Reaction kinetics and kinetic processes in modern liquid chromatographic reactors

On the Arrival Time Distribution of Reacting Systems in Ion Mobility Spectrometry

Ion mobility spectrometry (IMS) is a widely used gas-phase separation technique, particularly when coupled with mass spectrometry (MS). Modern IMS instruments often apply elevated reduced field strengths for improved ion separation and ion focusing. These alter the collision dynamics and further drive ion reaction processes that can change the analyte's structure. As a result, the measured arrival time distribution (ATD) can change with the applied reduced field strengths. In this work, we systematically study how the ion collision dynamics and the ion reaction dynamics, as a function of the reduced field strength, can alter the ATD. To this end, we investigate 2,6-di-tert-butylpyridine, methanol, and ethyl acetate using a home-built drift tube IMS coupled to a home-built MS and extensive first-principles Monte Carlo modeling. We show how elevated reduced field strengths can actually lower resolving power through increased ion diffusion and how the field dependency of the ion mobility can introduce uncertainties to collision cross sections (CCS) calculated from the measured mobilities. On top of the collision dynamics, we show how chemical transformation processes that alter the analyte's CCS, e.g., dynamic clustering or fragmentation, can lead to broadened, shifted, or non-Gaussian ATDs and how sensitive these processes are to the applied field strengths. We highlight how first-principles ion dynamics simulations can help to understand and even harness the mentioned effects.

An HPLC chromatographic reactor approach for investigating the hydrolytic stability of a pharmaceutical compound

The solution-phase hydrolysis kinetics of the Aprepitant (Emend) prodrug, Fosaprepitant Dimeglumine, were investigated using an HPLC chromatographic reactor approach. The term 'chromatographic reactor' refers to the use of an analytical-scale column as both a flow-through reactor and, simultaneously, as separation medium for the reactant(s) and product(s). Recently, we reported a novel mathematical treatment for the kinetic data obtained from chromatographic reactors, which we believe is superior to other treatments in terms of its accuracy, robustness and ease of implementation. In this work, we demonstrate that our treatment may be applied equally well to HPLC reactors, as previously we studied only GC reactors. It is found that the hydrolysis of Fosaprepitant Dimeglumine (FD) has an apparent activation energy of 107 kJ/mol when the reaction is investigated on-column, using the gradient elution conditions of the validated HPLC impurity profile method for this compound. For comparison, the activation energy determined for the same reaction occurring in a quiescent solution consisting of a fixed ratio of acetonitrile-0.1% v/v aqueous H3PO4 (50:50, v/v) is 91 kJ/mol, calculated using direct application of the Arrhenius equation. The data presented show that, when used as a screening tool, chromatographic reactors may be feasible for use in the pharmaceutical industry to quickly gauge the relative stabilities of various compounds with similar degradation pathways.

Stochastic simulation of reactive separations in capillary electrophoresis

A stochastic (Monte Carlo) simulation is used to investigate thermodynamic and kinetic contributions from the reversible A <--> B reaction in capillary electrophoresis (CE). The effects of equilibrium constant, rate constant, and electrophoretic mobility on the molecular zone profiles and the corresponding statistical moments are evaluated. As the reaction approaches steady state, the velocity of the zone is governed by the equilibrium constant and the electrophoretic mobilities of the reacting molecules. When the equilibrium constant is less than unity, the mean zone velocity is more similar to that of the reactant A. Conversely, when the equilibrium constant is greater than unity, the velocity is more similar to that of the product B. The extent of zone-broadening and asymmetry at steady state is dependent upon the equilibrium constant, the characteristic reaction lifetime, and the electrophoretic mobility difference between reacting molecules. If all other parameters are held constant, the plate height is greatest and skew is least when the equilibrium constant is unity. The plate height increases linearly with the characteristic reaction lifetime and electrophoretic mobility difference, whereas the skew is independent of these parameters. These conclusions have important implications for the elucidation of thermodynamic and kinetic information from experimental data.

Analytical solution of the ideal, nonlinear model of reaction chromatography for a reaction A-->B and a parabolic isotherm

The analytical solution of the ideal, nonlinear model of reaction chromatography for the simplest possible chemical reaction, A-->B and with a parabolic isotherm for the reagent, is derived for two types of boundary conditions, the injection of a rectangular concentration pulse of finite width (elution) and that of an instantaneous concentration jump (Riemann problem or breakthrough curve). The areas of the profiles of the reagent and of the product of the reaction are calculated in both the ideal and the nonideal cases. The effects of the nonlinear behavior of the equilibrium isotherm and of axial dispersion on the reagent profile are discussed using analytical and numerical methods.

