Liquid Chromatography: Theory and Methodology

A brief history and spectroscopic analysis of soy isoflavones

The production of soybean continues to increase worldwide. People are showing more interest in the beneficial health effects of soybeans than before. However, the origin and history of soybeans are still being discussed among many researchers. Chromatographic methods enable the desirable separation of a variety of isoflavones from soybeans. The structures of isolated soy isoflavones have been successfully identified in tandem with spectroscopic analytical instruments and technologies such as liquid chromatography, mass spectrometry, and nuclear magnetic resonance spectroscopy. The theoretical background behind spectroscopy may help improve the understanding for the analysis of isoflavones in soybeans and soy-derived foods. This review covers the origin of the English name of soybean and its scientific name, Glycine max (L.) Merrill, based on the evidence reported to date. Moreover, the reports of soy isoflavones discovered over a period of about 100 years have been briefly reviewed.

Probing the Entropic Effect in Molecular Noncovalent Interactions between Resin-Bound Polybrominated Arenes and Small Substrates

The associative interaction between resin-bound polybrominated arenes and small molecules was analyzed by using various spectroscopic techniques as well as a synthetic molecular model to establish the thermodynamics. The binding in acetonitrile was three orders of magnitude stronger than that in methanol, partly owing to the tertiary conformational gating of the resin that controls the entropic terms. By using the entropic superiority, the associative binding of up to 3×10⁴  m^-1 is achieved with the non-biological system. A modified Hill plot for the quantitative analysis of bindings was also devised, which enabled the interactions at the molecular level to be elucidated.

Expanding metabolic pathway for de novo biosynthesis of the chiral pharmaceutical intermediate L-pipecolic acid in Escherichia coli

BACKGROUND

The six-carbon circular non-proteinogenic compound L-pipecolic acid is an important chiral drug intermediate with many applications in the pharmaceutical industry. In the present study, we developed a metabolically engineered strain of Escherichia coli for the overproduction of L-pipecolic acid from glucose.

RESULTS

The metabolic pathway from L-lysine to L-pipecolic acid was constructed initially by introducing lysine cyclodeaminase (LCD). Next, L-lysine metabolic flux from glucose was amplified by the plasmid-based overexpression of dapA, lysC, and lysA under the control of the strong trc promoter to increase the biosynthetic pool of the precursor L-lysine. Additionally, since the catalytic efficiency of the key enzyme LCD is limited by the cofactor NAD⁺, the intracellular pyridine nucleotide concentration was rebalanced by expressing the pntAB gene encoding the transhydrogenase, which elevated the proportion of LCD with bound NAD⁺ and enhanced L-pipecolic acid production significantly. Further, optimization of Fe²⁺ and surfactant in the fermentation process resulted in 5.33 g/L L-pipecolic acid, with a yield of 0.13 g/g of glucose via fed-batch cultivation.

CONCLUSIONS

We expanded the metabolic pathway for the synthesis of the chiral pharmaceutical intermediate L-pipecolic acid in E. coli. Using the engineered E. coli, a fast and efficient fermentative production of L-pipecolic acid was achieved. This strategy could be applied to the biosynthesis of other commercially and industrially important chiral compounds containing piperidine rings.

New concepts and applications in the macromolecular chemistry of fullerenes

A new classification on the different types of fullerene-containing polymers is presented according to their different properties and applications they exhibit in a variety of fields. Because of their interest and novelty, water-soluble and biodegradable C(60)-polymers are discussed first, followed by polyfullerene-based membranes where unprecedented supramolecular structures are presented. Next are compounds that involve hybrid materials formed from fullerenes and other components such as silica, DNA, and carbon nanotubes (CNTs) where the most recent advances have been achieved. A most relevant topic is still that of C(60)-based donor-acceptor (D-A) polymers. Since their application in photovoltaics D-A polymers are among the most realistic applications of fullerenes in the so-called molecular electronics. The most relevant aspects in these covalently connected fullerene/polymer hybrids as well as new concepts to improve energy conversion efficiencies are presented.The last topics disccused relate to supramolecular aspects that are in involved in C(60)-polymer systems and in the self-assembly of C(60)-macromolecular structures, which open a new scenario for organizing, by means of non-covalent interactions, new supramolecular structures at the nano- and micrometric scale, in which the combination of the hydrofobicity of fullerenes with the versatility of the noncovalent chemistry afford new and spectacular superstructures.

