Probing the interactions between sterically demanding ferrihemes and histidine analogues. I. 2-D NMR studies of nonplanar ferriheme/bis(N-/2-methylimidazole) complexes; II. Synthesis of a trans-disubstituted tetraarylporphyrin with a bulky group near

In a hemoprotein, histidine and other aromatic amines that ligate to the heme may encounter steric interactions from the following: (1) the peptide chains that form the pocket; or (2) the porphyrin ring that may have been distorted by the protein matrix or a distal ligand. Models that mimic these steric interactions need to be synthesized and studied in detail. The results of two research projects—both dealing with porphyrins that interact sterically with the axially ligated aromatic amines—are presented in this dissertation.;In the first part, variable-temperature 2-D NMR studies of nonplanar ferriheme complexes are presented. Hemes are surprisingly flexible, and a significantly distorted conformation is accessible by the forces exerted by surrounding proteins and ligands. Iron(III) octaethyltetraphenylporphyrin ([Fe(III)OETPP]+) is a saddled macrocycle that could serve as a model for distorted hemes. Two low-spin [Fe(III)OETPP]+ complexes, one with two N-methylimidazole (N-MeIm) molecules as the axial ligands and the other with two 2-methylimidazole (2-MeImH), have been made, and their NMR spectra recorded as a function of temperature. Spectral assignment for the bis(N-MeIm) complex has been made through 2-D techniques. The NOESY spectra indicate that at about –40°C, the ring inversion halts while the axial ligand dissociation remains prevalent. The spectra of the bis(2-MeImH) complex at –85°C indicate that the dominant internal dynamics consists of ring inversion and a concerted rotation of the axial ligands.;In the second part, the synthesis of a porphyrin with two sterically large groups near the potential sites of axial ligation is presented. In a bis(aromatic amine)/iron(III) complex of the αβ-atropisomer of 5,15-di-p-tolyl-10,20-bis[2,3-[(((hydrotris(3,5-dimethylpyrazolyl)borato)oxomolybdenio)dioxy]phenyl]porphyrin (1), the axial ligands are expected to be rotationally fixed. The synthesis of 1 is described. The corresponding trans -ditolylbis(dimethoxyphenyl)porphyrin (2) was synthesized from p-tolualdehyde and 2,3- dimethoxyphenyldipyrromethane in acidified aqueous sodium dodecyl sulfate in 16–25% yield. The demethylation of 2 and the subsequent reaction with HB(3,5-Me2Pyz) 3(OCH2CH2O) afforded 1, characterized with FAB-MS (cation detection mode). The product most likely consists entirely of the αβ-atropisomer because of the steric constraint during the formation.