Structural characterization of a new class of penem beta-lactam antibiotics by triple quadrupole tandem mass spectrometry.

A series of new penem beta-lactam antibiotics, having at position 2 of the thiazoline ring an amido-substituted amino acid moiety spaced by a methylene group, was analyzed by collisionally activated dissociation (CAD) tandem mass spectrometry on a triple-quadrupole mass spectrometer, after ionization by cesium ion bombardment. This approach allowed complete structural characterization without any matrix interferences. All the compounds investigated underwent beta-lactam ring cleavage through a typical retro-Diels-Alder reaction, as well as cleavage of the 2-side chain at two different points, with charge retention on both sides of the molecule. Subsequent fragmentation reactions essentially involved combinations of the above processes.