-naproxen

An S-naproxen (S-NAP) molecularly imprinted monolithic stationary phase (MIMSP) with specific recognition for S-NAP and naproxen (NAP) was prepared by in situ technique, utilizing 4-vinylpridine (4-VP) as a function monomer, ethylene glycol dimethacrylate (EDMA) as a cross-linking agent, and low-polar solvents (toluene and dodecanol) as porogenic solvents. The selectivity of the polymers for S-NAP and NAP was evaluated by high performance liquid chromatography (HPLC). The binding characteristics were tested by Scatchard analysis. Racemic NAP could be specifically separated to some extent. At the same time, NAP could be separated from ibuprofen under optimized conditions. Scatchard analysis showed that two classes of binding sites existed in the S-NAP-imprinted polymers, with their dissociation constants estimated to be 1.045 and 5.496 μM, respectively. The results demonstrate that S-NAP and NAP can be recognized specifically on the obtained MIMSP.

Preparation and evaluation of molecularly imprinted polymer liquid chromatography column for the separation of Cathine enantiomers

In this study molecular imprinting technology was employed to prepare a specific affinity sorbent for the resolution of Cathine, a chiral drug product. The molecularly imprinted polymer (MIP) was prepared by non-covalent molecular imprinting with either (+) or (-)-Cathine (threo-2-amino-1-hydroxy-1-phenyl propane; norpseudoephedrine) as the template. Methacrylic acid and ethylene glycol di-methacrylate were copolymerized in the presence of the template molecule. The bulk polymerization was carried out in chloroform with 2,2'-azobisisobutyronitrile as the initiator, at 5 °C and under UV radiation. The resulting MIP was ground into powders, which were slurry packed into analytical columns. After removal of template molecules, the MIP-packed columns were found to be effective for the resolution of (±)-Cathine racemates. The separation factor for the enantiomers ranged between 1.5 and 2.4 when the column was packed with MIP prepared with (+)-Cathine as the template. A separation factor ranging from 1.6 to 2.9 could be achieved from the column packed with MIP, prepared with (-)-Cathine as the template. Although the separation factor was higher with that previously obtained from reversed-phase column chromatography following derivatization with a chiral agent, elution peaks were broader due to the heterogeneity of binding sites on MIP particles and the possible non-specific interaction.

Characterization of MIPs Using Heterogeneous Binding Models

New methods are presented for characterizing MIPs. These methods address the problems of quantitatively comparing the binding properties of different MIPs. Heterogeneous binding models were applied to MIPs based on an exponentially decaying distribution known as the Freundlich isotherm. The Freundlich isotherm was found to accurately model the binding isotherm of the majority of non-covalently imprinted MIPs. Using this model the experimental binding isotherm can be fit in log-log form to a linear equation from which the fitting parameters can be used to plot a quantitative affinity distribution which is a plot of the number of sites with respect to the binding constant of those sites. Comparison of MIPs using this methodology allowed for simpler and more accurate assessment of the binding properties than by previous methods such as the limiting slopes analysese of curved Scatchard plots.

Chiral Resolution of Racemic (±)-1,1'Bi (2-Naphthol) by Use of Molecularly Imprinted Polymers

Polymers imprinted with (S)-(-)-1,1’-bi (2-naphthol) and (R)-(+)-1,1’-bi (2-naphthol) have been prepared by non-covalent imprinting. A combinational procedure was used to optimize the functional monomer and crosslinker. A copolymer of 2-vinylpyridine and divinylbenzene resulted in the best chiral recognition. The ratio of template to functional monomer and solvent in the pre-polymerization mixture were also optimized. The imprinted polymers were used as stationary phases in high-performance liquid chromatography (HPLC). The molecularly imprinted polymers (MIPs) were more selective when prepared using a less polar solvent. Effective separations of the enantiomers of racemic (±)-1, 1’-Bi (2-naphthol) were achieved by use of acetonitrile as mobile phase; no cross-selectivity was observed. Interactions between functional monomers and template were investigated by 1H NMR spectroscopy. The results suggest that hydrogen-bonding between the functional monomer and the template and π-π stacking interaction between the cross-linker and the template may contribute to chiral recognition.

