ELECTRON IONIZATION MASS-SPECTROMETRIC ANALYSIS OF 5-NITRO OCTAETHYLPROPHYRIN - EVIDENCE FOR SCISSION OF THE PORPHYRIN MACROCYCLE

Analysis of the electron ionization mass spectrum (EIMS) of 5-nitro oc taethylporphyrin reveals that the molecular ion m/z 579 undergoes nove l fragmentations. The typical fragmentation pathway of beta-cleavage o f peripheral substituents is suppressed. Furthermore, the presence of the fragment ion m/z 375 provides strong evidence for a fragmentation pathway that involves cleavage of the porphyrin macrocycle. Such a fra gmentation pathway has not been reported previously for porphyrins bea ring beta-pyrrolic substituents. Subsequent EIMS and electron ionizati on tandem mass spectrometric analyses, both of the 5-nitro octaethylpo rphyrin and of its analogs isotopically labeled at the meso (bridge) p osition (C-13) or at the pyrrolic nitrogens (N-15) or at the nitro gro up (N-15), confirmed the ring scission process. The elemental composit ion of the significant fragment ions was confirmed by high-resolution EIMS. The anomalous behavior may be attributed to macrocyclic distorti on and/or the electron-withdrawing effect of the nitro group. (C) 1997 by John Wiley & Sons, Ltd.