193 nm ultraviolet photodissociation of deprotonated sialylated oligosaccharides.

The fragmentation patterns of deprotonated sialylated oligosaccharides and glycans from fetuin obtained upon collisionally induced dissociation (CID) and 193 nm ultraviolet photodissociation (UVPD) in a linear ion trap are presented. UVPD produced a more extensive series of cross-ring cleavage ions, such as A- and X-type ions, and dual-cleavage internal ions, including A/Y and X/B fragment ions. In addition, UVPD generated unique fragment ions which arise from site-specific cleavage of the triol substituent of the sialic acid residue. In contrast, CID produced more conventional glycosidic cleavages and relatively few A-type ions. UVPD of doubly deprotonated sialylated oligosaccharides produced mostly singly deprotonated fragment ions, whereas the product ions in the CID spectra were overwhelmingly doubly charged ions, an outcome attributed to the more extensive cleavages of sialic acid residues upon UVPD and products from electron photodetachment. The larger array of product ions, including those arising from extensive cross-ring cleavages and dual-cleavage ions, generated by 193 nm UVPD relative to CID gives greater confidence for identification of glycans. Several key site-specific cleavages by UVPD, such as ones involving the sialic acid moieties, provide evidence of glycan composition.