Replacement of acetonitrile by ethanol as solvent in reversed phase chromatography of biomolecules

Acetonitrile, which is a by-product of acrylonitrile synthesis, is the commonly used solvent in ion-pair reversed phase chromatography. In consequence of the decreasing demand for acrylonitrile due to the financial crisis, a worldwide shortage of acetonitrile is observed. Therefore, the aim of this study was to establish ion-pair reversed phase chromatographic assays using alternative eluents for acetonitrile and to decrease costs incurred hereby. We compared the performance of ion-pair reversed phase chromatography using acetonitrile with the alternative eluents methanol, ethanol and n-propanol, using monolithic reversed phase C5 as well as C18 chromatography columns. We used triethylammonium acetate (TEAA) and tetrabutylammonium sulfate (TBA) as representative cationic ion-pair reagents and trifluoroacetic acid (TFA) as representative anionic ion-pair reagent. For covering a large field of applications, we fractionated representative low, middle and high-molecular weight biomolecules, in particular dinucleoside polyphosphates, peptides, proteins and tryptic digested human serum albumin. Whereas the chromatographic characteristics of both methanol and n-propanol were partly insufficient, ethanol was characterised equally or partly even better in the matter of elution strength and separation quality compared to the eluent water-acetonitrile. In conclusion, ethanol is an appropriate alternative for acetonitrile in ion-pair reversed phase chromatography of biomolecules.

Retention mechanism of l-glutamide-derived noncrystalline stationary phases in reversed-phase high-performance liquid chromatography and application for separation of nucleic acid constituents

Double alkylated L-glutamide-derived noncrystalline stationary phases Sil-DSG and Sil-DBG have been prepared by coupling N',N''-dioctadecyl-N-[4-carboxybutanoyl]-L-glutamide (DSG) and N',N'',-dibutyl-N-[4-carboxybutanoyl]-L-glutamide (DBG) with aminopropylated silica (Sil-APS). TEM observations of DSG and DBG showed that lipid DSG can aggregate in organic solvents (methanol, chloroform, toluene, etc.) and self-assembled nano fibers are observed while such fibrous aggregations are not observed for DBG. The resulting chromatographic data have been provided information about its selective interaction with guest molecules (PAHs) in RP-HPLC. We have observed that the carbonyl groups in Sil-DSG exist in ordered state by forming a condensing thin layer over silica surface while DBG cannot form such an ordered state due to its lower order of short alkyl chain. The ordered carbonyl groups present in Sil-DSG promotes multiple carbonyl pi-benzene pi interactions with guest PAHs isomers which enhance the selectivity for these compounds. The contribution of pi-pi interactions was also supported by the substantial effects on the selectivity of benzene and nitrobenzenes. The effect of pi-electron containing solvent on the retention behavior of the PAHs was also studied. The selectivity for nucleic acid constituents, i.e. nucleosides and its bases were also evaluated by Sil-DSG and the selectivity for these compounds on Sil-DSG was compared with the selectivity of conventional polymeric ODS phase. It has been found that Sil-DSG provided higher selectivity for nucleic acid constituents than polymeric ODS and that HPLC packing materials can be efficiently employed for routine analysis of these compounds. The effect of methanol content on the separation behavior of nucleosides was also studied.

Interpreting the effects of temperature and solvent composition on separation of amino-acid racemates by chiral ligand-exchange chromatography

Routinely applied at both preparative and analytical scales, chiral ligand-exchange chromatography (CLEC) separates enantiomers capable of chelating a divalent transition-metal-ion through a pair of coordinating electronegative atoms. CLEC separation efficiencies are strongly dependent on column operating conditions, including temperature and mobile-phase solvent composition. Although previous empirical studies provide some useful guidelines for optimizing column operating conditions, the fundamental mechanisms underlying the unusually high sensitivity of CLEC performance to operating temperature and solvent composition remain poorly understood, limiting efforts to develop a comprehensive model for the technology. To address this problem, we report transport and chemical equilibria data for the separation of alpha-amino acids on a Nucleosil chiral-1 column presenting L-hydroxyproline as the immobilized ligand. Solute transport is found to be limited by pore diffusion at all column operating temperatures and solvent compositions, validating the existence of local equilibria throughout the column. Changes in separation performance are found to correlate with changes in chemical equilibria, emphasizing the need to carefully account for all speciation within the column when modeling CLEC and providing important fundamental data to achieve this goal. Each enantiomer participates in a large number of solution-phase complexes. As a result, the thermodynamic driving force for separation is unusually complex, allowing subtle changes in column operating conditions to mediate significant changes in speciation profiles and separation efficiency. A reaction-equilibria model accounting for all speciation within the CLEC column is proposed and used to estimate enantiomer partition coefficients and retention times.

