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Biomedical subjects

J Norberg

Publications and source records attributed to J Norberg.

9 recordsLinked to original sources
PubMed2000

Automated liquid membrane extraction for high-performance liquid chromatography of Ropivacaine metabolites in urine.

An automatic method for the determination of metabolites of Ropivacaine in urine was set up. It utilizes supported liquid membrane extraction for sample clean-up and enrichment, followed by ion-pair chromatography determination using UV detection. The extraction was very selective with no observed interfering compounds from the urine matrix, permitting simple isocratic chromatographic analysis. The detection limits for spiked urine samples were 2-18 nM for the different compounds. The repeatability was 1-3% (RSD) with an internal standard that was also extracted, and about twice without this standard. A throughput of 3.3 samples per hour was achieved and the liquid membrane was stable for more than a week.

Amides↗
PubMed2000

Microporous membrane liquid-liquid extraction coupled on-line with normal-phase liquid chromatography for the determination of cationic surfactants in river and waste water.

Membrane-based continuous liquid-liquid extraction combined on-line with normal-phase liquid chromatography is proposed for the determination of cationic surfactants in complex aqueous samples. The technique has the potential for complete automation. Selective enrichment of cationic surfactants from spiked river water and waste-water samples with simultaneous removal of matrix constituents, followed by a quantitative transfer of the extract onto a liquid chromatographic column and separation of the surfactant homologues yielding low detection limits, has been realised. The homologues of alkyldimethylbenzylammonium chloride (Dodigen 226) were chosen as model compounds in the method development. Dodigen homologues were ion-paired with heptanoic acid and extracted into chlorobutane by means of microporous membrane liquid-liquid extraction. It was thereby possible to attain an enrichment of over 250 times for one of the homologues, viz. the concentration in the organic liquid is 250 times higher than in the original sample. Detection limits for the three best-detected homologues of the mixture were in the range 0.7-5 microg/l in spiked river water samples. Ion-pair normal-phase liquid chromatography, again with heptanoic acid as counter-ion, gave the necessary separation of the surfactant homologues.

Cations↗
PubMed1998

Solvent influence on base stacking.

In this paper we present a detailed analysis of the base-stacking phenomenon in different solvents, using nanosecond molecular dynamics simulations. The investigation focuses on deoxyribo- and ribodinucleoside monophosphates in aqueous and organic solutions. Organic solvents with a low dielectric constant, such as chloroform, and solvents with intermediate dielectric constants, such as dimethyl sulfoxide and methanol, were analyzed. This was also done for water, which is highly polar and has a high dielectric constant. Structural parameters such as the sugar puckering and the base-versus-base orientations, as well as the energetics of the solute-solvent interactions, were examined in the different solvents. The obtained data demonstrate that base stacking is favored in the high dielectric aqueous solution, followed by methanol and dimethyl sulfoxide with intermediate dielectric constants, and chloroform, with a low dielectric constant.

Calorimetry↗
PubMed1997

On-line supported liquid membrane-liquid chromatography with a phenol oxidase-based biosensor as a selective detection unit for the determination of phenols in blood plasma.

The potential of on-line combination of supported liquid membrane extraction and column liquid chromatography with a phenol oxidase-based biosensor as a selective detection unit has been investigated for the determination of phenols in human plasma. The phenols are selectively extracted into a porous PTFE (polytetraflouroethene) membrane impregnated with a water-immiscible organic solvent and further into an alkaline acceptor phase. Via an ion-exchange interface, the analytes are transferred to a reversed-phase column where they are separated and detected using the biosensor. No sample pretreatment before the extraction, except centrifugation, is made. Due to the high selectivity both in the extraction and in the detection steps and to the fact that the demands on the chromatographic separation are low, a quick separation using an eluent with a low concentration of organic modifier can be made, without affecting the biosensor response. Detection limits below the 50 microg/l level in blood plasma were obtained for the three model compounds, phenol, p-cresol and 4-chlorophenol.

Biosensing Techniques↗
PubMed1996

Glass transition in DNA from molecular dynamics simulations.

