Chiral assembly of a pair of free base porphyrins and peroxidase-like activity of iron(III) porphyrins in four-α-helix bundle structures with dimerized two-α-helix polypeptides

Abstract

A 5-(4α-bromoacetamidophenyl)-10,15,20-tritolylporphyrin was incorporated into a single-chain two-α-helix polypeptide containing 29 amino acid residues via the thiol side chain of a Cys residue. Two molecules of two-α-helix polypeptide connecting free base porphyrin (H₂-porphyrin) were strongly associated to form a four-α-helix bundle structure by hydrophobic interaction among α-helix segments and porphyrin rings in aqueous solution. The dimer formation was demonstrated by gel filtration chromatography and various spectroscopic measurements. The Soret band in the UV/vis spectrum was broadened and red-shifted in aqueous solution. In the fluorescence spectrum with excitation at the Soret band the emission at 650 nm was quenched to 40% of the intensity measured in methanol. Especially, at the Soret band was observed a strongly split circular dichroism (CD) signal, which demonstrated the chiral assembly of a pair of porphyrins in a left-handed sense. With increasing methanol content, the intensity of this split signal was decreased and finally diminished probably due to dissociation of the porphyrin moieties in the peptides. The dimerized Fe(III)-porphyrin-linked two-α-helix polypeptide was examined for biomimetic peroxidase-like activity with H₂O₂ or 3-chloroperbenzoic acid (MCPBA) as the oxidant. The k_cat/K_M value for the oxidation of 3,7-dimethylamino-10-methylcarbamoylphenothiazine by the polypeptide with MCPBA was increased by 5000 times compared to that with H₂O₂. This fact suggests that Fe(III) porphyrin was located in the hydrophobic core and more easily accommodated an organic oxidant than does H₂O₂. The interior of the four-α-helix bundle structure may be further designed to mimic various haemproteins.