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Biomedical subjects

A Berthod

Publications and source records attributed to A Berthod.

31 records · Page 2Linked to original sources
PubMed1996

Comparison and modeling study of vancomycin, ristocetin A, and teicoplanin for CE enantioseparations.

The structurally related glycopeptide antibiotics vancomycin, ristocetin A, and teicoplanin can all be used as chiral selectors in capillary electrophoresis (CE). Both experimental and modeling studies were done to elucidate their similarities and differences. There are identifiable morphological differences in the aglycon macrocyclic portions of these three compounds. In addition, there are other structural distinctions that can affect their CE enantioselectivity, migration times, and efficiency. Teicoplanin is the most distinct of the three and is the only one that is surface active. Its aggregational properties appear to affect its enantioselectivity among other things. The similar but not identical structures of the three glycopeptides produce similar but not identical enantioselectivities. This leads to the empirically useful "principle of complementary separations", in which a partial resolution with one chiral selector can be brought to baseline with one of the others. Overall, ristocetin A appears to have the greatest applicability for CE enantioseparations.

Anti-Bacterial Agents↗
PubMed1996

Facile liquid chromatographic enantioresolution of native amino acids and peptides using a teicoplanin chiral stationary phase.

The glycopeptide antibiotic teicoplanin is shown to be a highly effective stationary phase chiral selector for the resolution of underivatized amino-acid and imino-acid enantiomers. Fifty four of these compounds (including all chiral protein amino acids) as well as a number of dipeptides were resolved. Hydro-organic mobile phases are used and no buffers or added salts are needed in most cases. Hence the purified analytes are easily isolated in pure form, if needed, by evaporating of the solvent. The effect of pH, organic modifier type and amount are discussed. The enantioselective separation mechanism is examined using both molecular modeling and retention data. The strongest stereoselective interaction is for carboxy-terminated D-amino-acids. In case of peptides, it is not necessary for these to be a D-, D-, terminal sequence for strong interactions. In some cases, including Ala-Ala, the L-, D-, terminal sequence showed greater interaction with the teicoplanin chiral stationary phase.

Amino Acid Sequence↗
PubMed1995

Determination and use of Rohrschneider-McReynolds constants for chiral stationary phases used in capillary gas chromatography.

The overall polarity of 22 chiral stationary phases (CSPs) used in capillary open tubular gas chromatography columns was estimated using the first five Rohrschneider-McReynolds constants. Most of the columns (i.e., 18) were wall-coated, and four were of the wall-immobilized or of the so-called "bonded" type. A wall-coated capillary squalane column was specially prepared as a polarity reference column. All but two of the CSPs were based on derivatized cyclodextrins (CDs) of different sizes. The overall properties of the CSPs are discussed in terms of the five Rohrschneider-McReynolds constants and their average values. It was found that the derivatized cyclodextrin CSP polarity increased with the CD ring size. The bonded CSPs were significantly less polar than their coated homologues due to the apolar polymer used to immobilize the CD rings. The retention behavior of 14 compounds was studied at 100 degrees C on the capillary columns. Retention parameters are clearly related to the McReynolds constants. Conversely, the enantiomeric resolution capability of a given stationary phase is not related to the constants. The enantioselective resolution mechanism critically depends on the solute structure and on the nature of the CSP.

Chromatography, Gas↗
PubMed1993

Microbore liquid chromatography with electrochemical detection for the control of phenolic antioxidants in drugs and foods.

Antioxidants are added to foods and drugs to inhibit their oxidation. As these additives are somewhat toxic, it is necessary to control the amount added to any food or drug. Liquid chromatography (LC) is a powerful tool for this purpose. Many antioxidants are electroactive molecules which enables the advantages of electrochemical detection or selectivity and sensitivity to be realized. The interest of analysts in microbore LC arises from the low mobile phase volumetric flow rates involved, the reduced on-column samples together with reduced chromatographic dilution and high efficiency. Coupling of microbore LC with electrochemical detection adds another advantage: the decrease of electrode ageing. The problem of extra-column band broadening with microbore column is discussed in the present communication. A micro LC-electrochemical detection system is constructed and tested using catecholamines. The limit of detection (LOD) for noradrenaline using a 0.7 mm bore column is found to be 0.1 pg injected in 0.2 microliter (0.6 femtomoles). Three phenolic antioxidants are studied: tert-butyl-p-hydroxyanisole (BHA), di-tert-butyl-hydroxytoluene (BHT) and n-propyl gallate (3,4,5-trimethoxybenzoic acid propyl ester). The dynamic range is four orders of magnitude with LODs down to 0.1 femtomoles (20 fg injected) with a 0.3 mm bore column. No electrode response change is observed after 60 injections of 3 ng BHA over 6 days. Antioxidants are determined in different pharmaceutical preparations and foodstuffs (chewing gums, dried potato flakes). The agreement between the manufacturer stated concentrations and observed results is found to be satisfactory.

Antioxidants↗
PubMed1992

Empirical procedure that uses molecular structure to predict enantioselectivity of chiral stationary phases.