Molecular dynamics simulations of the oligonucleotide duplex d(CGCGCG)2 in aqueous solution are used to investigate the glass transition phenomenon. The simulations were performed at temperatures in the 20 K to 340 K range. The mean square atomic fluctuations showed that the behavior of the oligonucleotide duplex was harmonic at low temperatures. A glass transition temperature at 223 K to 234 K was inferred for the oligonucleotide duplex, which is in agreement with experimental observations. The largest number of hydrogen bounds between the polar atoms of the oligonucleotide duplex and the water molecules was obtained at the glass transition temperature. With increasing temperature we observed a decrease in the average lifetime of the hydrogen bonds to water molecules.

Base Sequence↗
PubMed1996

Internal mobility of the oligonucleotide duplexes d(TCGCG)2 and d(CGCGCG)2 in aqueous solution from molecular dynamics simulations.

In this paper we present longitudinal relaxation times, order parameters and effective correlation times for the base and sugar carbons in both strands of the oglionucleotide duplexes d(TCGCG)2 and d(CGCGCG)2, as calculated from 400 ps molecular dynamics trajectories in aqueous solution. The model-free approach (Lipari and Szabo, 1982) was used to determine the amplitudes and time scales of the internal motion. Comparisons were made with NMR relaxation measurements (Borer et al., 1994). The order parameters could acceptably be reproduced, and the effective correlation times were found to be lower than the experimental estimates. Reasonable T1 relaxation times were obtained in comparison with experiment for the nonterminal nucleosides. The T1 relaxation times were found to depend mainly on the order parameters and overall rotational correlation time.

Base Sequence↗
PubMed1995

Potential of mean force calculations of the stacking-unstacking process in single-stranded deoxyribodinucleoside monophosphates.

The free energy of the stacking-unstacking process of deoxyribodinucleoside monophosphates in aqueous solution has been investigated by potential of mean force calculations along a reaction coordinate, defined by the distance between the glycosidic nitrogen atoms of the bases. The stacking-unstacking process of a ribodinucleoside monophosphate was observed to be well characterized by this coordinate, which has the advantage that it allows for a dynamical backbone and flexible bases. All 16 naturally occurring DNA dimers composed of the adenine, cytosine, guanine, or thymine bases in both the 5' and the 3' positions were studied. From the free-energy profiles we observed the deepest minima for the stacked states of the purine-purine dimers, but good stacking was also observed for the purine-pyrimidine and pyrimidine-purine dimers. Substantial stacking ability was found for the dimers composed of a thymine base and a purine base and also for the deoxythymidylyl-3',5'-deoxythymidine dimer. Very poor stacking was observed for the dCpdC dimer. Conformational properties and solvent accessibility are discussed for the stacked and unstacked dimers. The potential of mean force profiles of the stacking-unstacking process for the DNA dimers are compared with the RNA dimers.

DNA↗
PubMed1994

Stacking-unstacking of the dinucleoside monophosphate guanylyl-3',5'-uridine studied with molecular dynamics.

Molecular dynamics simulations were carried out on two conformations of the dinucleoside monophosphate guanylyl-3',5'-uridine (GpU) in aqueous solution with one sodium counterion. One stacked conformation and one with the C3'-O3'-P-O5' backbone torsion angle twisted 180 degrees to create an unstacked conformation. We observed a relatively stable behavior of the stacked conformation, which remained stacked throughout the simulation, whereas the unstacked conformation showed major changes in the backbone torsion and glycosidic angles. During the simulation the unstacked conformation transformed into a more stacked form and then back again to an unstacked one. The calculated correlation times for rotational diffusion from the molecular dynamics simulations are in agreement with fluorescence anisotropy and nuclear magnetic resonance data. As expected, the correlation times for rotational diffusion of the unstacked conformation were observed to be longer than for the stacked conformation. The 2'OH group may contribute in stabilizing the stacked conformation, where the O2'-H...O4' hydrogen bond occurred in 82.7% of the simulation.

Computer Simulation↗