Electrospray mass spectrometry of isomeric tetrakis(N-alkylpyridyl)porphyrins and their manganese(III) and iron(III) complexes

Manganese(III) complexes of isomeric tetrakis(N-alkylpyridyl)porphyrins (N- alkyl = N-methyl, M or N-ethyl, E), MnTM(E)-2(3,4)-PyP5+, are being develop ed as superoxide dismutase (SOD) mimics. Simultaneously, techniques for the ir purification, identification and characterization are being pursued. Ele ctrospray mass spectrometry (ESMS) proved to be an excellent method for ide ntification and characterization of this group of water-soluble cationic po rphyrins. The multiply charged parent ion is observed for both the metal fr ee ligands and their corresponding manganese complexes. The other major pea ks in the mass spectra result from loss of N-alkyl groups, reduction of the metal center, axial coordination of chloride or hydroxo ion in the case of the Fe porphyrin, loss of metal and deprotonation of pyrrolic nitrogens. A s a result of inductive and resonance effects, which stabilize the ortho is omer, almost no loss of N-alkyl groups from the manganese complex or from i ts parent ligand was observed. The relative intensity of the multiply charg ed molecular ion (MnTM)-T-III-3(4)-PyP5+/5 was 100% in the case of the meta and para isomers. Although manganese porphyrins display a low preference t oward axial ligation, favorable electrostatics at the metal center of the o rtho isomer gives rise to 100% relative intensity of the species that has c hloride axially ligated at the manganese site, (MnTM)-T-III(E)-2-PyPCl4+/4. When the stronger preference of iron porphyrins toward axial ligation comb ines with the ortho effect, the monohydroxo iron porphyrin (FeTM)-T-III-2-P yP(OH)(4+)/4 dominates the ESMS of an aqueous acetonitrile solution at pH 7 .8. Copyright (C) 2000 John Wiley & Sons, Ltd.