Adsorption Equilibria of Benzodiazepines on a Hybrid Polymeric Chiral Stationary Phase

A combined kinetic and thermodynamic approach for the interpretation of continuous-flow heterogeneous catalytic processes

The heterogeneous proline-catalyzed aldol reaction was investigated under continuous-flow conditions by means of a packed-bed microreactor. Reaction-progress kinetic analysis (RPKA) was used in combination with nonlinear chromatography for the interpretation, under synthetically relevant conditions, of important mechanistic aspects of the heterogeneous catalytic process at a molecular level. The information gathered by RPKA and nonlinear chromatography proved to be highly complementary and allowed for the assessment of optimal operating variables. In particular, the determination of the rate-determining step was pivotal for optimizing the feed composition. On the other hand, the competitive product inhibition was responsible for the unexpected decrease in the reaction yield following an apparently obvious variation in the feed composition. The study was facilitated by a suitable 2D instrumental arrangement for simultaneous flow reaction and online flow-injection analysis.

Absolute configuration and biological profile of two thiazinooxadiazol-3-ones with L-type calcium channel activity: a study of the structural effects

In the framework of our interest in racemic thiazinooxadiazol-3-ones we determined the absolute configuration and the biological activity as L-type calcium channel blockers of two compounds that differ in the length of the acetal chain, which could affect the pharmacological profile. We observed an interesting inversion of the stereoselectivity, with the activity residing on the R-form for a short chain compound (n = 1) and on the S-form for a long chain one (n = 12). The length of the linear acetal chain appears to be able to invert the stereoselectivity of such a class of compounds, and in silico simulations suggested that this different behaviour might be explained by different hydrophilic and hydrophobic interactions with the binding site.

Adsorption Mechanisms in Normal-Phase Chromatography. Mobile-Phase Modifier Adsorption from Dilute Solutions and Displacement Effect

The adsorption equilibria of methanol from diluted solutions of the alcohol in dichloromethane (the weak solvent of the binary mixture) have been investigated in normal-phase conditions on an hybrid polymeric chiral stationary phase. This was done by analyzing the shapes of nonlinear perturbations of methanol, measured with a refractive index detector from a column in equilibrium with pure dichloromethane. Experiments have been set up to demonstrate the displacement effect, predictable from the adsorption data, by a nonlinear pulse of methanol on a racemic mixture of temazepam injected at trace level. A most used method in fundamental studies of band behavior in chromatography has been used for modeling the system. An extraordinary increase of the apparent column efficiency has been observed due to the displacement phenomenon, with the peak of the second eluted enantiomer noticeably higher and narrower than that of the first eluted component.

Heterogeneity of adsorption mechanisms in chiral normal-phase liquid chromatography

The adsorption equilibria of two commonly employed strong mobile phase modifiers, ethyl acetate and 2-propanol, on a polysaccharide-based chiral stationary phase have been studied by modeling nonlinear perturbation peaks measured after equilibration of the column with hexane (the weak component of the binary mixture). The investigation of adsorption processes from dilute solutions for species that are strongly retained on the stationary phase could be performed by this approach. On the opposite, limits of traditional linear perturbation technique for isotherm determination, in presence of strong interactions, have been evidenced. Alcohol adsorption has been modeled by a single Langmuir isotherm, while the ester has required a BiLangmuir model. These findings have found to be in a semi-quantitative agreement with available spectroscopic data about 2-propanol and ethyl acetate adsorption on thin silica sol-gel films in contact with a weak solvent. Experimental features observed for racemic separation on polysaccharide-based chiral stationary phases, such as the dependence of the separation factor on the amount and type of the employed additive, have been explained in light of these measurements.

Empirical development of a binary adsorption isotherm based on the single-component isotherms in the framework of a two-site model

The adsorption behavior of mixtures of the enantiomers of 2,2,2-(trifluoro)-1-(9-anthryl)-ethanol (TFAE) on a quinidine carbamate bonded stationary phase was studied as an example of competitive binary adsorption on a Pirkle-type chiral adsorbent. A model of the adsorption isotherms is proposed and discussed. Binary adsorption isotherms derived by combination of the single-component isotherms of the two enantiomers in the framework of the classical bi-Langmuir model allow the correct prediction of the retention times of the elution bands of the components of binary mixtures but fail properly to predict the separation of the two enantiomers. Use of a quadratic model was needed to improve the agreement between calculated and experimental chromatograms of binary mixtures. The existence of a third type of adsorption sites besides the high-energy enantioselective and the low-energy nonselective sites was assumed. These sites have a low interaction energy, exhibit some affinity toward (S)-TFAE, but none toward (R)-TFAE.

Twenty years of research on silica-based chiral stationary phases

This review provides an overview of twenty years of pioneering work (from 1985 to 2005) of our research group in the preparation and application of enantioselective packing materials for HPLC. After a brief introduction to the rational design of a new chiral stationary phase, a detailed presentation in chronological order of appearance in the literature is given of the currently developed repertoire of chiral stationary phases and their typical applications. Emphasis is placed on the different synthetic strategies exploited to obtain highly efficient, stable, and versatile chiral stationary phases.

Adsorption of the enantiomers of 3-chloro-1-phenyl-propanol on silica-bonded chiral quinidine carbamate

The interactions of 3-chloro-1-phenyl-propanol with a quinidine carbamate-bonded chiral stationary phase under NPLC conditions were studied by measuring the adsorption isotherm data of its enantiomers by frontal analysis, modeling these data with a suitable isotherm model, and comparing the experimental overloaded elution band profiles with those calculated with this isotherm and the equilibrium dispersive model of liquid chromatography. The affinity energy distribution was calculated from the adsorption isotherm data. The results show that the surface of the adsorbent is heterogeneous and exhibits a bimodal adsorption energy distribution. This fact is interpreted in terms of the presence of two different types of adsorption sites on the stationary phase, nonselective and enantioselective sites. Albeit the bi-Langmuir isotherm model successfully accounts for the single-component data corresponding to both enantiomers, the competitive bi-Langmuir isotherm model does not allow an accurate prediction of the overloaded band profiles of the racemic mixture. Thermodynamic data are drawn for explanation. Some aspects of the retention mechanism are discussed in the light of the data obtained.