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The interplay between quantum coherence and environment in the photosynthetic electronic energy transfer and light-harvesting: a quantum chemical picture

Final Report Summary - ENLIGHT (The interplay between quantum coherence and environment in the photosynthetic electronic energy transfer and light-harvesting: a quantum chemical picture)

Photosynthetic organisms such as bacteria, algae and plants have developed sophisticated supramolecular complexes to achieve an optimal use of sunlight.
All this starts when solar light is absorbed by specific pigments (mainly chlorophylls) bound at high concentrations in the so-called antenna pigment-protein complexes. Before the excited pigments can return to the ground state, the electronic excitation is transferred within the protein and to other neighboring proteins until it reaches the reaction centers, where it is used to initiate the charge separation. The extraordinary efficiency of these transfers is mostly due to the unique architecture of the antenna complexes, which carry out the light-harvesting (LH) function. In such complexes, structure, dynamics and function are inextricably linked. Revealing the molecular bases of these architectures requires a new theoretical and computational approach, which accounts for the large network of interactions among the different components (the pigments, the protein and the external environment) and the coupling between the electronic degrees of freedom and the nuclear motions of all the parts. To develop such an approach is formidably challenging both from the theoretical and the computational point of view. Only a strategy based upon an integration of models with different length and time scales can achieve the required completeness of the description.
The project has designed and coded this multiscale strategy by integrating hybrid methods, which combine accurate quantum chemical descriptions and classical models, with molecular dynamics simulation and theories of quantum transport.

By applying this strategy to very different LH systems of bacteria, algae and plants, we have succeeded in identifying common key elements and important specificities. The former can be explained as the necessary features, which identify the same common function while the latter are due to the very different external conditions the different organisms have to adapt to live.
One of the main common aspects we have revealed and quantified is the fundamental role that the heterogeneity in the electrostatic and polarization properties of the protein has in tuning the energy ladder among the different pigments. Secondly, we have shown that in all antenna complexes, the temperature-dependent fluctuations of the environment translate into a dramatic effect in the creation, migration and trapping of the excitations, which are central to the functionality of the light-harvesting apparatus.
When moving to an even more detailed level of description, however, we have shown that each specific organism has “developed” its own way to harvest sunlight. By using different types of pigments (chlorophylls versus carotenoids versus bilins) and/or different structural arrangements, the LH complexes are able to tune their energetic properties through a system-specific coupling between electronic and vibrational degrees of freedom. This coupling is controlled by (i) the fluctuating electrostatic and polarization effects due to the embedding environment, and (ii) the simultaneous presence of slow and fast nuclear motions, which modulate the energy fluctuations of the involved electronic levels.
It is exactly this system-specific control that allows the LH function to be maximized but also to be flexible and robust at the same time. Both robustness and flexibility with respect to the changes in the light condition and the temperature are in fact necessary for the organism to live and grow.