Hemoproteins are metalloproteins which possess iron porphyrin as a prosthetic group (cofactor). These proteins have received much attention not only as biocatalysts but also as promising building blocks for development of new types of biomaterials. Particularly, the heme cofactor in several hemoproteins such as myoglobin, horseradish peroxidase, cytochrome P450, and cytochrome b₅₆₂ are replaceable with an artificial heme analog.  To functionalize the hemoproteins, our group has focused on the cofactor-substitution with several porphyrinoid metal complexes.¹⁾  Furthermore, we have recently introduced a heme moiety onto the myoglobin surface via covalent linkage to obtain a supramolecular protein polymer via interprotein heme–heme pocket interaction.²⁾  In this presentation, we report the recent results of the hemoprotein assembly which is a model of a light-harvesting system.³⁾

     A hexameric tyrosine-coordinated heme protein (HTHP) containing photosensitizer molecules was prepared in an effort to construct a biomolecule with engineered photochemical properties.  Zn protoporphyrin IX (ZnPP) and Zn chlorin e₆ (ZnCe6) are inserted into the apoprotein of HTHP to yield reconstituted proteins which maintain the original hexameric structure of HTHP.  Femtosecond transient absorption measurements indicate the occurrence of rapid singlet–singlet annihilation within a few picoseconds for each protein completely reconstituted with the six photosensitizer molecules.  This finding supports the fact that the photo-induced energy migration occurs whtin the protein with the zinc complex.  The fluorescence quenching efficiencies provided by methyl viologen as an electron acceptor for the completely reconstituted proteins with ZnPP and ZnCe6 are 2.3 and 2.6 fold-higher than that of the corresponding partially photosensitizer-inserted proteins, respectively.  This indicates that energy migration occurs among the photosensitizers bound within the protein matrices. These findings are expected to lead to development of new protein-based artificial light harvesting systems. 

1)  Hayashi, T. In Coordination Chemistry in Protein Cages; Ueno, T.; Watanabe, Y., Eds.; Wiley: Hoboken, US, 2013; Chapter 4, pp 87–110.

2)  Oohora, K.; Onoda, A.; Hayashi, T. Chem. Commun. 2012, 48, 11714–11726. 

3)  Oohora, K.; Mashima, T.; Ohkubo, K.; Fukuzumi, S.; Hayashi, T. Chem. Commun. 2015, 51, 11138–11140.