Pigments Controlling the Quantum Efficiency of Photosynthetic Light Harvesting

The biological process of photosynthesis is the foundation upon which all life on Earth is supported. Since the advent of oxygenic photosynthesis, oxygen-evolving organisms have provided all heterotrophic organisms with both food and the oxygen required to utilize it. More recently, photosynthetic organisms have provided humans with a huge variety of useful compounds, including fuels, pharmaceuticals, fibres, pigments, and animal feeds and it has become apparent that plant-derived commodities will only increase in importance in the future green economy. The proliferation of oxygenic photosynthesis on Earth may be attributed to the efficient evolutionary design of the molecular machinery of photosynthesis, along with their adaptability with respect to changing environmental conditions. The photosynthetic membranes of the chloroplasts, commonly known as the thylakoid membranes, are the most complex of all biological membranes, being greatly enriched in a diverse collection of various protein complexes. These protein complexes represent the necessary molecular machinery of complex, multi-step process of photosynthesis, carrying out such diverse tasks as light-harvesting, electron transport and the synthesis of vital biochemical compounds. In order to fulfil these roles the various membrane proteins bind a number of cofactors such as chlorophylls, carotenoids, lipids, water, and various metal ions. One of the major pigment-lipoprotein complexes found within the thylakoid membrane is the light-harvesting complex of photosystem-II, LHCII. This complex, which is a trimer of three identical proteins, each binding 18 photosynthetic pigment molecules, collects light energy received by the thylakoid membrane and transfers it to the photosynthetic reaction centres. In addition to LHCII there are two minor light-harvesting complexes known as CP26 and CP29. In addition to this role LHCII, has been found to play an important role in regulating the amount of energy that is delivered to the reaction centre. This is achieved via the dissipation of excess energy absorbed during periods of intense illumination, a process commonly referred to as photoprotection. Recently we have discovered that the currently available structure of LHCII corresponds to the structure of a photoprotective conformation of the complex. This finding is of particular importance since it offers unique structural insights into how the thylakoid membrane senses and responds to excessive illumination, and hence how photosynthetic systems protect themselves against photodamage. It has also been suggested that the minor antennas, CP26 and CP29, rather than LHCII are responsible for photoprotection. The aim of this proposed work is to understand how the photosynthetic pigments known as xanthophylls regulate the amount of light energy delivered to the reactions centres by the major and minor antenna complexes. This work is divided into two copuled parts, each of which will employ a unique combination of methods from the fields of experimental biophysics, theoretical physics, and quantum chemistry to complete. First, it is essential to understand how varying the specific xanthophyll compliment of LHCII affects the rate of energy dissipation in the antenna complex. This will be achieved via spectroscopic measurements of the antenna complexes and theoretical modelling of the transfer of excitation energy. Second, we will investigate whether specific sites in LHCII, CP26, or CP29 are responsible for the dissipation of excess energy. This will require detailed theoretical modelling of the electronic properties of the photosynthetic pigments, theoretical simulation of the transfer and dissipation of energy within the antenna complexes, and detailed spectroscopic mapping of the energy transfer pathways within the antenna complexes of both mutants and natural specimens.

Who will benefit from this research? 1. The principle end users of this research will be in the commercial sectors associated with nanotechnology, and solar energy industries. In addition, applications may arise in biofuel and crop industries. 2. Agricultural agencies, both in the UK and abroad are also potential beneficiaries. How will they benefit from this research? 1. In the very near future, there will be a strongly increasing demand for sustainable energy for our society. The sun is the far biggest source of energy, and photosynthetic organisms in both land and aquatic environments are the foundations of a bio-based economy. 2. This project will investigate fundamental characteristics about how light energy is absorbed and transduced by plants, and how the process is regulated to provide both highly efficient solar energy conversion and protection from environmental stress. In a bio-based economy, photosynthesis is the direct basis for the generation of food, chemical feedstocks and biofuels from plants and microalgae. It is increasingly recognized that improvements in photosynthesis are needed in all of these cases and, in a dynamic environment, the response of photosynthesis to irradiance is an important trait. Already programmes are beginning which have the improvement in photosynthetic processes as the target, with a timescale of 5-10 years. 3. There is also considerable interest in utilizing the unique properties of biomolecules in various new technologies - e.g. in the construction of nanoscale devices in which biomolecules are printed onto circuits interfacing with man-made components. In the case of the light harvesting complexes investigated in this project there is scope for new kinds of solar energy converters which utilise the remarkable plasticity of these molecules in smart energy conversion. Rapid progress is being made in the technologies underpinning the construction of sophisticated hybrid systems. One foresees practical outputs within 10 years. 4. A key element in the knowledge transfer process will be the training of young scientists with the necessary set of skills. Researchers are needed who have sufficient intellectual luggage to produce the innovations that are needed for a successful bio-based economy. The multidisciplinary nature of this research project which combines state of the art spectroscopy and theotical physics and chemistry applied to a problem of demonstrated physiological importance will help train young researchers in such a way that they will be able to work at the forefront in such innovations. What will be done to ensure that they benefit from this research? Dissemination of results. Publications in peer-reviewed international journals; Oral and poster contributions at international scientific meetings and workshops; the project will be described on the PI's website; (http://queenmaryphotosynthesis.org/~ruban/); publicity of important finding via press releases from QMUL. Exploitation. In order to interface effectively with various potential end users, we would like to involve Professor Peter Horton. Following 30 years of research in the area of this project, Professor Horton now advises and consults on the practical applications of photosynthesis research, and is involved with a number of projects in the agricultural and bioenergy fields. He is currently biosolar advisor to the Faculty of Science at the University of Sheffield (http://photosynthesis.peterhorton.eu/). Training. The training opportunities provided by this project will be greatly augmented by the participation of the PI in the HARVEST Marie Curie training Network of the EU FP7 programme. HARVEST brings together 15 top institutes from various disciplines working on the elementary regulation mechanisms in oxygenic photosynthesis, as well as academic groups and commercial enterprises working on new methodologies suitable for industrial and commercial exploitation of biosolar energy.