Structural Characterization of “Tailed Picket-Fence Porphyrins” by High-Energy Fast Atom Bombardment Collision-Induced Dissociation Mass Spectrometry/Mass Spectrometry

The applicability of high-energy fast atom bombardment (FAB) collision-induced dissociation mass spectrometry/mass spectrometry (CID-MS/MS) to the structural characterization of side-chains of porphyrin derivatives with a “tailed picketfence” structure was examined. These porphyrins showed a molecular ion [M]^+• as well as a protonated molecule [M + H]⁺ and another at [M + 2]⁺ in the positive ion mode. Doubly-charged ions were observed when the “tail” has an imidazole ring.

The linked-scan spectrometry indicated that the “tail” of the porphyrin is eliminated first as a neutral species from the pyrrole-2-position as a β-type cleavage, after which each of the four pivaloyl “picket-fences” are cleaved off in series from the remainder of the molecule-related ions. The spectrum also clearly showed charge-remote product ions in the low mass region, in addition to charge-mediated product ions representing the precise side chain structure of the “tailed picket-fence” porphyrin. FAB CID-MS/MS spectra of [M]^+•, [M + 1]⁺ and [M + 2]⁺, using the EBEB-type four-sector tandem mass spectrometer, show similar fragmentation patterns to each other with 1 u difference in parallel. The CID-MS/MS of [M + 1]⁺ and [M + 2]⁺ showed higher product ion intensities than the corresponding product ion intensities of [M]^+•. CID-MS/MS, using an EBEB-type four-sector tandem mass spectrometer, showed charge-remote product ions much more clearly than those observed in B/E-constant linked-scan spectra.