Quantum effects in photosynthesis
Photosynthesis is a process that is vital to all life on earth, with a 3 billion year evolutionary history of plant life photosynthesising both on land and deep in the ocean. During photosynthesis, light coming from the sun is converted into usable chemical energy. Studying photosynthesis allows us to understand this incredibly important process in more detail, as well as potentially using mechanisms in futuristic solar cells.
Although the overall process of photosynthesis is fairly inefficient, with a practical maximal efficiency of light to energy conversion of 8-9%, the initial step of capturing a photon with a light harvesting molecule and the transportation to a reaction centre happens with an efficiency of near unity [1,2]. There has been speculation for many years that there are quantum effects present in photosynthesis, with the unexpectedly high photon capture efficiency quoted as a reason. Quantum coherence present in antenna complexes would allow for fast and efficient excitonic transport required for the observed efficiencies, due to a possible wavelike search of paths from the exciton location to the reaction centre .
Long-lived quantum coherence in photon transfer is supported by experimental observations such as the interference oscillatory signals present in ultrafast optical spectroscopy experiments carried out on the Fenna-Matthews-Olson (FMO) protein . This view, however, faces continuous controversy and is challenged by other observations in which the oscillatory signals are not present leading to the proposal of alternative mechanisms of energy transfer. Investigating whether quantum mechanisms are involved in the initial photon capture by the antenna complex of different microorganisms apart from green-sulphur bacteria represents one of the methods that would contribute greatly for elucidating the phenomenon.
Eveliny, a biologist, will initially focus on the light-harvesting complexes (LH1 and LH2) of non-sulfur purple bacteria (see Fig. 1) such as Rhodopseudomonas acidophila, a tractable experimental system for which molecular tools are available . The initial hypothesis is that quantum signatures may be present depending on the protein investigated and experimental conditions tested and, therefore, an 'all or none' quantum explanation may be an oversimplification of reality. Eveliny will investigate the nature of energy transport by conducting a systematic study of the structure and function of LH proteins using i) computational methods (through structural modelling), ii) in vitro biophysical characterisation of combinatorial libraries of purified LH proteins and iii) in vivo screening of LH variants under different light conditions to analyse whether long-lived quantum coherence may be evolutionary favoured.
Michael is a physicist conducting optical characterisation experiments on organic molecule-nanomaterial hybrid systems analogous to the light harvesting structures typically investigated in spectroscopic studies. Initially, the electronic couplings between variants of porphyrin, an organic light harvester found in chlorophyll, and carbon nanotubes will be studied. There is also scope for some theoretical band-gap modelling. Michael will be employing characterisation techniques such as Photoluminescence, Raman scattering, and femtosecond pump-probe studies. The experimental techniques also allow for electron-hole recombination and agglomeration quenching effects to be studied. Experimental work will be conducted at the Advanced Technology Institute, also located at the University of Surrey.
In photosynthesis, the capture and transfer of light energy in the light harvesting complexes (LH1 and LH2) has an extremely high efficiency that cannot be fully explained by classical energy transport mechanisms.
Recent work however, has indicated that the interaction between vibrational and electronic dynamics can create and maintain quantum coherence in biological systems, which may contribute to the efficiency of such phenomena. Specifically, studies on the LH2 complex have demonstrated long-lived coherence, evidenced by the detection of “quantum beating” in 2D optical spectroscopy.
My project aims to investigate the role of vibrational motions in the quantum coherence of light harvesting complexes of the photosynthetic, non-sulphur, purple bacteria Rhodobacter sphaeroides. Using metabolic methods, heavy isotopes will be incorporated into the bacteriochlorophyll molecules present in Rhodobacter’s light harvesting complexes with the intention of perturbing coherence and thus reducing the efficiency of energy transfer.
Metabolically labelled bacteriochlorophyll pigments will be purified from Rhodobacter and have their optical properties examined using techniques such as Ramen spectroscopy and 2D optical spectroscopy.
University of Surrey
Professor Ravi Silva
Director, Advanced Technology Institute (ATI) and Head of NanoElectronics Centre
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Fig. 1. Rhodopseudomonas acidophila strain 7050 growing in modified Pfenning agar.
Fig. 2. View looking down on the top (periplasmic surface) of the LH2 complex from Rhodopseudomonas acidophila strain 10050. Taken from .