Abstract
The aim of this work was to further develop the use of the protein-free iron porphyrin Microperoxidase-8 (MP8) as a haemoprotein model and to exploit its potential for studying weak and non-bonding interactions in aqueous solution. MP8 contains a proximal histidine ligand and is monomeric in 20% aq. MeOH solution. MP8 was found to form low spin 6-coordinate complexes with a range of nitrogenous bases, including amines and amino acids; such complexes are models for the b cytochromes and cytochrome f. Evidence was obtained for axial coordination of pyridine and aniline. The influence of the trans ligand on the pKa of the proximal histidine ligand was studied and a trans effect order determined. FerroMP8 can also be used as a model for infra red and UV-visible studies of axial ligation of deoxy-Hb and -Mb. The higher logK values and red-shifted Soret maxima for formation of MP8-Phe (4. 76) and MP8-Trp (5. 76) compared to non-aromatic amino acids (e. g. MP8-Gly 3. 46) indicated π-π interaction of the aromatic side chains with the porphyrin ring. This novel approach uses the Fe-N bond as a molecular anchor for studying porphyrin-side chain interactions. Studies of oligopeptide complexes of haemin and MP8 were carried out to further probe the role of key amino acids in haemoproteins. MP8 was shown to form donor-acceptor complexes with N-methyl pyridinium ion and a range of antimalarial drugs including phenanthrene-based molecules (relevant to clinical mode of action). Evidence for interaction of chloroquine and quinine with haemoglobin was also obtained. Aqueous urea and guanidinium ion were shown to interact directly with aromatic model compounds (e. g. benzene) to form stoichiometric adducts; equilibrium constants were determined. This is of relevance to hydrophobic solubilization and protein denaturation by urea and guanidinium and to NH-π interactions in haemoproteins.