Hemoprotein models based on a covalent helix-heme-helix sandwich 4. Discrimination of paramagnetic Fe(III)-Mimochrome I Δ and Λ isomers by NMR spectroscopy

Abstract

A 1H NMR study of the paramagnetic Fe(III)-Mimochrome I, a porphyrin peptide compound based on a covalent helix-porphyrin-helix sandwich, prototype of a new class of heme-protein models, is reported. The study demonstrates that Fe(III)-Mimochrome I exists, in d 7-DMF solution at millimolar concentration, as two hexacoordinated Δ and Λ isomers. It represents the first observation of two isomeric forms of a bis-histidine Fe(III)-porphyrin peptide adduct. 1H NMR studies in d 7-DMF solution of the paramagnetic Fe(III) deuteroporphyrin dimethylester (DPDME), either as acetate (Ac) or chloride salts, are also reported for comparison. In the acetate form this compound gives detectable different mixed-ligand isomers by imidazole addition, while the chloride form gives preferentially the bis-imidazole complex.