A theory of the fundamental adaptive properties of natural light harvesting
Photosynthesis is the transformation of solar energy into carbohydrates by plants, algae, etc. and as such is the foundation of the biosphere. It is the process from which humanity derives its food, fuel, pharmaceuticals, fibres and dyes, the process at the heart of the world’s economy and the focus of an intense global research effort.
Photosynthetic life possesses the capacity for highly efficient light-harvesting which is facilitated by the antenna/reaction centre architecture: A large modular assembly of pigment-binding proteins (light-harvesting complexes or ‘LHCs’) that serve to absorb the diffuse ‘rain’ of light energy and concentrate and deliver it to the reaction centres (RCs), the small subset of chlorophyll pigment molecules that carry out the first steps of photosynthesis. The photosynthetic architecture of plants is one of the most interesting light-harvesting structures in nature, not just for its efficiency but for its inherent adaptability in the face of changing environmental conditions. Far from being a passive, static structure it is highly adaptable, allowing for incredibly efficient (around 80%) energy capture in poor light while possessing the ability to defend itself from photo-damage in periods of intense illumination via the harmless dissipation of excess energy. The almost unique combination of high efficiency and adaptive flexibility arguably represents the most effective and subtle solar energy-harvesting strategy currently known. The molecular design principles behind the evolution of this architecture are reflected at the level of fine control of the network of intermolecular interactions within individual LHCs (the intrinsic switch) and in the variable integration of many different LHCs into a flexible, modular light-harvesting machinery (the dynamic photosynthetic membrane.
Understanding light-harvesting at the molecular level from a purely experimental perspective is difficult as data are typically coarse-grained and ambiguous. This ambiguity can be removed by establishing the fundamental theoretical foundations of the molecular processes of light-harvesting. Presently, this theoretical framework is incomplete as it explicitly neglects two central features of the light-harvesting architecture: The role of the secondary photosynthetic pigments, the red/orange carotenoids; and the variable structure of the modular LHC assembly as it exists in the living organism. The first omission is a result of the difficulty in constructing a theoretical model of the electronically complex carotenoids. However, Dr Duffy has recently published several papers establishing a computational framework that allows for the inclusion of these pigments into current models of LHCs, giving a theoretical picture of how the carotenoids are central to both light-harvesting and the intrinsic switch. A realistic description of the dynamic photosynthetic membrane is now possible due to recent, non-invasive measurements of intact membranes.
The project will establish the theoretical foundations of natural light-harvesting by:-
- Objective 1: The student will use a combination of computational chemistry and molecular dynamics to predict atomic structures for LHCs with altered xanthophyll compositions. Experimentally it has been shown that alterations in the xanthophyll composition of LHCs can profoundly alter their light-harvesting and defensive capabilities. Via modelling the excitation energy transfer dynamics of these mutant complexes the student will establish the theory of how the intrinsic light-harvesting/photoprotective capacities of the LHCs can be controlled by alterations in xanthophyll composition. The biological roles of xanthophyll variety (including the xanthophyll cycle) will be studied.
- Objective 2: The student will develop a coarse-grained of energy of energy-transfer/dissipation in a realistic model of the dark- and light-adapted PSII membrane, as obtained from existing Freeze Fracture EM data from intact chloroplasts. The model will make predictions of the role of protein organisation in light-harvesting adaptability.
- Chmeliov J, Bricker WP, Lo C et al. (2015) . An 'all pigment' model of excitation quenching in LHCII.PHYSICAL CHEMISTRY CHEMICAL PHYSICS vol. 17, (24) 15857-15867. 10.1039/c5cp01905b.
- Duffy CD, Chmeliov J, Macernis M et al. (2013) . Modeling of fluorescence quenching by lutein in the plant light-harvesting complex LHCII.J Phys Chem B vol. 117, (38) 10974-10986. 10.1021/jp3110997.
- Duffy CD, Valkunas L, Ruban AV (2013) . Light-harvesting processes in the dynamic photosynthetic antenna.Phys Chem Chem Phys vol. 15, (43) 18752-18770. 10.1039/c3cp51878g.
- Ruban AV, Johnson MP, Duffy CD (2012) . The photoprotective molecular switch in the photosystem II antenna.Biochim Biophys Acta vol. 1817, (1) 167-181. 10.1016/j.bbabio.2011.04.007.