Synthesis, ligand binding and biomimetic oxidations of deuterohaemin modified with an undecapeptide residue

Abstract

Deuterohaemin [(3,7,12,17-tetramethylporphyrin-2,18-dipropionato)iron(III)] has been covalently linked to the undecapeptide Ala-Phe-Ser-Phe-Glu-Ala-Gln-Gly-Gly-Leu-Ala at one of the propionic acid side chains. The binding equilibria between the resulting deuterohaemin-undecapeptide complex and imidazole occur in two steps; the first molecule of the ligand binds with high affinity (K₁= 1500 dm³ mol^–1), while for the second molecule the affinity is markedly lower (K₂= 220 dm³ mol^–1), indicating that folding of the peptide chain in the monoimidazole adduct severely limits the accessibility of the sixth iron co-ordination position. The complex exhibits both catalase and peroxidase activity towards reducing substrates in the presence of hydrogen peroxide. In catalytic sulphoxidations of a series of para-substituted thioanisoles, p-XC₆H₄SMe, by hydrogen peroxide a good correlation has been found between the relative rates and the Hammett σ_p values, indicating that direct oxygen transfer from the active oxoiron species to the sulphide is probably operative. The kinetics of the catalytic oxidation of tyrosine by hydrogen peroxide in the presence of the complex, producing the oxidative coupling dimer o,o′-dityrosine, was also studied. It is consistent with a mechanism involving the initial binding of the phenolic substrate to the active catalyst to form an intermediate complex, and its subsequent breakdown in the rate-determining step of the catalytic cycle. The results obtained in the biomimetic oxidations are compared with those of the corresponding peroxidase-catalysed reactions.