NEGATIVE-ION FAST ATOM BOMBARDMENT TANDEM MASS-SPECTROMETRY OF SODIUM-SALTS OF MONENSINS AND RELATED COMPOUNDS

The negative-ion collisionally induced dissociation fast atom bombardment mass spectra of sodium salts of monensins A and B, their 3-O-demethyl derivatives, laidlomycin and 26-deoxylaidlomycin are reported. The nature of the R(3) substituent appears to be important for the formation of sodium-containing product ions. Fragment ion series observed in the mass spectra of the [M - H](-) ions of monensins are characterized by alkali metal retention in contrast to monensins, the product ions observed in the corresponding spectra of laidlomycios are sodium free. The affinity of monensins to alkali metals is a useful feature that may be used for the structure determination of unknown monensins, their metabolites and derivates. The structure of a novel natural monensin was elucidated based on its mass spectrometric behaviour.