Bärbel Gehrcke

Stereochemical effects of PCBs in the marine environment: Seasonal variation of coplanar and atropisomeric PCBs in blue mussels (Mytilus edulis L.) of the german bight☆

The toxic potential of coplanar polychlorinated biphenyls (PCBs), expressed by their toxic equivalent (TEQ) values according to Safe and to the recent WHO/ICPS recommendations, was determined in blue mussel (Mytilus edulis L.) samples collected at six sites in the Weser, Jade, and Elbe river estuaries during the spring and the autumn period. The maximum ∑TEQ value of about 1.5 ng g−1 EOM was found in the Elbe estuary, however, at all sampling sites the ∑TEQ values were alarming during both seasons. The non-ortho coplanar congener PCB 126 turned out to give a highest toxic potential in blue mussels. Based upon the present data set the possibilities and limitations of the TEQ concept are critically discussed. Furthermore, the stereochemical aspect of atropisomeric PCBs is investigated for the first time for marine biota. Five of the nine atropisomeric congeners present in commercial PCB formulations were found in blue mussels at all six sampling sites, i.e. PCB 88, PCB 149, PCB 183, PCB 174, and PCB 171, where their concentrations were significantly lower during the autumn than during the spring period. Furthermore, the enantiomers of PCB 149 in all mussel samples collected during the spring and autumn period were separated, which revealed that only weak enzymatic degradation of atropisomeric PCBs occurs in blue mussels. Although sufficient evidence is available that chirality of atropisomeric PCBs plays an important role in many recognition events associated with enzymatic processes, a risk assessment concept is lacking that might compete with the ‘toxic equivalence factor’ concept applied to coplanar PCBs.

Migration of secondary tert-butyldimethylsilyl groups in cyclomalto-heptaose and -octaose derivatives

When 2,6-di-O-tert-butyldimethylsilylated cyclomalto-oligosaccharides (cyclodextrins) are treated with allyl or methyl iodide and NaH in dry tetrahydrofuran, O-2-->O-3 migration of the secondary 2-O-tert-butyldimethylsilyl groups occurs, leading to 2-O-alk(en)yl-3,6-di-O-tert-butyldimethylsilyl-cyclodextrin derivatives. The detection and identification of the reaction step during which migration occurred is described and possible mechanisms of migration are discussed.

Determination of the enantiomers of ifosfamide and its 2- and 3-N-dechloroethylated metabolites in plasma and urine using enantioselective gas chromatography with mass spectrometric detection

A rapid, sensitive, enantioselective gas chromatographic method has been developed for the quantitation of the enantiomers of ifosfamide (IFF) and its 2- and 3-dechloroethylated metabolites (2-DCE-IFF and 3-DCE-IFF) in human and animal plasma and human urine. IFF and the two dechloroethylated metabolites were extracted into chloroform, enantioselectively resolved by gas chromatography on a chiral stationary phase based upon heptakis(2,6-di-O-methyl- 3-O-pentyl)-beta-cyclodextrin and quantitated using mass-selective detection with selected-ion monitoring. The limits of quantitation for the enantiomers of IFF, 2-DCE-IFF and 3-DCE-IFF in plasma were 250 and 500 ng/ml respectively. In urine, the limits of quantitation for the enantiomers of IFF, 2-DCE-IFF and 3-DCE-IFF were 500 ng/ml. The method can detect concentrations as low as 250 ng/ml of each enantiomer of 2- and 3-DCE-IFF in plasma and urine. The intra- and inter-day coefficients of variation for this method were with one exception less than 8%. The assay was validated for enantioselective pharmacokinetic studies in humans and rats and is the first reported enantioselective assay for the measurement of the enantiomers of 2- and 3-DCE-IFF in plasma.

Pharmacokinetics of (R)- and (S)-cyclophosphamide and their dechloroethylated metabolites in cancer patients.

