Liquid chromatographic separation and stereoselective detection of L- and D-amino acids with catalytic reaction detection using immobilized enzymes

D-Amino acids in mammals and their diagnostic value

Substantial amounts of D-amino acids are present in mammalian tissues; their function, origin and relationship between pathophysiological processes have been of great interest over the last two decades. In the present article, analytical methods including chromatographic, electrophoretic and enzymatic methods to determine D-amino acids in mammalian tissues are reviewed, and the distribution of these D-amino acids in mammals is discussed. An overview of the function, origin and relationship between the amino acids and pathophysiological processes is also given.

Bienzymatic amperometric sensor for protein assay in milk

A new bienzymatic amperometric sensor is proposed for the assay of the protein content of milk. The sensor is based on two enzymes: carboxypeptidase A and L-amino acid oxidase. The response characteristics obtained for this sensor (detection limit of 1.5 micromol/L, linear concentration range between 1.8 and 2.8 micromol/L), as well as high selectivity over possible interferences from milk, made it applicable as a detector in flow injection analysis (FIA). The response characteristics obtained in the non-equilibrium conditions (FIA system) are: detection limit of 1.5 micromol/L and linear concentration range between 2 and 3.5 micromol/L. Without FIA, the average recovery of proteins from milk and milk products is 99.06 +/- 0.07% and, by utilization of FIA, it increased to 99.73 +/- 0.03. The sensor proved a good reliability for the assay of proteins in milk and milk products.

Analytical chemistry and biochemistry of D-amino acids

The methodologies for the analysis of D-amino acids in biological materials have been reviewed, including the use of enzymes, gas and liquid chromatography with chiral stationary phases and diastereomer derivatization with chiral reagents followed by GC or HPLC separation. The distribution of D-amino acids in the body, their origin, metabolism and possible roles in human diseases are discussed.

Enantiomeric derivatization for biomedical chromatography

Derivatization reactions aimed at creating the basis for the chromatographic resolution of biologically and pharmaceutically important enantiomers are reviewed, with emphasis on the literature published in the last 10 years. Three main aspects of chiral derivatization are discussed. (a) Enantiomers containing suitable functional groups (amino, carboxyl, hydroxyl, epoxy, etc.) are transformed into covalently bonded diastereomeric derivatives using homochiral derivatizing agents. The diastereomers formed (esters, amides, urethanes, urea and thiourea, etc., derivatives) can be separated on achiral stationary phases. The derivatization reactions often afford further advantages, such as the improvement of chromatographic properties and the detectability of the solutes using UV and fluorimetric detectors. (b) Covalent but achiral derivatization is often necessary even with the use of chiral stationary phases enabling in principle direct enantioseparations (Pirkle-type columns, cyclodextrin-bonded phases, glycoprotein column and functionalized cellulose columns). The main goals of these derivatization reactions (which are analogous to those discussed above), are to introduce functional groups into the molecule of the enantiomers that improve the possibilities for chiral interactions or block functional groups to avoid non-specific interactions. (c) In the broader sense, the dynamic formation of diastereomers using chiral mobile phase additives (cyclodextrins, various reagents to form diastereomeric ion pairs, adducts, mixed metal complexes) can also be considered to be chiral derivatization reactions and is therefore briefly discussed also.

Enzyme-based biosensor as a selective detection unit in column liquid chromatography

A reagentless enzyme electrode based on co-immobilized alcohol oxidase and horseradish peroxidase was used as the working electrode in an amperometric flow-through cell connected to a column liquid chromatographic (CLC) system for the selective detection of methanol and ethanol. The enzymes were covalently immobilized in carbon paste (graphite-phenylmethylsilicone oil) in the presence of polyethylenimine. Electrodes prepared from the enzyme-modified carbon paste were optimized with respect to their sensitivity and selectivity. Different membranes were cast or electropolymerized directly on the surface of the electrode to increase the long-term stability of the biosensor. The compatibility with the reversed-phase chromatographic system was established. A PLRP-S polymer-based separation column was used with phosphate buffer as the mobile phase. The selectivity of the enzyme electrode was also determined by injecting some easily oxidizable and possibly interfering species normally present in biological samples. The enzyme electrode was also used in an on-line system, consisting of a microdialysis probe as the sampling unit, the CLC system and the biosensor detection device, for the selective following of the ethanol produced when a paper pulp industrial waste water was fermented with Saccharomyces cerevisiae.

A reagentless amperometric biosensor for alcohol detection in column liquid chromatography based on co-immobilized peroxidase and alcohol oxidase in carbon paste

A reagentless carbon paste electrode chemically modified with covalently bound alcohol oxidase and horse-radish peroxidase was examined as a selective sensor in flow injection and column liquid chromatography. A combination of carbodiimide, glutaraldehyde, and polyethyleneimine was used for immobilizing the enzymes in the paste. The surface of the electrodes was protected by first forming a layer of electropolymerized ortho-phenylenediamine followed by deposition of a cation exchange membrane (Eastman AQ 29D). The electrodes were used for detection of hydrogen peroxide, methanol, ethanol, propanol, isopropanol, and butanol. Preliminary investigations of the use of this sensor for bioprocess control are reported.

Study of a reagent- and mediator-less biosensor for D-amino acids based on co-immobilized D-amino acid oxidase and peroxidase in carbon paste electrodes

A biosensor for the analysis of D-amino acids is described. Carbon paste (graphite/paraffin oil) was chemically modified with immobilized D-amino acid oxidase and either horse-radish peroxidase or fungal peroxidase from Arthromyces ramosus. The two enzymes dissolved in buffer, together with an amine containing oligomer or polymer, were adsorbed on dry graphite. Prior to immobilization, the graphite was heat treated at 700 degrees C for 15 s to promote an efficient electron transfer between graphite and the peroxidase. The mixture was dried before addition of the pasting liquid. The sensor is based on the fact that the hydrogen peroxide produced by the action of D-amino acid oxidase is electrocatalytically reduced through the action of the peroxidase. The amine containing compound acted as a stabilizer and activator of the enzymes in the paste. The enzyme electrode was investigated as a sensor for D-phenylalanine and hydrogen peroxide in a flow through electrochemical cell connected to a single line flow injection system. The influences on the response by different additives to the paste and pH are reported. Linear calibration curves were obtained between 0.1 and 1.4 mM for D-phenylalanine and 5 and 1000 microM for hydrogen peroxide at an applied potential of -50 mV vs. Ag/AgCl. The sensor was also active for the following D-amino acids: D-alanine, D-valine, D-leucine, D-isoleucine, D-serine, D-aspartic acid, D-glutamic acid, D-lysine, D-histidine, D-arginine, D-tryptophan, D-methionine, and D-proline.

Bioselective detection in liquid chromatography by the use of immobilized enzymes

The combination of liquid chromatography and immobilized enzyme reactors (LC/IMER) is a rapidly developing field of research. The enzymes are used to catalyse chemical reactions and thereby facilitate selective detection. In this instance the chemical derivatizations are performed in the post-column mode. The selectivity is demonstrated with respect to interfering compounds present in complex samples.