Photosynthesis – the light of life

Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems
Engel and colleagues
Nature vol 446, 12th April 2007
This summary is taken from Nature. Whilst the detailed chemistry is unlikely to appeal to most Nutritionists this is a fundamentally important process for life . It is therefore worth having a look at and even 10% uptake is valuable. Nutrition is a science and also we should be educated .
If nothing else knowing of the Fenna-Matthews-Olson (FMO) bacteriochlorophyll complex, will give you an edge in any Pub quiz.
Photo synthetic complexes are exquisitely tuned to capture solar light efficiently, and then transmit the excitation energy to reaction centres, where long term energy storage is initiated. The energy transfer mechanism is often described by semi classical models that invoke ‘hopping’ of excited-state populations along discrete energy levels’”. Two-dimensional Fourier transform electronic spectroscopy”’ has mapped” these energy levels and their coupling in the Fenna-
Matthews-Olson (FMO) bacteriochlorophyll complex, which is found in green sulphur bacteria and acts as an energy ‘wire’ connecting a large peripheral light-harvesting antenna, the chlorosome, to the reaction centre. . The spectroscope data clearly document the dependence of the dominant energy transport pathways on the spatial properties of the excited-state wavefunctions of the whole bacteriochlorophyll complex. But the intricate dynamics of quantum coherence, which has no classical analogue, was largely neglected in the analyses—even though electronic energy transfer involving oscillatory populations of donors and acceptors was first discussed more than 70 years ago, and electronic quantum beats arising from quantum coherence in photo synthetic complexes have been predicted and indirectly observed. Here we extend previous two-dimensional electronic spectroscopy investigations of the FMO bacteriochlorophyll complex, and obtain direct evidence for remarkably long-lived electronic quantum coherence playing an important part in energy transfer processes within this system. The quantum coherence manifests itself in characteristic, directly observable quantum beating signals among the excitons within the Chlorobium tepidum FMO complex at 77 K. This wavelike characteristic of the energy transfer within the photo synthetic complex can explain its extreme efficiency, in that it allows the complexes to sample vast areas of phase space to find the most efficient path..
The system is essentially involved in a process wherein many senses are sensed at the same time and allowing the most effective transfer of energy to the correct locus.
This mechanism contrasts with a semi classical ‘hopping’ mechanism through which the excitation movesstepwise from exciton state to exciton state, dissipating energy at each step, which would be similar to a classical search where only one state can be occupied at any one time. Such a mechanism also raises the possibility of non-local events.
The FMO light-harvesting complex provides an opportunity to apply more complete energy transfer theories that invoke non-markovian dynamics and include coherence transfer. Such theories need to include wavelike energy motion owing to long-lived coherence terms, alongside the population transfer included in current models. Further, the observed preservation of coherence in this photosynthetic system requires us to redefine our description of the role of electron-phonon interactions within photosynthetic proteins. In particular, the protein may not only enforce the structure that gives rise to the couplings, but also modulate those couplings with motions of charged residues and changing local dielectric environments, which will change exciton energies and promote coherence transfer.

Martin Eastwood
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