Gas phase reactions of ß-substituted hetero-Diels–Alder adducts of meso-tetraphenylporphyrin using tandem mass spectrometry

• ESI-MS/MS of ß-substitued porphyrins with heterocyclic moieties.
• These heterocyclic moieties have pharmacological activities.
• Main fragmentations occur via retro hetero-Diels–Alder pathway.
• Unexpected fragmentations were observed for Zn(II) complexes.
• Differentiation of isomers were possible only for Zn(II) complexes.

ß-Substitued-meso-tetraphenylporphyrins with 5,10-dioxobenzo[g]- or 5,6-dioxobenzo[h]chromene, pyrano[3,2-c]coumarin and benzopyran moieties and the corresponding Zn(II), Cu(II) and Ni(II) complexes were studied by electrospray mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS). These heterocyclic moieties have well established pharmacological activities and as such the introduction of these motifs into the ß-pyrrolic position of the porphyrin macrocycle can alter the properties of the macrocycle and can produce new molecules with dual functions. The free base and Zn(II) complexes showed, in the ESI-MS spectra, the [M+H]+ ions while the Cu(II) and Ni(II) complexes showed the M+
• ions. The [M+H]+ and M+
• ions were induced to fragment and the corresponding ESI tandem mass spectra (MS/MS) were analyzed. The main fragmentation mechanism occurs in general via the retro hetero-Diels–Alder pathway while unexpected fragmentations or rearrangements were observed principally with the Zn(II) complexes. The analysis of the fragmentation pattern of all complexes indicates that the presence or absence of the carbonyl function in the ß-substituent led to the formation of secondary fragments. The differentiation of the isomers 2a and 2b was only possible by comparison of their MS/MS spectra.

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