Chemical ionization mass spectrometry of some representative 16-membered ring macrolide antibiotics.

Chemical ionization mass spectrometry, using isobutane as a reactant gas, is shown to be useful in the structural characterization of representative 16membered ring macrolide antibiotics. The spectra of spiramycin I, spiramycin III, and niddamycin contain relatively intense protonated molecule ion peaks (MH+) making establishment of molecular formulae straightforward. There are relatively few fragment ion peaks in comparison with the corresponding electron impact mass spectra, but these peaks are nonetheless highly significant. The major fragmentations observed involve sequential loss of the sugar moieties and loss of other small molecules such as water, acetic acid, and methanol. Scission of carbon-carbon bonds is uncommon. The nature of these reactions and the mechanisms responsible are discussed briefly.