The Quantum Design of Photosynthesis
Professor Rienk van Grondelle of the VU University Amsterdam in The Netherlands, was on the Westville campus recently to deliver a public lecture on the Quantum Design of Photosynthesis.
A quantum biologist, van Grondelle was visiting UKZN as a guest of Professor Francesco Petruccione, who is Director of UKZN’s Quantum Research Group and holds the South African Research Chair in Quantum Information Processing and Communication.
In his lecture, van Grondelle explained that photosynthesis had found an ultrafast and highly efficient way of converting the energy of the sun into electrochemical energy.
‘The solar energy is collected by Light-Harvesting Complexes (LHC),’ he said. ‘Then, the electronic excitation is transferred to the Reaction Centre (RC), where the excitation energy is converted into a charge separated state with almost 100% efficiency.’
Van Grondelle said that separation of charges created an electrochemical gradient across the photosynthetic membrane, which ultimately powered the photosynthetic organism.
He predicted that the understanding of the molecular mechanisms of light harvesting and charge separation, would provide a template for the design of efficient artificial solar energy conversion systems.
‘Both the LHCs and the RCs are highly specialised proteins that bind pigments (chlorophylls, carotenoids) and are organised in the photosynthetic membrane, in plants the thylakoid membrane,’ he said. ‘In plants two photosystems, Photosystem II (PSII) and Photosystem I (PSI), each with their own LHCs, operate in series, capable of light-driven water oxidation and NADP+ reduction. Photosynthetic green and purple bacteria make do with a single RC and cannot oxidize water.’
Van Grondelle explained that upon excitation of the photosynthetic system, the energy was delocalised over several cofactors creating collective excited states (excitons) that provide efficient and ultrafast paths for energy transfer using the principles of quantum mechanics.
‘In the reaction centre the excitons become mixed with charge transfer (CT) character (exciton-CT states), which provide ultrafast channels for charge transfer.’
Van Grondelle pointed out that both the LHC and the RC had to cope with a counter effect, namely, disorder.
‘The slow protein motions (static disorder) produce slightly different conformations which, in turn, modulate the energy of the exciton-CT states,’ he explained. ‘In this scenario, in some of the LHC/ RC complexes within the sample ensemble the energy could be trapped in some unproductive states leading to unacceptable energy losses.’
Van Grondelle went on to show how the LHCs and RCs have found a unique solution for overcoming this barrier, in that they use the principles of quantum mechanics to probe many possible pathways at the same time and to select the most efficient one that fits their realisation of the disorder.
He compared this to a taxi driver finding the correct pathway through the chaotic alternatives of Amsterdam’s roadways.
‘During photosynthesis, plants use electronic coherence for ultrafast energy and electron transfer and have selected specific vibrations to sustain those coherences,’ he said. ‘In this way photosynthetic energy transfer and charge separation have achieved their amazing efficiency. At the same time these same interactions are used to photoprotect the system against unwanted byproducts of light harvesting and charge separation at high light intensities.’
Van Grondelle, together with Professor Roberta Croce, heads the Biophysics of Photosynthesis programme at the VU University Amsterdam.
He is one of the most influential experimental physicists working on the primary physical processes of photosynthesis world-wide.
Using the tools of ultrafast spectroscopy van Grondelle has made major contributions to elucidate the fundamental physical mechanisms that underlie photosynthetic light harvesting and charge separation.
He has developed theoretical tools to infer the effective electronic and molecular structure and dynamics from complex spectroscopic data. His work recently led to a fundamental new understanding of light-driven charge separation in the oxygen-evolving, photosynthetic reaction centre of plants.
Using multi-dimensional electronic spectroscopy he has been able to show that in photosynthesis ultrafast charge separation is driven by specific molecular vibrations that allow electronic coherences to stay alive.
In proposing his explicit molecular model for photoprotection, van Grondelle has demonstrated that the major plant light harvesting complex operates as a nanoswitch, controlled by its biological environment.
These results, of utmost importance for mankind’s understanding of photosynthesis, inspire technological solutions for artificial and/or redesigned photosynthesis, as a possible route towards sustainable energy production on a global scale.
Van Grondelle has published 535 papers in international, peer-reviewed journals that in total have attracted over 25K citations (h-index 80, WoS). In addition he is the co-author of three textbooks.