Characterization of erythromycin analogs by collisional activated dissociation and infrared multiphoton dissociation in a quadrupole ion trap.

The effectiveness of two activation techniques, collision activated dissociation (CAD) and infrared multiphoton dissociation (IRMPD), is compared for structural characterization of protonated and lithium-cationized macrolides and a series of synthetic precursors in a quadrupole ion trap (QIT). Generally, cleavage of the glycosidic linkages attaching the sugars to the macrolide ring and water losses constitute the major fragmentation pathways for most of the protonated compounds. In the IRMPD spectra, a diagnostic fragment ion assigned as the desosamine ion is a dominant ion that is not observed in the CAD spectra because of the higher m/z limit of the storage range required during collisional activation. Activation of the lithium-cationized species results in new diagnostic fragmentation pathways that are particularly useful for confirming the identities of the protecting groups in the synthetic precursors. Multi-step IRMPD allows mapping of the fragmentation genealogies in greater detail and supports the proposed structures of the fragment ions.