Supramolecular Metalloporphyrin Assemblies Mediated By a Hexameric Hemoprotein Unit

Abstract

Supramolecular protein assemblies provide unique structures, thermodynamic behaviors, and attractive functions derived from reversible interactions between protein-based building blocks. In nature, there are many kinds of unique protein assemblies including rings, sheets, tubes and cages, which are formed by multiple interactions such as electrostatic interactions, hydrophobic contacts, coordination etc. These beautiful composites have encouraged chemists to generate new bionanomaterials and then a lot of artificial protein assemblies have been reported over last decade.Hemoproteins possess a cofactor, protoporphyrin IX iron complex (heme b), which is bound in the heme pocket via Fe–axial ligand coordination and non-covalent interaction with large affinities. It is well known that the heme cofactor is removable from the heme pocket under acidic conditions to yield a corresponding apoprotein. Furthermore, the apoprotein can be converted to a reconstituted protein upon the addition of an artificial cofactor [1]. Our group has focused on a specific interaction between heme and the heme pocket of a hemoprotein to construct supramolecular hemoprotein assembles.A hexameric tyrosine-coordinated heme protein (HTHP) provides a ring structure containing six domains. Each domain has a heme-binding site named a heme popcket. The heme cofactors are bound in the heme pockets, showing a circular aggregate of the heme cofactor within protein matrix. First, we replaced the heme cofactor with Zn protoporphyrin IX (ZnPP) or Zn chlorin e6 (ZnCe6) to obtain a photosensitizer assembly mediated by protein. The measurement of femtosecond transient absorption spectra upon pulse irradiation showed a rapid singlet-singlet annihilation within a few picoseconds for the reconstituted HTHP. This finding supports the fact that the photoinduced energy migration occurs within the protein. Furthermore, the fluorescence quenching efficiency determined by the addition of methyl viologen as an electron acceptor also support the existence of the photoinduced energy migration for the photosensitizer-inserted protein. These findings are expected to lead to development of new protein-based artificial light harvesting systems [2].Furthermore, our group recently tried to construct supramolecular assembly of HTHP to obtain 1D, 2D and 3D protein assembled structures formed by non-covalent multi interaction between modified HTHP. In the case of the linear HTHP assembly, we introduced pyrene moiety onto the surface of HTHP and then provided the stable dimer of HTHP [3]. The 2D and 3D structures have already been prepared as an artificial light-harvesting system [4].In this presentation, we want to report photochemical property of HTHP reconstituted with the Zn photosensitizer and the trial of the construction of the highly dimensional HTHP assembly.