FAB-MS Experiments Research Articles

Synthesis, structure (NMR and mass spectrometry) and conformational analysis of heterocyclic analogues of dibenzo[a,e]cycloocta-1,5-diene: 5,6,12,13-tetrahydrobispyrazolo[1,2-a:1′,2′-e][1,2,5,6]tetraazocinediium dihalides

Several 5,6,12,13-tetrahydrobispyrazolo[1,2-a:1′,2′-e][1,2,5,6] tetraazocinediium dihalides 4a–d and 8 are prepared from pyrazole, 3,5-dimethylpyrazole, 4-(1-adamantyl)pyrazole and campho[2,3-c]pyrazole by stepwise alkylation with 1,2-dibromoethane or 1,2-dichloroethane. Their structural characterization has been achieved by NMR and mass spectrometry. Dynamic NMR spectroscopy allowed the measurement of the barrier for the chair–chair interconversion in the case of the parent compound 4a and the 1,3,8,10-tetramethyl derivative 4b. These barriers as well as the preferred chair conformation are rationalized through semi-empirical and molecular mechanics calculations with regard to dibenzo[a,e]cycloocta-1,5-diene. The study of doubly charged bispyrazolium salts allows demonstration of their reduction by addition of a hydride ion [C+++ H–→(C + H)+] during FABMS experiments.

Isolation and structure elucidation of five new asterriquinones from Aspergillus, Humicola and Botryotrichum species.

Five new quinone pigments have been discovered from the fermentation broth of Aspergillus, Humicola and Botryotrichum species isolated from different soil samples. These compounds inhibit serine proteases of the coagulation pathway. Their structures which differ in the identity and position of a 5-carbon side chain on the indole moiety have been elucidated based on NMR and FAB-MS experiments.

Panclicins, novel pancreatic lipase inhibitors. II. Structural elucidation.

Panclicins A-E are novel and potent pancreatic lipase inhibitors produced by Streptomyces sp. NR 0619. Their structures have been elucidated based on NMR and FAB-MS experiments. The relative configurations have also been determined by NMR experiments. The absolute stereochemistry has been determined by the chiral HPLC analysis of the hydrolysates of panclicins A and B and by modified Mosher's method on a derivative of panclicin A. They are structurally related to beta-lactone esterase inhibitors of microbial origin, lipstatin, valilactone, ebelactones and esterastin. Panclicins also contain a beta-lactone structure with two alkyl chains, one of which has an N-formylalanyloxy or N-formylglycyloxy substituent.

Mechanism of fragmentation of organic molecules during keV particle bombardment

A simplified scheme is presented to distinguish between surface and gas-phase fragmentation of molecular ions in SIMS or FABMS experiments. The scheme is based on the measurement of the polar angle distribution of ejected parent and daughter cluster-ions and is derived from straightforward considerations of classical mechanics. Examples are given for chlorobenzene, benzene and pyridine adsorbed onto Ag(111) at 153 K.