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

✍ Scribed by Ines Batinić-Haberle; Robert D. Stevens; Irwin Fridovich

• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: English
• Weight: 135 KB
• Volume: 04
• Category: Article
• ISSN: 1088-4246
• DOI: 10.1002/(sici)1099-1409(200004/05)4:3<217::aid-jpp198>3.0.co;2-e

✦ Synopsis

Manganese(III) complexes of isomeric tetrakis(N-alkylpyridyl)porphyrins (N- alkyl = N- methyl , M or N- ethyl , E ), MnTM ( E )-2(3,4)- PyP^5+^, are being developed as superoxide dismutase (SOD) mimics. Simultaneously, techniques for their purification, identification and characterization are being pursued. Electrospray mass spectrometry ( ESMS ) proved to be an excellent method for identification and characterization of this group of water-soluble cationic porphyrins. The multiply charged parent ion is observed for both the metal-free ligands and their corresponding manganese complexes. The other major peaks 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. As a result of inductive and resonance effects, which stabilize the ortho isomer, almost no loss of N-alkyl groups from the manganese complex or from its parent ligand was observed. The relative intensity of the multiply charged molecular ion Mn^III^TM -3(4)- PyP^5+^/5 was 100% in the case of the meta and para isomers. Although manganese porphyrins display a low preference toward axial ligation, favorable electrostatics at the metal center of the ortho isomer gives rise to 100% relative intensity of the species that has chloride axially ligated at the manganese site, Mn^III^TM ( E )-2- PyPCl^4+^/4. When the stronger preference of iron porphyrins toward axial ligation combines with the ortho effect, the monohydroxo iron porphyrin Fe^III^TM -2- PyP ( OH )^4+^/4 dominates the ESMS of an aqueous acetonitrile solution at pH 7.8.