Differentiation of positional isomers of nitro meso-tetraphenylporphyrins by tandem mass spectrometry

We studied by tandem mass spectrometry two isomers of nitro meso-tetrapheny lporphyrin, one with a nitro group in the pam position of a phenyl ring and the other with the same group in a beta -pyrrolic position, and their copp er complexes. Collisional activation of the molecular ions of both free-bas e porphyrins and of their copper complexes produces an array of product ion s that permit ready differentiation of the two positional isomers. The diag nostic ions, when the nitro group is in a beta -pyrrolic position, may be p roduced through intramolecular and double cyclization processes, triggered by the interaction of the nitro substituent with the neighboring meso-pheny l ring. These diagnostic ions do not form when the nitro group is in the pa ra position. The gas-phase processes have precedents in solution chemistry. (C) 2001 American Society for Mass Spectrometry.