Separation of stereoisomers of dinuclear metal complexes by binding affinity chromatography using non-duplex DNA

Affinity chromatography--using non-duplex DNA as the affinity ligand--has been used as a highly efficient means of separating stereoisomers of dinuclear polypyridyl ruthenium(II) complexes.

Hydrophilic interaction liquid chromatography–mass spectrometry for the analysis of paralytic shellfish poisoning (PSP) toxins

Hydrophilic interaction liquid chromatography (HILIC) was examined for the separation of paralytic shellfish poisoning (PSP) toxins using the stationary phase TSK-gel Amide-80. The parameters tested included type of organic modifier and percentage in the mobile phase, buffer concentration, pH, flow rate and column temperature. Using mass spectrometric (MS) detection, the HILIC column allowed the determination of all the major PSP toxins in one 30 min analysis with a high degree of selectivity and sensitivity. The high percentage of organic modifier in the mobile phase and the omission of ion pairing reagents, both favored in HILIC, provided limits of detection (LOD) in the range 50-100 nM in selected ion monitoring (SIM) mode on a single quadrupole LC-MS system. LOD in selected reaction monitoring (SRM) mode on a sensitive triple quadrupole system were as low as 5-30 nM. Excellent linearity of response was observed.

Kinetico–mechanistic studies of C–H bond activation on new Pd complexes containing N,N′-chelating ligands

The hybrid imine/amine palladium(II) coordination complexes [PdX2(kappa2-N(imino),N(amino))](X = Cl, AcO; kappa2-N(imino),N(amino)= 4ClC6H4CHNCH2(CH2)nN(CH3)2, n= 1, 2) have been prepared in different isomeric forms which include E/Z arrangement around the C[double bond]N bond of the hybrid ligand and {Pd(kappa(2)-N(imino),N(amino))} ring conformation. The crystal structures of four of them, E-1AcO, Z-1AcO, E-2AcO and E-2Cl, have been determined and the solution behaviour in acetic acid, the common cyclometallating solvent, for all these systems studied. The complexes in acetic acid solution are shown to maintain the structure determined by X-ray crystallography, as they do in deuterated chloroform. Nevertheless, a partial opening equilibrium of the {Pd(kappa2-N(imino),N(amino))} ring is observed by NMR experiments. When the complexes are held in solution for longer periods the corresponding cyclometallated derivatives, 1AcO-CM, 2AcO-CM, 1Cl-CM and 2Cl-CM, containing the {Pd(kappa2-C,N(imino))} palladacycle are obtained, as characterized by 1H NMR spectroscopy. In these compounds the total opening of the N(amino) moiety of the ligand has occurred. The C-H bond activation process has been studied kinetico-mechanistically at different temperatures, pressures and acid concentrations; the results agree with the need of an opening of the chelate ring in [PdX2(kappa2-N(imino),N(amino))] prior to the proper cyclometallation reaction. The values of the enthalpies of activation are higher than those observed for known N-monodentated cyclometallating ligands, as should correspond to the contribution of a ligand dechelation pre-equilibrium. The entropies and volumes of activation are also indicative of this predissociation that include an important amount of contractive ordering. The presence of small amounts of triflic acid in the reaction medium accelerates the reaction to the value observed for N(imino)-monodentate systems, indicating that the full opening of the chelate ring has taken place. For the badly oriented isomeric forms of the ligand in the chelated complex (Z), the cyclometallation process is even more slow and corresponds directly to the reorganization of the ligand to its cyclopalladation-active (E) conformation.

A new type of chemically bonded phase for reversed-phase HPLC

A new type of ether-bonded packing for reversed-phase HPLC (RP-HPLC) was synthesized by reacting 1-octanol with beta-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, followed by coupling the product onto porous silica. The prepared packing was characterized by elemental analysis, solid-state 13C NMR, and Fourier transform infrared (FT-IR) spectroscopy. Chromatographic evaluations were performed by using a mixture of organic compounds as the analyte and methanol-water as binary mobile phase. The influence of the composition of organic modifier on the retention behavior of basic compounds was studied. The hydrolytic stability of the packing between pH 2.5-7.5 was also investigated. The results showed that the new stationary phase has excellent chromatographic properties and good hydrolytic stability.