Kinetic analysis and subambient temperature on-line on-column derivatization of an active aldehyde

The chromatographic analysis of aldehydes under typical reversed-phase conditions may be a challenging task due to an equilibrium process leading to the formation of a gem diol species regardless of acidic or basic conditions. Initially, a reversed-phase HPLC gradient elution was developed to determine the amount of a n acetylenic aldehyde in a reaction mixture. Significant fronting was observed under acidic and basic conditions even at -5 degrees C. In order to circumvent this problem, a reversed-phase HPLC gradient method on a C18 column at subambient temperature was developed using diethylamine as a mobile phase additive for on-line on-column derivatization of the aldehyde moiety. The on-line on-column reaction rate for the derivatization of the aldehyde with diethylamine was determined as a function of column temperature. An Arrhenius plot was constructed and the activation energy was calculated. The chromatographic behavior of the derivatized acetylenic aldehyde and products formed in-situ in the chromatographic system were studied at various temperatures ranging from -10 to 60 degrees C. It was found that the reaction products could be controlled by adjusting the column temperature. Different reaction pathways were identified as a function of temperature. The products and the reaction pathways were characterized by NMR, LC-MS and UV spectra.

Chromatographic reactors based on biological activity

In the last decade there were many papers published on the study of enzyme catalyzed reactions performed in so-called chromatographic reactors. The attractive feature of such systems is that during the course of the reaction the compounds are already separated, which can drive the reaction beyond the thermodynamic equilibrium as well as remove putative inhibitors. In this chapter, an overview of such chromatographic bioreactor systems is given. Besides, some immobilization techniques to improve enzyme activity are discussed together with modern chromatographic supports with improved hydrodynamic characteristics to be used in this context.

Centrifugal partition chromatographic reaction for the production of chiral amino acids

The use of a centrifugal partition chromatographic reactor is investigated for the production of chiral amino acids from racemic mixtures. Chirally selective enzymatic hydrolysis of N-acetyl-L-methionine into acetic acid and L-methionine was carried out in the chromatographic reactor to demonstrate the concept of integrated reaction and separation in centrifugal partition chromatography (CPC). The products L-methionine and acetic acid, as well as the unconverted substrate, N-acetyl-D-methionine are obtained separately. An aqueous two-phase system, consisting of PEG 600, potassium phosphate and water was successfully applied as liquid-liquid two-phase system in CPC. A model is presented, which describes the reaction chromatograms on the basis of the independently measured partition and mass transfer coefficients of the individual (reacting) components. The model appears to be a valuable tool for optimizing the reaction-separation process.

Extended binding sites of cyclophilin as revealed by the interaction with HIV-1 Gag polyprotein derived oligopeptides

Oligopeptides derived from the gag polyprotein (Pr55gag) of human immunodeficiency virus type 1 (HIV-1) segment were used to evaluate the extension of the putative binding region for the complex of Pr55gag and the human cytosolic peptidyl prolyl cis/trans isomerase (PPIase) 18 kDa cyclophilin (Cyp18). Five N-terminally acetylated, C-terminally amidated oligopeptides containing one (HIV-1 Gag218-224; 1), two (HIV-1 Gag218-226 and HIV-1 Gag217-224; 2 and 3, respectively), three (HIV-1 Gag217-226; 4) or four (HIV-1 Gag213-237; 5) proline residues were synthesized. Using competition experiments with a standard substrate the binding affinities to Cypl8 of the synthesized peptides were determined. The IC50 value of 184 microM for the 25-mer peptide 5 was fivefold or more lower than those of the peptides 1-4 lacking one or more prolines. Failure of competition in assays containing enzymes of other PPIase families by millimolar concentrations of 5 revealed a Cyp18 specific interaction involving the active site of the enzyme. In its far UV circular dichroism, aqueous solutions of 5 display properties of random coil conformation, but spectra were also consistent with a small contribution of proline specific secondary structures. However, a proline-rich peptide typical of forming left-handed polyproline II helices did not compete for the active site of Cypl8. The results demonstrate that the putative binding region of HIV-1 gag polyprotein has a certain degree of binding affinity to the PPIase site of Cyp18, and may add a previously unrecognized topological component to the known subsite specificity of cyclophilins.

Solid-phase derivatization reactions for biomedical liquid chromatography

Polymeric reagents have been developed for performing off- and on-line derivatizations of numerous organic analytes in HPLC-detection modes. Such reagents utilize ionic or covalent attachment of labile tags that possess specific detector enhancement properties: ultraviolet, electrochemical, fluorescence, and so forth. Specific synthetic procedures have evolved to generate various linkages of the tag to the underlying, polymeric support, usually involving activated ester connections (leashes). The polymer itself may play a number of roles in the nature of the overall reactions, such as hydrophobic-hydrophillic exclusion, pore size restriction, stabilization of the attachment leashes, and protection of the tags from hydrolysis in aqueous media. The basic, underlying chemistry of polymeric reagents has evolved to the point where it is possible to engineer the polymer support itself, the attachment leash, and the various tags that are then transferred to the analyte molecules. These procedures have now reached the stage of commercialization and practical applicability for real-world drugs and bioorganics in complex biofluid type samples. Polymer supported reagents can now be used for direct injection of biofluids with solid-phase (hydrophobic) extraction of the analytes of interest, followed by sample cleanup, derivatization, elution onto the HPLC column, peak compression, gradient HPLC elution, multiple detection, and final data interpretation with quantitation. This review summarizes much or most of what has been described in the scientific literature over the past decade in the various areas where polymeric reagents are being used for derivatization in HPLC and in capillary electrophoresis as well.