István Ilisz

Application of chiral derivatizing agents in the high-performance liquid chromatographic separation of amino acid enantiomers: a review.

The past 20 years has seen an explosive growth in the field of chirality, as illustrated by the rapid progress in the various facets of this intriguing field. The impetus for advances in chiral separation has been highest in the past decade and this still continues to be an area of high focus. This paper reviews indirect separation approaches, i.e. derivatization reactions aimed at creating the basis for the chromatographic resolution of biologically and pharmaceutically important enantiomers, with emphasis on the literature published in the last 12 years. The main aspects of the chiral derivatization of amino acids are discussed, i.e. derivatization on the amino group, transforming the molecules into covalently bonded diastereomeric derivatives through the use of homochiral derivatizing agents. The diastereomers formed (amides, urethanes, urea, thiourea derivatives, etc.) can be separated on achiral stationary phases. The applications are considered, and in some cases different derivatizing agents for the resolution of complex mixtures of proteinogenic d,l-amino acids, non-proteinogenic amino acids and peptides/amino acids from peptide syntheses or microorganisms are compared.

Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspensions. II. Effect of charge-trapping species on product distribution

Abstract The photodegradation of phenol was investigated in the presence of TiO 2 (anatase) as photocatalyst in near-UV-irradiated aqueous unbuffered suspensions. The distribution of degradation products and a reduction in total organic carbon are reported, focusing on the influence of charge-trapping species (O 2 , Ag + and H 2 O 2 ). In the presence of dissolved O 2 , the degradation proceeds predominantly via OH ⋅ , where hydroxylated aromatics were detected: catechol, hydroquinone and 1,2,4-trihydroxybenzene; in the presence of Ag + , the direct hole oxidation dominates, with p -benzoquinone as the only identified transient product; in the presence of H 2 O 2 , both OH ⋅ and direct oxidation by positive holes contribute to the degradation of phenol. Besides the identified compounds, the formation of presumed ring-opening products occurs simultaneously. In contrast with the general view, it is pointed out that the appearance of aliphatic products in the early stage of the oxidation process is not unambiguous proof of the participation of direct hole oxidation in the degradation mechanism.

Removal of 2-chlorophenol from water by adsorption combined with TiO2 photocatalysis

Abstract The results of a laboratory-scale study of an environmentally friendly water treatment method is presented, where the organic model pollutant 2-chlorophenol (2-CP) was first adsorbed and then removed by a direct photolytic or heterogeneous photocatalytic process. The adsorbent was an organically treated (with hexadecylpyridinium chloride) clay mineral (montmorillonite), and the photocatalyst was Degussa P25. The total organic carbon and total inorganic chloride contents were measured to monitor the mineralization process, while the degradation of 2-CP and the formation of intermediates were followed by HPLC. No negative effect of the suspended adsorbent was observed either in the direct photolytic process or in the heterogeneous photocatalytic degradation process. Although direct photolysis allows the highest degree of 2-CP removal, in this case the degradation of the organic pollutant is accompanied by destruction of the adsorbent. The removal of 2-CP through the combination of adsorption and heterogeneous photocatalysis was achieved in a reasonable acceptable time interval, and can therefore be suggested as an efficient, cost-effective and environment-friendly water treatment facility.

Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspensions. I: Effect of charge-trapping species on the degradation kinetics

Abstract The effects of charge-trapping species on the kinetics of phenol decomposition were studied in near-UV-irradiated aqueous TiO 2 (anatase) suspensions in a batch photoreactor. The influence of catalyst loading, initial phenol concentration, dissolved O 2 concentration, Ag + content and H 2 O 2 concentration on the rate of phenol degradation is reported. The observed heterogeneous degradation of phenol followed apparently zero-order kinetics up to ca. 70% conversion. The Langmuir–Hinshelwood kinetic model successfully described the influence of the initial phenol concentration and dissolved O 2 concentration on the rate of heterogeneous photooxidation of phenol. The data obtained by applying the Langmuir–Hinshelwood treatment are consistent with the available kinetic parameters. The results of the experiments in the presence of Ag + indicated that the phototransformation of phenol can proceed via direct electron transfer, neither dissolved O 2 nor its reduction forms playing a significant role in the degradation mechanism.

Retention mechanism of high-performance liquid chromatographic enantioseparation on macrocyclic glycopeptide-based chiral stationary phases

The development of methods for the separation of enantiomers has attracted great interest in the past 20 years, since it became evident that the potential biological or pharmacological applications are mostly restricted to one of the enantiomers. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of high-performance liquid chromatography (HPLC), thin-layer chromatography and electrophoresis. The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these macrocyclic glycopeptide antibiotics and, through their application, endeavors to demonstrate the mechanism of separation on macrocyclic glycopeptides. The sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.

