Picking apart the role of network organisation in biological photosynthesis

Enough sunlight falls on the Earth in one hour to power the planet for a year; given this astonishing fact, it is no surprise that a major goal in the chemical sciences is the search for new and improved chemical systems that can efficiently capture sunlight to drive electricity production and/or chemical storage. The widespread adoption of cheap, efficient and sustainable solar technologies will have major implications for UK society, most notably in reducing dependence on foreign oil imports; in recognition of the impact and potential of solar technologies, the UK government is currently committed to powering 4 million homes by solar energy alone by 2020.

Of course, nature has already developed an outstanding system for deriving energy from sunlight: photosynthesis. The light-harvesting (quantum) efficiency of photosynthetic pigment-protein complexes is close to 100%, but varies anywhere from 1–100% in organic semiconductor-based solar cells; both system types are built from simple building blocks familiar to organic chemistry, such that the difference in efficiency points to the clear importance of organisation in guiding emergent properties.

As a result, there is much to be learnt from nature’s light-harvesting apparatus in developing artificial systems that mimic this performance; this is exactly the aim of this project. For example, how does the efficient and directed energy flow in biological PPCs emerge from the organisation of relatively simple light-harvesting molecules (chromophores)? What kind of spatial organisation is most favourable? And how does nature avoid competitive pathways that would reduce the light-harvesting efficiency? In this project, we will use computer simulations to investigate these questions, with the aim of divining nature’s own ‘recipe’ for efficient light-harvesting.

Biological photosynthetic energy transport complexes exhibit a wide range of molecular organisation motifs, from the seemingly disordered arrangement of chromophores (coloured tiles) amongst the protein scaffold (grey) of the Fenna-Matthews-Olson complex to the highly-symmetric arrangement observed in purple bacteria (Rhodospirilum molischanum). Unravelling the role of network organisation in these systems is the goal of this project.

A recent proof-of-concept study of the Fenna-Matthews-Olson (FMO) complex found in green sulfur bacteria [J. Chem. Phys., 143, 105101 (2015)] demonstrated that the influence of molecular organisation in photosynthetic systems can be understood using a novel combination of quantum simulations of energy transport along with network-based analysis. For example, our initial work has already suggested that FMO exhibits a surprising degree of robustness and redundancy in its energy transport network; up to 50% of the chromophore molecules can be removed before significant decrease in energy transport efficiency is observed. This Leverhulme Trust Research Project Grant will enable us to significantly expand this work by studying a much broader range of photosynthetic complexes from a variety of organisms. By investigating correlations between photosynthetic complex structure and energy transport efficiency, we hope that this project can suggest new directions for exploiting molecular organization in the development of, for example, new artificial solar cell technologies.

Dr Scott Habershon
University of Warwick
Research Project Grant