FAST-ATOM-BOMBARDMENT AND TANDEM MASS-SPECTROMETRY OF MACROLIDE ANTIBIOTICS

Molecular weights of macrolide antibiotics can be determined from eith er (M + H)+ or (M + Met)+, the latter desorbed from alkali metal salt- saturated matrices. The ion chemistry of macrolides, as determined by tandem mass spectrometry (MS/MS), is different for ions produced as me tallated than those formed as (M + H)+ species. An explanation for the se differences is the location of the charge. For protonated species, the charge is most likely situated on a functional group with high pro ton affinity, such as the dimethylamino group of the amino sugar. The alkali metal ion, however, is bonded to the highly oxygenated aglycone . As a result, the collision-activated dissociation spectra of protona ted macrolides are simple with readily identifiable fragment ions in b oth the high and low mass regions but no fragments in the middle mass range. In contrast, the cationized species give complex spectra with m any abundant ions, most of which are located in the high mass range. T he complementary nature of the fragmentation of these two species reco mmends the study of both by MS/MS when determining the structure or co nfirming the identity of these biomaterials.