Problems connected with band-broadening in size-exclusion chromatography with dual detection

The sources of band-broadening (axial dispersion) in size-exclusion chromatography (SEC) of polymers and its influence on data obtained using dual detection of concentration and molecular weight are reviewed. Special attention is paid to the combination of a multiangle light-scattering photometer and a differential refractometer as detectors. The local polydispersity is discussed in the relation to the band-broadening as well as to heterogeneity of the polymer with respect to molecular weight and hydrodynamic volume.

Relations between the separation coefficient, longitudinal displacement and peak broadening in size exclusion chromatography of macromolecules

The separation of a polymer by size exclusion chromatography is described as a series of interactions, i.e. consecutive establishments of equilibria between polymer fractions in the mobile and stationary phases followed by displacements of mobile phase containing the polymer. The elution curve is derived as the longitudinal concentration profile in the column observed in one position in space during the time of the analysis. The mean value of elution volume of a particular polymer species turns out to be the interstitial volume of the separation system divided by the mean fraction of polymer in the mobile phase. The number of the displacement-equilibrium steps can be estimated from the limiting values of the variance of the spreading function.

Comparative study of the linear solvation energy relationship, linear solvent strength theory, and typical-conditions model for retention prediction in reversed-phase liquid chromatography

This paper describes two new retention models for predicting retention under different reversed-phase liquid chromatography (RPLC) conditions. The first one is a global linear solvation energy relationship (LSER) that expresses retention as a function of both solute LSER descriptors and mobile phase composition. The second is a so-called "typical-conditions model" that expresses retention under a given chromatographic condition as a linear function of retention under different so-called "typical" conditions. The global LSER was derived by combining the local LSER model and the linear solvent strength theory (LSST) of RPLC. Compared to local LSER and the LSST models, the global LSER model requires far fewer retention measurements for calibrating the model when different solutes and different mobile phase compositions are involved. Its fitting performance is equal to the local LSER model but worse than that of LSST. The poor fit of the global LSER results primarily from the local LSER model and not from the LSST model. The typical-conditions model (TCM) was developed based on a concept of multivariate space that is conceptually compatible with LSER. However, no LSER descriptors are used in the TCM approach. The number of input conditions needed in the typical-conditions model is determined by the chemical diversity of the solutes and the conditions involved. Principal component analysis (PCA) and iterative key set factor analysis (IKSFA) were used to find the number of typical conditions needed for a given data set. Compared to LSER, LSST, and global LSER, the typical-conditions model is more precise and requires fewer retention measurements for calibrating the model when different solutes and different stationary and/or mobile phases are involved.

Comparison of capillary electrophoresis and reversed-phase liquid chromatography for determination of the enantiomeric purity of an M3 antagonist

The chiral separation of an M3 antagonist was investigated using capillary electrophoresis (CE) with various sulfated cyclodextrins and by reversed-phase liquid chromatography with derivatized cellulose, derivatized amylose, and two protein stationary phases. Operational parameters for each technique, such as the concentration of the chiral selectors, background electrolyte (or mobile phase) pH and type, organic modifiers, injection mode and temperature were varied in order to achieve a desired elution order and to meet a 0.1% limit of quantitation (LOQ) criteria. Based on the advantages and disadvantages of each technique, a practical CE method using sulfated gamma-cyclodextrin was selected. The method was validated in terms of linearity, LOQ, accuracy, ruggedness and precision.

Analysis of complex formation between crown ethers and potassium ion by determining retention factors in reversed-phase high-performance liquid chromatography

Retention factors in reversed-phase high-performance liquid chromatography (RP-HPLC) were utilized for the analysis of complex formation reaction of four crown ethers with potassium ion in water-methanol media. The crown ethers were injected as analytes, potassium ion was present in the eluent, and three types of reversed-phase columns were used. The retention factor of the crown ethers decreased with increasing concentrations of potassium ion in the eluent, which is attributed to the change in the species of the crown ether from a neutral ligand to a positively charged complex. The complex formation constants were determined by analyzing the changes in the retention factor with a non-linear least-squares method.

