Structural characterization of glycoporphyrins by electrospray tandem mass spectrometry.

Porphyrin derivatives having a galactose or a bis(isopropylidene)galactose structural unit, linked by ester or ether bonds, were characterized by electrospray tandem mass spectrometry (ES-MS/MS). The electrospray mass spectra of these glycoporphyrins show the corresponding M + H ions. For the glycoporphyrins with pyridyl substituents and those having a tetrafluorophenyl spacer, the doubly charged ions M + 2H were also observed in ES-MS with high relative abundance. The fragmentation of both M + H and M + 2H ions exhibited common fragmentation pathways for porphyrins with the same sugar residue, independently of the porphyrin structural unit and type of linkage. ES-MS/MS of the M + H ions of the galactose-substituted porphyrins gave the fragment ions M + H - C(2)H(4)O(2), M + H - C(3)H(6)O(3), M + H - C(4)H(8)O(4) and M + H - galactose residue. The fragmentation of the M + 2H ions of the porphyrins with galactose shows the common doubly charged fragment ions porphyrin + H, M + 2H - C(2)H(4)O(2), M + 2H - C(4)H(8)O(4), M + 2H - galactose residue and the singly charged fragment ions M + H - C(3)H(6)O(3) and M + H - galactose residue. The fragmentation of the M + H ions of glycoporphyrins with a protected galactosyl residue leads mainly to the ions M + H - CO(CH(3))(2), M + H - 2CO(CH(3))(2), M + H - 2CO(CH(3))(2) - CO, M + H - C(10)H(16)O(4) and M + H - protected galactose. The doubly charged ions M + 2H fragment to give the doubly charged ions porphyrin + H and the singly charged ions M + H - protected galactose residue and M + H - CO(CH(3))(2). For the porphyrins where the sugar structural unit is linked by an ester bond, M + 2H, ES-MS/MS showed a major and typical fragmentation corresponding to combined loss of a sugar structural unit and further loss of water, leading to the ion M + 2H - sugar residue - H(2)O, independently of the structure of the sugar structural unit. These results show that ES-MS/MS can be a powerful tool for the characterization of the sugar structural unit of glycoporphyrins, without the need for chemical hydrolysis.