The complete pharmacokinetics (PK) of (R)- and (S)-cyclophosphamide (CP) and their dechloroethylated (DCE) metabolites have not been reported to date. We collected plasma and urine samples from 12 cancer patients and determined concentrations of both enantiomers of CP and DCE-CP using a chiral GC-MS method. All concentrations of (R)-CP, (S)-CP, (R)-DCE-CP, and (S)-DCE-CP were simultaneously modeled using an enantiospecific compartmental PK model. A population PK analysis was performed. Enantiospecific differences between (R)- and (S)-CP were found for the formation clearance of CP to the DCE metabolites (Clf: 0.25 (R) vs. 0.14 (S) L/h). No difference was found between enantiomers for Cl40H, Cld, Cl(m)R, ClT, or T1/2. In contrast to the adolescent and adult group of patients, a child (6 years old) appeared to have a very different PK and metabolic profile (Bayesian control analysis). Proportions of the (R,S)-CP doses transformed to the (R)-DCE- and (S)-DCE-CP were much higher (R: 25 vs. 1.9%, and S: 38 vs. 3.6%), while formation of active metabolites was much lower (R: 42 vs. 74%, and S: 48 vs. 77%). CP appears to be enantioselectively metabolized to the DCE metabolites. This PK model can evaluate the proportion of a CP dose that is transformed to toxic or active metabolites. It may therefore be used to optimize CP treatment, to identify important drug interactions and/or patients with an abnormal metabolic profile.

Resolution of chiral [2.2]paracyclophanes by enantioselective gas chromatography

Abstract [2.2]Paracyclophanes possessing a substituent on the aromatic ring are dissymmetric compounds due to the restricted rotation of the benzene rings. For interconversion of enantiomers temperatures above 200 °C are necessary. We have succeeded in resolving the enantiomers of some mono-substituted [2.2]paracyclophanes by enantioselective gas chromatography using 2,3-di-O-methylated β-cyclodextrins with bulky O-(1,1,2-trimethylpropyl)-dimethylsilyl or O- t .butyl-dimethylsilyl substituents in the 6-position of the glucose units as chiral stationary phases.

Determination of the enantiomeric composition of mexiletine and its four hydroxylated metabolites in urine by enantioselective capillary gas chromatography

The enantiomers of mexiletine and four of its hydroxylated metabolites were directly separated by capillary gas chromatography using a heptakis(6-O-tert-butyl-dimethylsilyl-2,3-di-O-methyl)-beta- cyclodextrin column. The method was applied to the analysis of urine samples from cancer patients who were treated with racemic mexiletine as part of a study of the use of mexiletine in the relief of neuropathic pain. Samples analyzed before and after deconjugation of the urine with beta-glucuronidase/arylsulfatase showed a high stereoselectivity in the formation and conjugation of these compounds.

Gas chromatographic separation of the enantiomers of bromocyclen in fish samples

Abstract In muscle tissue of rainbow trouts from Danish fish farms and of three different fish species (orfe, bream, and pike) from the river Stor (northern Germany) remarkably high concentrations (between 0.09 and 1.23 mg/kg fat) of the chiral insecticide bromocyclen were found. For the first time, the separation of the bromocyclen enantiomers in environmental samples was achieved with the help of chiral capillary gas chromatography using modified cyclodextrin phases. A preferential enzymatic metabolism of the first eluting (−)-enantiomer was verified. The toxic implication of this result as well as the sources for the contamination are not yet known.

Enantioseparation of 5-monosubstituted hydantoins by capillary gas chromatography - investigation of chemical and enzymatic racemization

Abstract The chemical as well as the enzymatic racemization of R - and S -5-monosubstituted hydantoin derivatives play an important role in the chemoenzymatic synthesis of optically pure D - or L -amino acids. To study these reactions, specifically with 5-alkylhydantoins enantiomer-separation by capillary gas chromatography, octakis(2,6-di-O-methyl-3-O-pentyl)-γ-cyclodextrin as a chiral stationary phase was used. It will be shown that this method allows the sensitive and reproducible detection of a wide range of substrates in a short time. Examples are given for the chemical racemization of different 5-alkylhydantoins as well as for their enzymatic racemization by a hydantoin racemase of Arthrobacter sp. DSM 3745. The enantioseparation of different 5-monosubstituted hydantions by capillary gas chromatography, using oktakis(2,6-methyl-3-O-pentyl)-γ-cyclodextrin as chiral stationary phased was examined. The method described should allow not only to study chemical, but also enzymatic racemization processes of 5-monosubstituted alkyl-hydantoins.

Direct gas chromatographic enantiomeric resolution of juvenile hormones I – II

Abstract Enantioselective capillary gas chromatography with heptakis(2,6- di-O-methyl-3-O-pentyl)-β-cyclodextrin as a chiral stationary phase was used to resolve the enantiomers of juvenile hormones I - III. The assignment of the order of elution of the enantiomers was performed by comparison with synthetic reference compounds.