Electrospray Tandem Mass Spectrometry of β-Nitroalkenyl Meso-Tetraphenylporphyrins

Abstract

β-Nitroalkenyl meso-tetraphenylporphyrins [β-TPPCHC(NO2)R)], as free-bases and Zn(II) complexes, were studied by electrospray mass spectrometry (ESI-MS). Under this ionisation condition the [M + H]+ ions are formed. The fragmentation pattern of the resulting [M + H]+ ions were studied by electrospray tandem mass spectrometry (ESI-MS/MS). The ESI-MS/MS of β-nitroalkenylporphyrins, either as free-bases or as Zn(II) complexes, show several interesting features, distinct from the typical behaviour of nitro compounds. For the studied compounds, common main fragmentation patterns are observed, namely characteristic losses of NO2•, HNO2, 2OH•, RNO2, RCNO, RCNO2, RCH2NO2, C6H5• plus NO2• and the formation of the protonated macrocycle, [TPP + H]+ or [ZnTPP + H]+. However, depending on the presence or absence of the metal and the nature of the R substituent, important differences are observed on the relative abundances of the ions formed by the same fragmentation pathway. The presence of bromine in the alkenyl group leads to a peculiar behaviour, since the main fragmentation pattern corresponds to the combined elimination of the bromine atom with the typical nitro group fragments. When R = Br, the loss of the nitro group occurs in low relative abundance (11-16%). However, when R = CH3, the relative abundance of the ion due to the loss of HNO2 changes drastically from 100%, observed for the free-base porphyrin, to 29% in the case of the Zn(II) complex. These variations of the relative abundance of the fragment corresponding to the loss of the nitro moiety (typically considered as a diagnostic fragment) can induce to an erroneous interpretation of their MS/MS spectra. Some fragmentations are observed only for the free-base porphyrins, namely the loss of •CH(NO2)R and HNO2 plus C2H2, while the loss of OH•, H2O, OH• plus H2O and RCCH plus H2O is observed only for the complexes. Unusual and unexpected fragmentations are also observed, namely the losses of RCNO, RCNO2 and HNO2 plus C2H2. This work demonstrates that valuable structural information about the β-nitroalkenyl substituents linked to meso-tetraarylporphyrins can be achieved using MS/MS. These results can also be useful for the interpretation of the mass spectra of other nitroalkenyl substituted compounds.