A total of 121 racemic compounds were separated in the normal-phase mode on a (S)-(1-naphthylethyl)carbamoylated beta-cyclodextrin (S-NEC-beta-CD) bonded phase and 74 on the R equivalent (R-NEC) chiral stationary phase (CSP). All compounds are of the type that have four substituents on a stereogenic center, rather than an "axis of chirality". It is shown that the binary solvent pair used as the mobile phase has a significant influence on chiral recognition. However, the proportions of the components of a specific pair have little effect. From the results, the individual contributions to chiral recognition by these CSPs were estimated for 81 different substituents of the stereogenic center. Varying the arrangement of these 81 substituents could produce over 1.6 million compounds. Hydrogen was chosen as the reference substituent and was assigned a 0 cal/mol free energy. The chiral recognition increased when sp2-hybridized carbons were connected to the stereogenic center. Conversely, sp3-hybridized carbons decreased the enantioselectivity. Amido groups increased the chiral recognition, especially when associated with pi-acid (3,5-dinitrobenzoyl) or pi-basic (naphthyl) groups. This approach does not allow one to know which enantiomer elutes first. However, the "substituent energy" list for chiral compounds can be used to obtain an estimated value for the enantioselectivity of a compound by adding the energy contributions of the four substituents connected to the stereogenic center. In this way one can predict a priori whether or not a compound will separate on a CSP and estimate its separation factor (alpha). Theoretically, this approach can be used for most CSPs, provided a sufficient data base is generated on them.

Carbamates↗
PubMed1990

Solid-surface room-temperature phosphorescence detection for high-performance liquid chromatography.

In the present study, we have optimized the use of a two nebulizer system for solid-surface room-temperature phosphorescence (SSRTP) as a selective, permanent record detector for high-performance liquid chromatography (HPLC). The chromatographic parameters and the analytical figures of merit of five well known phosphorescent compounds were compared to those obtained by ultraviolet detection. Calibration curves with satisfactory linear dynamic ranges and limits of detection in the nanogram and subnanogram level showed the feasibility of the SSRTP detector for HPLC. In addition, overlapped compounds were individually identified demonstrating that the selectivity of the proposed detector can be a useful feature in case of incomplete chromatographic separations of complex mixtures.

Benzopyrenes↗
PubMed1990

Sensitive, indirect photometric detector for high-performance liquid chromatography using a light-emitting diode.

A low-noise detector for indirect photometric detection has been constructed using a highly stable source--a light-emitting diode (LED). Use of the detector is demonstrated for reversed-phase liquid chromatography by adding methylene blue to the mobile phase to make a background signal. The indirect determination of alcohols by their effect on methylene blue concentration distribution is demonstrated, and an investigation is made into the conditions for high sensitivity. Because the source exhibits low noise, the detection limits for alcohols are as low as more complex and expensive detection methods, despite the lower radiant power of the LED. Detection limits for nine alcohols are below micrograms injected amounts.

Alcohols↗
PubMed1990

Cyclodextrin chiral stationary phases for liquid chromatographic separations of drug stereoisomers.

Many active drugs are racemic mixtures. Because the two enantiomers of a racemate often cause different pharmacological responses, the use of optically pure isomers is desirable and may be soon required. Cyclodextrin-bonded silica gel can be used as chiral stationary phase (CSP) in liquid chromatography. The enantiomers of 25 different racemic drugs were separated on such CSPs in the reversed-phase mode. The principal features of the cyclodextrin chiral recognition mechanism are recalled and some information on future trends for cyclodextrin CSPs is provided.

Chromatography, Liquid↗
PubMed1990

Determination of non-metallic elements by capacitively coupled helium microwave plasma atomic emission spectrometry with capillary gas chromatography.

A capacitively coupled microwave helium plasma with a tubular tantalum electrode was evaluated as an element selective detector for gas chromatography (GC). The end of a 10-m bonded fused capillary column was directly inserted into the tubular electrode without any switching system. A heated copper tube was used to house the part of the GC column that protruded from the oven. The optimisation of operating parameters, line selection, background emission and horizontal and vertical observation position is described. Analytical figures of merit including sensitivity, reproducibility, signal to background ratio, selectivity, dynamic range and limit of detection (LOD), were evaluated for carbon, hydrogen, chlorine and bromine emission. Limits of detection in the low ng range (20 pmol) were obtained for halogenated compounds using carbon emission, whereas LODs in the 0.1 micrograms range (2 nmol) were obtained using chlorine or bromine emission lines.

Bromine↗
PubMed1988

Dry adsorbed emulsions: an oral sustained drug delivery system.

The oral sustained drug delivery system "dry adsorbed emulsion" was defined as an organized dispersion of hydrophilic and hydrophobic particles whose structure was initiated by the structure of a water-in-oil (W/O) emulsion. Sodium salicylate was dissolved in the aqueous phase of the primary W/O emulsion as an active drug. The aqueous phase of the W/O emulsion was adsorbed by a hydrophilic silica and then a hydrophobic silica was added to the preparation to obtain a stable and solid pulverulent form. The physicochemical structure of a "dry adsorbed emulsion" was described and observed by electron microscopy. The effect of different oils, castor oil and a silicone oil, on the sustained drug release was studied at two different pH values, 1.2 and 7.4, to simulate the gastric and intestinal medium, respectively. The properties of these forms were retained for more than one year at room temperature storage.

Adsorption↗
PubMed1988

Analysis by surface enhanced Raman spectroscopy on silver hydrosols and silver coated filter papers.

Surface enhanced Raman spectrometry (SERS) is an analytical technique with a sensitivity comparable to that of conventional molecular absorption or fluorescence spectroscopy, with the additional major advantage of selectivity inherent in vibrational spectroscopies. The analytical application of flowing silver hydrosols is described. Under the controlled experimental conditions of flow injection analysis, it was possible to detect as low as 30 ng of p-aminobenzoic acid. The linear range was two orders of magnitude (1-100 microg ml(-1)) with a signal reproducibility of 3.2%. Silver coated filter paper is another SERS active substrate that is simple to prepare and handle. The SERS spectra of several nitrogen-containing molecules were obtained on these substrates. The effects of laser power and paper hydration are described. The relative advantages of both substrates are compared.

Journal Article↗