Binding site characteristics of 17β-estradiol imprinted polymers

The variety of applications utilizing molecularly imprinted polymers (MIPs) requires synthetic strategies yielding different MIP formats including films, irregular particles, or spheres, along with precise knowledge on the specific material characteristics, such as binding capacity and binding efficiency of these materials. In response to this demand, MIPs are prepared in different formats by variation of the polymerization methodology. It is commonly agreed that micro- and sub-microspheres are particularly advantageous MIP formats, due to their monodispersity and facile synthesis procedures in contrast to conventional imprinted polymers prepared by bulk polymerization. However, the differences in actual rebinding characteristics of different MIP formats based on molecular interactions under a variety of binding/rebinding conditions have not been studied in detail to date. Consequently, the present work details an analytical strategy generically applicable to MIP systems for rebinding studies including equilibrium binding, non-equilibrium binding, and release experiments enabling more profound understanding on the molecular interactions between the imprinted materials and the template molecules. In this study, three MIP formats were considered for the same template molecule, 17beta-estradiol: irregularly shaped particulate polymers prepared by bulk polymerization and grinding, microspheres, and sub-microspheres. The latter two formats were synthesized via precipitation polymerization using different processing strategies. The morphologies and porosities of the resulting imprinted materials were characterized by scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) analysis, respectively. The obtained results indicate that microspheres prepared by precipitation polymerization provide superior rebinding properties during equilibrium binding in contrast to bulk polymers and sub-microspheres, and that the rebinding properties are different during equilibrium binding versus non-equilibrium binding. The median binding affinity constant determined during non-equilibrium rebinding is higher than the values obtained from equilibrium rebinding. Furthermore, the binding site distribution appears more homogeneous thief derived from non-equilibrium rebinding, as reflected in a heterogeneity index of m=0.725. Moreover, it is hypothesized that the specific interactions between template and monomers are related to the porosity of the imprinted polymers, which implies that the amount of binding sites and the pore sized distribution of the imprinted materials are a critical factor in achieving the desired MIP performance in various analytical applications. The BET results indicate that particles prepared with lower cross-linker-to-template ratio have a reduced surface area. Furthermore, it can be expected that there are less specific binding sites available at particles with reduced surface area and pore volume given similar distribution of the binding sites, as confirmed by the equilibrium binding isotherm studies. The pore size distribution results reveal that control of the pore size in the range of 100-180 A is essential to obtain the desired retention properties and Gaussian peak shape during HPLC analysis of small molecules.

Chiral discrimination and enantioselective analysis of drugs: an overview

Molecular recognition of different enantiomers of a drug has become of increasing importance in the last decade due to the racemic switch strategy adapted by the pharmaceutical industry. Different analytical techniques to carry out enantioselective analysis of chiral compounds have been suggested in the literature. In the following, a brief overview of different techniques used for enantioselective analysis is given. Challenging aspects of these techniques, such as the quality of analytical information received from each technique, advantages, and disadvantages are discussed. Alternatives (enantioselective membranes, amperometric biosensors, molecularly imprinted polymers (MIPs)), capable of meeting the requirements of industrial processes, in terms of productivity, cost-effectiveness, and environmental issues are critically reviewed.

Monodispersed, molecularly imprinted polymers as affinity-based chromatography media

This review article deals with preparation methods for spherical and monodispersed molecularly imprinted polymers (MIPs) in micrometer sizes. Those methods include suspension polymerization in water, liquid perfluorocarbon and mineral oil, seed polymerization and dispersion/precipitation polymerization. The other methods are the use of beaded materials such as a spherical silica or organic polymer for grafting MIP phases onto the surfaces of porous materials or filling the pores of silica with MIPs followed by dissolution of the silica. Furthermore, applications of MIP microspheres as affinity-based chromatography media, HPLC stationary phases and solid-phase extraction media, will be discussed for pharmaceutical, biomedical and environmental analysis.

Enantiomeric separation using an l-RNA aptamer as chiral additive in partial-filling capillary electrophoresis

In this paper, we report the chiral resolution of arginine using an anti-arginine l-RNA aptamer chiral selector in partial-filling CE. The effects of the capillary temperature, sample load, and aptamer plug length on the enantiomeric separation were assessed. Very high chiral resolving capability was observed at low or moderate capillary temperatures (the target peak being not detected in the separation window), whereas the practical chiral resolution was achieved only at high enough temperatures (50-60 degrees C). Over this high-temperature range, the electrophoretic behavior of the target enantiomer appeared to result from a combination of binding site heterogeneity, slow desorption kinetics, and concentration overload of aptamer binding sites. From additional thermal UV melting experiments, three RNA conformations were identified for the 50-60 degrees C temperatures. It was suggested that the presence of these different RNA conformations was a plausible source of the binding site heterogeneity.