Modeling the effect of solvation on solute retention in reversed-phase liquid chromatography

The retention of a homologous series of alkylbenzenes was determined on octyl and octadecyl reversed-phase columns in several polar organic liquids. Free energies of transfer were calculated by the SM5.0R classical solvation model for each organic liquid tested and for several alkanes. The relationships between the measured retention factors and the calculated free energies of transfer were then investigated. Although the natural logarithms of the retention factor and the calculated free energies of transfer were linearly correlated, the obtained free energies of transfer of the solutes did not completely explain the retention behavior of the solutes. Nonetheless, even in these pure organic liquids, the energetics of RPLC retention behaved very similarly to those of partitioning.

Use of atmospheric pressure ionization mass spectrometry in enantioselective liquid chromatography

Recent advances in mass spectrometry have rendered it an attractive and versatile tool in industrial and academic research laboratories. As a part of this rapid growth, a considerable body of literature has been devoted to the application of mass spectrometry in studies involving enantioselectivity, molecular recognition, and supramolecular chemistry. In concert with separation techniques such as capillary electrophoresis and liquid chromatography, mass spectrometry allows rapid characterization of a large array of molecules in complex mixtures. A majority of these findings have been made possible by the introduction of 'soft-ionization' techniques such as electrospray ionization interface. Other techniques such as atmospheric pressure chemical ionization mass spectrometry have been widely used as a rugged interface for quantitative liquid chromatography-mass spectrometry. Herein, we present a brief overview of the above techniques accompanied with several examples of enantioselective capillary electrophoresis- and liquid chromatography-mass spectrometry in drug discovery and development. Although the emphasis of this article is on quantitative enantiomeric chromatography-mass spectrometry, we envisage that similar strategies are adaptable in qualitative studies.

Advances in the determination of inorganic anions by ion chromatography

The time period covered for this review includes articles published from 1997 to 1999, with the addition of a few classic references. The purpose of the review is to include the most relevant works from each topic area of the determination of inorganic anions by ion chromatography, including new sample pretreatments, new separation methods, new detection systems and the latest applications in the field of environmental, water, foods, etc. samples. Experimental conditions such as stationary phase, eluent, detection mode, as well as matrix are summarized in a table.

Repeatability and reproducibility of retention data and band profiles on reversed-phase liquid chromatography columns. III. Results obtained with Kromasil C18 columns

The reproducibility of the retention data and the band profiles was investigated with Kromasil C18 columns (silica-based monomeric type reversed-phase packing material). High precision data were obtained and statistically compared among five columns from the same batch (column-to-column reproducibility) and six columns, one from each of six different batches (batch-to-batch reproducibility). These data were acquired under five different sets of chromatographic conditions, for a group of 30 neutral, acidic and basic compounds selected as probes following an experimental protocol previously described. Data characterizing the retention time, the retention factor, the separation factor, the column efficiency and the peak asymmetry for the different probe compounds are reported. Factors describing the silica surface interaction with the selected probe compounds, such as the hydrophobic interaction selectivity, the steric selectivity, and the separation factors of basic compounds at different pH values were also determined. The influence of the underlying silica on these data and correlations between the chromatographic and physico-chemical properties of the different batches are discussed.

Separation and identification of polar degradation products of benzo[a]pyrene with ozone by atmospheric pressure chemical ionization-mass spectrometry after optimized column chromatographic clean-up

The environmental relevance of oxidized degradation products of polycyclic aromatic hydrocarbons (PAHs) increases due to enhanced combustion of organic matter and fossil fuels. For PAHs consisting of more than three condensed aromatic rings, soot aerosols are the main carrier, on the surface of which they can react with trace gases like ozone. In this study the clean-up procedure and analysis of ozonized benzo[a]pyrene (B[a]P) was optimized. B[a]P and its degradation products were preseparated into three fractions. Different reversed-phase materials were evaluated for high-performance liquid chromatographic separation. Among these, a phenyl-modified silica material proved best-suited and the chromatographic separation was optimized on this material. For the detection of separated degradation products, liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (LC-APCI-MS) was used. With this method, 29 components could be characterized. Besides the three known main degradation products (B[a]P-1,6-dione, B[a]P-3,6-dione, B[a]P-6,12-dione, B[a]P-4,5-dione and 4-oxa-benzo[d,e,f]chrysene-5-one (B[def]C-lactone), were identified for the first time with the help of reference substances. B[def]C-lactone is known as a substance with a mutagenic potential similar to B[a]P. Several other compounds could be tentatively identified.