Ralph Krebber

Characterization of cyclomalto-hexaose and -heptaose derivatives by the reductive-cleavage method

Abstract The substitution patterns of cyclomalto-hexaose and -heptaose derivatives carrying alkyl, acyl, and carbamoyl substituents have been investigated by the reductive-cleavage method. The modified cyclomalto-hexaoses or -heptaoses were treated with triethylsilane and trimethylsilyl trifluoromethanesulfonate to give the corresponding 1,5- and 1,4-anhydroglucitol derivatives that were acetylated or, in the case of acetyl derivatives, trifluoroacetylated and analysed by g.l.c.-m.s. For the alkylated compounds, minute amounts of products formed by under- or over-alkylation, or of isomeric components, could be detected. Partial reduction of the acyloxy groups to alkyloxy groups and cleavage of acyl substituents were observed. Carbamoyl substituents were stable under the conditions of reductive cleavage.

Nontoxic and toxic oligopeptides with D-amino acids and unusual residues in Microcystis aeruginosa PCC 7806

Toxic and nontoxic peptides were isolated from the cyanobacterium Microcystis aeruginosa PCC 7806 by a procedure including extraction of cells with water-saturated 1-butanol, chromatography of the extract on silica gel plates and high performance liquid chromatography (HPLC) on Partisil-5. The toxin was shown to be only a minor constituent, being negatively charged and thus separable by electrophoresis, within the HPLC-purified fraction. It contained erythro-β-methyl-D-Asp, D-Glu, D-Ala, L-Leu, and L-Arg known to be part of the Microcystis peptide-toxin with Mr 994. The major part of the HPLC-purified fraction was assigned, however, to a nontoxic peptide with a Mr of 956. Partial hydrolysis studies of the nontoxic peptide(s) revealed amino acid sequences composed of D-Glu, N-methyl-Phe, and 3,4-dehydro-Pro, aside from the common L-amino acids. Cyclic linkage in the nontoxic peptide(s) appears likely.

Determination of the absolute configuration of α-damascone and α-ionone from black tea by enantioselective capillary gas chromatography

Abstract The (S)-configuration was assigned to α-damascone and α-ionone, important flavour components in black tea, by enantioselective capillary gas chromatography using octakis(3-O-methyl-2,6-di-O-pentyl)-γ-cyclodextrin as a chiral stationary phase.

Two closely related peptide toxins in axenically grown Microcystis aeruginosa PCC 7806.

Summary Two closely related peptide toxins from axenically grown Microcystis aeruginosa PCC 7806 were purified using molecular mass, charge and solubility as criteria. Water extracts of cells were applied to reversed phase C18 cartridges, and toxic fractions eluting between 20% and 100% methanol in water (vol/vol) were subjected to gel filtration. The fraction with an apparent M r of about 1 kDa was further separated by analytical paper electrophoresis and preparative ion exchange chromatography. Acidic, ninhydrin-negative toxic material was finally purified by reversed phase high performance liquid chromatography yielding two distinct toxic peptides. The LD 100 for mice was determined at 0.075 mg per kg body weight for each toxin. The major toxin (T17) with M r 994 was composed of equimolar amounts of erythro -β-methyl-D-aspartic acid, D-glutamic acid, D-alanine, L-leucine, L-arginine, N-methyl dehydroalanine and 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-4,6-decadienoic acid (Adda). The other toxin (T16) with M r 980 had an identical composition except for a replacement of erythro -β-methyl-D-aspartic acid by D-aspartic acid. Mass spectrometric data suggested cyclic linkage for both toxins.

Preparative separation of enantiomeric polychaete sex pheromones

result indicates that [2A3C]leucine was metabolized in M. alba cell cultures to [1AaC]acetyl CoA, which subsequently participates in the triketide synthesis (Fig. 4). On the other hand, incorporation of the 13C from L-[2-13C]leucine into SIT was not observed as in the case of prenyl moieties of CAL. This finding is compatible with the results obtained by Overton et al. on the biosynthesis of sesquiterpenoids in Andrographis paniculata cell cultures [7].

Gas Chromatographic Enantiomer Separation of Chiral Ketones - Order of Elution and Configurational Stability of Oxime Derivatives

SummaryOximes of chiral ketones can be separated into enantiomers by capillary gas chromatography on the chiral polysiloxanes XE-60-L-valine-(S)- or (R)-α-phenylethylamide. Usually predominantly E-isomers are formed and are better separated than Z-isomers. From time-dependent gas chromatographic measurements it can be concluded that the oximes are configurationally stable and that racemization during oxime formation is negligible. The order of elution of enantiomers depends on the structure of the ketone and is different for aliphatic and alicyclic compounds.

Stereoisomere Aromastoffe@@@Stereoisomeric flavour compounds: XLVII. Direkte chirospezifische HRGC-Analyse natrlicher ?-Lactone@@@XLVII. Direct chirospecific HRGC-analysis of natural ?-lactones

Summary5-Alkylatedδ-lactones are well known flavour compounds in animal fats and also some special kinds of fruit. Recently, the first direct stereo-differentiation of chiralδ-lactones was reported using Oktakis(3-O-butyryl-2,6-di-O-pentyl)-γ-cyclodextrin as the chiral stationary phase [4]. The enantiomeric separation ofδ-lactones up to the long-chain compoundsδ-trideca-(δ-C13−) andδ-tetradeca-(δ-C14−)-lactone is demonstrated. By means of optically pure references, the order of elution S(I), R(II) was determined for the important flavour compoundsδ-octa-,δ-deca-, andδ-dodecalactone. Analysis of the steam distillate of some dairy products (milk, cream, butter), of margarine and of coconut has been carried out, in order to yield the enantiomeric distribution of C8-, C10- and C12-δ-lactones by HRGC and multidimensional gas chromatography. Characteristic and product-specific ratios ofδ-lactone enantiomers are detected. The pattern of enantiomeric composition ofδ-octa-,δ-deca-,δ-dodecalactone in coconuts differs completely from that found in the dairy products.Zusammenfassung5-Alkyl-substituierteδ-Lactone finden sich als aromarelevante Substanzen in tierischen Fetten, aber auch in einigen Früchten. Vor kurzem gelang die erste direkte gaschromatographische Stereodifferenzierung einigerδ-Lactone an der chiralen Phase Oktakis(3-O-butyryl-2,6-di-O-pentyl)-γ-cyclodextrin [4]. Es wird nun die Enantiomerentrennung derδ-Lactone bis zu den langkettigen Verbindungenδ-Trideca-(δ-C13-)- undδ-Tetradeca-(δ-C14-)-lacton aufgezeigt. Über optisch reine Referenzsubstanzen konnte die Elutionsfolge S(I), R(II) der drei wichtigsten aromarelevanten Vertreter dieserδ-Lactonklasse bestimmt werden. Die Enantiomerenverteilung der C8-, C10-, C12-δ-Lactone im Wasserdampfdestillat verschiedener Milchprodukte (Milch, Sahne, Butter), in Margarine sowie im Kokosnußfleisch wurde exemplarisch mit Hilfe der HRGC, auch unter Einsatz der multidimensionalen Gaschromatographie (MDGC), untersucht. Es zeigten sich charakteristische, substratspezifische Enantiomerenverteilungen. Die Verteilungsmuster der untersuchten Spiegelbildisomeren aus Kokosnüssen unterschieden sich wesentlich von denen der Milchprodukte.