Recent advances in the direct and indirect liquid chromatographic enantioseparation of amino acids and related compounds: A review

Amino acids are essential for life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of peptides and proteins, giving rise to complex three dimensional structures through disulfide bonds or crosslinked amino acids. Peptides are frequently cyclic and contain protein as well as non-protein aminoacids in many instances. Since most of the proteinogenic α-amino acids contain a chiral carbon atom (with the exception of glycine), the stereoisomers of all these amino acids and the peptides in which they are to be found may possess differences in biological activity in living systems. The impetus for advances in chiral separation has been highest in the past decade and this still continues to be an area of high focus. The important analytical task of the separation of isomers is achieved mainly by chromatographic methods. This review surveys indirect and direct HPLC separations of biologically and pharmaceutically important enantiomers of amino acids and related compounds, with emphasis on the literature published from 2005.

Degradation of naproxen by UV, VUV photolysis and their combination

Naproxen is a widely used nonsteroidal anti-inflammatory drug. Recently, this medicine was detected both in natural waters (up to 1.5 μg L(-1)) and in sewage treatment plant effluents (up to 5.2 μg L(-1)). Moreover, naproxen is only partly eliminated by classical processes used in sewage treatment plants. Therefore, its degradation is of utmost interest. Advanced oxidation processes proved to be the most suitable methods for the elimination of persistent organic contaminants. In this work ultraviolet (UV, 254 nm), vacuum ultraviolet photolysis (VUV, 172 nm) and their combination (UV/VUV, 254/185 nm) were investigated. The efficiency of the methods increased in the following order: UV < VUV < UV/VUV photolysis. However, VUV irradiation was found to mineralize the contaminant molecule most effectively. The chemical structures of three out of four aromatic by-products and of some aliphatic carboxylic acids were presumed. The effects of dissolved O2 and the initial concentration of naproxen on the degradation were also investigated.

Chiral derivatizations applied for the separation of unusual amino acid enantiomers by liquid chromatography and related techniques.

Amino acids are essential for life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of peptides and proteins, giving rise complex three dimensional structures through disulfide bonds or crosslinked amino acids. Peptides are frequently cyclic and contain proteinogenic as well as nonproteinogenic amino acids in many instances. Since most of the proteinogenic α-amino acids contain at least one stereogenic center (with the exception of glycine), the stereoisomers of all these amino acids and the peptides in which they are to be found may possess differences in biological activity in living systems. The impetus for advances in chiral separation has been highest in the past 25 years and this still continues to be an area of high focus. The important analytical task of the separation of isomers is achieved mainly by chromatographic and electrophoretic methods. This paper reviews indirect separation approaches, i.e. derivatization reactions aimed at creating the basis for the chromatographic resolution of biologically and pharmaceutically important enantiomers of unusual amino acids and related compounds, with emphasis on the literature published from 1980s. The main aspects of the chiral derivatization of amino acids are discussed, i.e. derivatization on the amino group, transforming the molecules into covalently bonded diastereomeric derivatives through the use of homochiral derivatizing agents. The diastereomers formed (amides, urethanes, urea and thiourea derivatives, etc.) can be separated on achiral stationary phases. The applications are considered, and in some cases different derivatizing agents for the resolution of complex mixtures of proteinogenic d,l-amino acids, non-proteinogenic amino acids and peptides/amino acids from peptide syntheses or microorganisms are compared.

Enantiomeric separation of nonproteinogenic amino acids by high-performance liquid chromatography

Amino acids are essential for life, and have many functions in metabolism. One particularly important function is to serve as the building blocks of peptides and proteins, giving rise complex three dimensional structures through disulfide bonds or crosslinked amino acids. Peptides are frequently cyclic and contain proteinogenic as well as nonproteinogenic amino acids in many instances. Since most of the amino acids contain a chiral carbon atom, the stereoisomers of all these amino acids and the peptides in which they are to be found may possess differences in biological activity in living systems. The development of methods for the separation of enantiomers has attracted great interest, since it became evident that the potential biological or pharmacological applications are mostly restricted to one of the enantiomers. The important analytical task of the separation of isomers is achieved mainly by chromatographic and electrophoretic methods. This special review surveys indirect and direct high-performance liquid chromatographic (HPLC) methods of biologically and pharmaceutically important enantiomers of nonproteinogenic amino acids and related compounds, with emphasis on the literature published from the beginning.

The photochemical behavior of hydrogen peroxide in near UV-irradiated aqueous TiO2 suspensions

Abstract The kinetic behavior of H 2 O 2 decomposition in near UV-irradiated aqueous TiO 2 suspensions was investigated. The effects of the H 2 O 2 concentration, the catalyst loading, the dissolved oxygen concentration, and the light intensity on the initial photodecomposition rate were studied. The observed kinetics were treated by both the Langmuir–Hinshelwood and the Freundlich model; the latter proved more successful for characterization of the system. The H 2 O 2 photodegradation rate was found to be directly proportional to the light intensity and it increased with the catalyst concentration up to a saturation limit. The concentration of dissolved oxygen had no significant effect on the photodecomposition rate.