Creating Artificial Photosynthetic Reaction Centres: Amphiphilic Porphyrin Protein Maquette Complexes

The emulation of photosynthesis, the efficient and sustainable utilization of solar energy using renewable materials to produce hydrogen and oxygen from water or convert carbon dioxide into a chemical feedstock represents one of the great scientific challenges of the 21st Century. Creating photosynthetic-like processes in devices could not only provide a new generation of economical photovoltaic devices but also lead to sustainable hydrogen production through water splitting as well as fuel and food production through carbon dioxide fixation.

 The challenge in building a useful ‘artificial photosynthetic’ assembly is not simply using or mimicking the natural photosynthetic apparatus but utilizing new materials to create and, if possible, improve the structural properties and functionality of the biological system. In 1994, Dutton et al. developed the methodology for the facile production of de novo synthetic protein helices¹ (maquettes), structurally simpler analogs of natural redox proteins, which have proved extremely useful for the study of porphyrin behaviour and interactions in proteins (heme, chlorophyll, light-activated zinc porphyrins).² It has been demonstrated that not only is a maquette-bound porphyrin more efficiently photo-oxidized than the analogous free porphyrin but also that light-induced electron transfer between the porphyrin complex and an added acceptor is faster and higher yielding.³ As the maquettes can be assembled on a variety of surfaces such as gold or titanium dioxide, they provide a unique platform on which to build and study a light harvesting reaction centre reproduction.

 Over the last 10 years, we have developed syntheses of single porphyrins and porphyrin arrays and utilized the resulting materials as light harvesters in dye sensitized solar cells bound through carboxyl-based linkers to titanium dioxide.⁴ However, the introduction of porphyrins into water-soluble maquettes requires the development of amphiphilic porphyrins and porphyrin arrays.  Here we present the syntheses and incorporation of single porphyrin and amphiphilic porphyrin dimers into maquettes and the characterisation of the resulting porphyrin-maquette assemblies.  In order to assess the potential of the porphyrin maquette and as a first step in the development of an artificial photosynthetic reaction centre, we have bound a porphyrin maquette to titanium dioxide and used it as a photoanode in a solar cell; we will discuss the characteristics of this first artificial protein-based photovoltaic device.

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