Engineering purple bacterial photovoltaic complexes for device applications

This project is concerned with the first attempts to convert a protein that powers biological photosynthesis into a material that can be used at the heart of an efficient solar cell. The project comes at a time when there is growing interest in alternative technologies for providing mankind with increasing amounts of energy in ways that will not exacerbate environment change. Mankind is facing an energy crisis. It is estimated that our global energy demand will double by the middle of the 21st century, and could more than treble by 2100. For well-understood reasons increased burning of fossil fuels will not be able to meet this soaring energy demand, and although many of the alternative energy sources currently being considered may address need in the short term, they suffer from limited capacity, uneven geographical distribution, adverse environmental impact and political/security problems. The only source of alternative energy freely available across the whole planet is solar energy. Biological photosynthesis consumes around 250 terawatts of the sun's output annually, dwarfing the current 16 terawatt energy demand of the global population, but still only a tiny fraction of the 120,000 terawatts of solar energy incident on the Earth every year. Humans have always been reliant on the products of photosynthesis for the bulk of their energy, in a variety of ways, but there is now increasing interest in clean technological approaches to harvesting this free and largely benign source of energy. Photovoltaics, the direct generation of electric current from light energy, has become a familiar concept with the public through the advent of silicon-based solar panels. However this technology has its limitations, and there is a pressing need to develop alternative photovoltaic materials that are lighter, cheaper to produce, less reliant on rare elements, and are more efficient under non-optimal conditions. It is likely that in the future solar energy will be harvested through a wide range of technologies, each suited to particular conditions. One approach that has seen a rapid growth in interest over the last ten years is the use of natural photovoltaic proteins as a photovoltaic material. Plants, algae and bacteria exploit light energy through nanometer-sized proteins called reaction centres. These proteins capture light energy and convert it into a form that the organism can use, and in the key event light energy is used to move an electron along the length of the reaction centre, leaving a positive charge at one end and a negative charge at the other. Just as with the terminals of a battery, or a silicon solar panel, this difference in charge can be used as a source of power. It is already known that if reaction centres are carefully removed from a plant or bacterium, and attached in a particular way to an electrode and illuminated, an electric current can be generated. The next stage in the evolution of this technology is to adapt natural reaction centres as more effective photovoltaic materials by altering their structures and various aspects of how they deal with light energy and electrons. This project is a first step in this direction, seeking to convert the reaction centre from a photosynthetic bacterium into a material that will produce larger photocurrents and is better suited to operation inside a solar cell rather than inside a bacterium. The attractive aspect of exploiting natural reaction centres in this way is that they convert harvested light energy into an electrical potential difference with an efficiency that is close to 100 %, they can be synthesised in large quantities through natural processes, and they have capacity for self-assembly and self-protection/repair. The longer term goal of the research is to develop new materials for harvesting solar energy, either through direct adaptation of natural proteins, or by providing new insights that will inspire new generations of photovoltaic materials.

Concerns over burgeoning energy demands and energy security have produced increased interest in alternative means of harnessing freely available solar energy through the development of novel photovoltaic materials. Through natural photosynthesis, most of the biological activity on the planet is powered by a process in which harvested light energy is used to separate charge across an energy-transducing membrane. The quantum yield with which reaction centres (RCs) catalyse this process is close to unity, and in the last ten years a number of research groups in the USA, China, Germany, Israel and Japan have explored methods for incorporating of native RCs into photovoltaic devices. The applicant has also conducted research in this area with collaborators in the UK and The Netherlands, and this application takes this work in new directions. This project will explore ways in which the structure and mechanism of the purple bacterial RC can be adapted for efficient operation in a solar cell rather than in a bacterial cell. We will engineer the RC to interface more effectively with an electron-donating electrode surface. We will simplify the redox chemistry of the acceptor side of the RC by opening up the QA site to oxidation by redox mediators and diverting electron flow from the unnecessarily complex two-electron/two-proton chemistry catalysed at the QB site. We will explore the use of GxxxG helix dimerisation motifs and salt bridge interactions to induce RCs to form higher order complexes, and use gene fusions to explore the formation of novel covalently-linked RC multimers. We will, for the first time, analyse the impact that the protein/lipid environment of the RC has on its stability, mechanism and photovoltaic capacity. Finally, we will determine how mutations that are known to change the rate of charge separation impact on RC function when operating in a protective bilayer and in the presence of light harvesting complexes.

Relevance to BBSRC priorities. This project is an excellent fit with the BBSRC priorities NANOSCIENCE THROUGH ENGINEERING TO APPLICATION: BIONANOTECHNOLOGY and SYNTHETIC BIOLOGY. In the former, the research addresses the interest areas 'development or reconstruction of both 2-D and 3-D nanoscale systems to mimic the biological environment, taking into account molecular structure, function and orientation' and 'understanding of how biology can be interfaced with electronic, mechanical and optical systems to create devices with applications'. In the latter, the research addresses the issues 're-design of existing, natural biological systems for useful purposes' and 'create novel biological functionality', areas of application aligning with 'new and renewable sources of energy, novel materials and sensors'. The project also has relevance to the BBSRC priority area BIOENERGY, seeking to develop a biological system in a novel manner for the efficient exploitation of solar energy. Who will benefit from this research? It is anticipated that data and materials from the project will be of interest to researchers with broader interests in alternative photovoltaic materials, and adaptation of biological reaction centres and other membrane proteins for a range of device applications. It will also be of benefit to researchers interested in how photosystems work in their natural setting, rather than in 'bubbles of soap'. The appointed RA will also benefit from the training offered by the interdisciplinary research programme. Significant travel funds are being requested to allow the appointee to gain hands-on experience of the spectroscopic and photovoltammetric techniques employed in the laboratories of collaborating groups. How will they benefit from this research? In the longer term, practical materials and devices for harnessing solar energy in a carbon neutral way, developed through the type of basic research described in this application, will have an enormous impact on society through increased energy security. The project will generate two main types of output, knowledge and materials. Knowledge will be disseminated through the routes outlined below, and will inform the design of future protein-based solar cells. Findings from the project will also be of interest to researchers with broader interest in interfacing biological materials with surfaces, and large scale engineering of membrane proteins for device or synthetic biology applications Regarding materials, the applicant's approach in the past has been to make materials such as bacterial strains expressing mutated reaction centres freely available to other academic researchers, provided that there isn't a very clear conflict of interest. In the case of the present project the goal is to generate materials that can be exploited in a device, and so the approach will be to continue to make materials available on request (subject to the usual material transfer agreements) unless the properties of the material warrant protection through a patent. The University of Bristol have a Research & Enterprise Development office that provides excellent support to academic staff in areas such a protection of intellectual property. What will be done to ensure that they have the opportunity to benefit from this research? Findings from the project will be disseminated principally through publication in high-impact journals and presentations at international conferences. The University of Bristol has a Public Relations Office tasked with publicising research appearing in high-impact journals, and we will make use of this resource. Subject to the caveat outlined above, materials generated during the project such as plasmids, bacterial strains, and engineered reaction centres and RC-LH1 complexes will be made available to other researchers on request, and through new collaborative links where feasible.