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NEWS

Monday, March 11th, 2013  

From 09:30:00 AM
to 06:00:00 PM

Main hall, Physics and Astronomy Dept., University of Florence (Italy)

Published on-line at 11:19:46 AM on Monday, March 4th, 2013

Quantum transport in light-harvesting bio-nanostructures

An open access and broad audience workshop on Quantum Biology.

Very recently fascinating ultra-fast spectroscopy experiments have shown that quantum mechanics plays a crucial role in explaining the remarkable efficiency (almost 100%) and robustness of energy transport in biological light-harvesting nanostructures, involved in natural photosynthesis. This triggered also several theoretical studies on how Nature exploits quantum coherence and environmental noise to implement such transport mechanisms. Therefore, these experimental and theoretical investigations, giving rise to the new field of Quantum Biology, are expected to allow us to have a deeper understanding of the role of quantum effects in biology, whose practical implementations might be crucial, in the 21st century, for novel and more powerful renewable energy technologies based on quantum phenomena.

This open access and broad audience workshop will provide some mainly introductory lectures on this topic, from both the theoretical and experimental side, by the main international leaders of research areas such as quantum transport phenomena, ultra-fast spectroscopy, natural and artificial light-harvesting complexes, quantum effects in biological photosynthetic systems, solar energy and quantum information technologies.

Timetable

  • 09:30-09:50 - Welcome
  • 09:50-10:30 - "Spectroscopic signatures of quantum-coherent energy transfer" (Elisabetta Collini, Padova Univ., Italy)
  • 10:30-11:10 - "Structure-based modeling of light-harvesting in photosynthesis" (Thomas Renger, Linz Univ., Germany)
  • 11:10-11:40 - Coffee break
  • 11:40-12:20 - "Noise assisted transport: Concepts, challenges and tools" (Martin Plenio, Ulm Univ., Germany)
  • 12:20-13:00 - "Vibrational structures, electronic coherence and efficient transport in photosynthetic complexes" (Susana Huelga, Ulm Univ., Germany)
  • 13:00-15:30 - Lunch
  • 15:30-16:10 - "Photosynthesis exploits quantum coherence for efficient solar energy conversion" (Elisabet Romero, VU Amsterdam Univ., Netherlands)
  • 16:10-16:50 - "Introduction to non-linear spectroscopy: What is in it, and what is not" (Tomas Mancal, Prague Univ., Czech Republich)
  • 16:50-17:20 - Coffee break
  • 17:20-18:00 - "Quantum Coherence explored at the level of individual light-harvesting complexes" (Niek van Hulst, ICFO, Spain)

Lectures

09:40-10:20 - Elisabetta Collini (Padova Univ., Italy): "Spectroscopic signatures of quantum-coherent energy transfer" - One of the most surprising and significant advances in the study of the photosynthetic light-harvesting process is the discovery that the electronic energy transfer might involve long-lived electronic coherences, also at physiologically relevant conditions. This means that the transfer of energy among different chromophores does not follow the expected classical incoherent hopping mechanism, but that quantum-mechanical laws can steer the migration of energy. The implications of such quantum transport regime, although currently under debate, might have a tremendous impact in our way to think about natural and artificial light-harvesting. Central to these discoveries has been the development of new ultrafast spectroscopic techniques, in particular two-dimensional electronic spectroscopy, which is now the primary tool to obtain clear and definitive experimental proof of such effects. In this talk an overview of the experimental techniques developed with the purpose of attaining a more detailed picture of the coherent and incoherent quantum dynamics relevant to energy transfer processes will be provided, not limited to the two-dimensional electronic spectroscopy. With the idea of summarizing the experimental and theoretical basic notions necessary to introduce the field, the connection between experimental observables and coherence dynamics will be analysed in detail for each technique, highlighting how electronic coherences could be manifested in different experimental signatures. Similarities and differences among coherent signals as well as advantages and disadvantages of each approach will be critically discussed. Current opinions and debated issues will be emphasised and some possible future direction to address still open questions will be suggested.

10:20-11:00 - Thomas Renger (Linz Univ., Germany): "Structure-based modeling of light-harvesting in photosynthesis" - Light-Harvesting in photosynthesis is a fascinating topic for many reasons. First of all one touches the basis of life on earth. Second, high-resolution crystal structures of many photosynthetic pigment-protein complexes exist and their optical properties are known as well. In order to bridge the gap between the crystal structures and the optical experiments probing their function, two essential problems need to be solved. On one hand, theories of optical spectra and excitation energy transfer have to be developed that take into account the pigment-pigment (excitonic) and the pigment-protein (exciton-vibrational) coupling on an equal footing. On the other hand, the parameters entering these theories need to be calculated from the structural data. I will give a summary of recent approaches to solve the above problems and present applications.

11:30-12:10 - Martin Plenio (Ulm Univ., Germany): "Noise assisted transport: Concepts, challenges and tools" - In this lecture I will discuss the concepts of noise assisted transport as a first example of the beneficial interaction between electronic and vibrational degrees of freedom in biological systems which will be explored much further in the subsequent lectures. I will show that biological systems operate optimally in an intermediate regime that is not well captured by current theoretical approaches. I will then outline how this challenge to theory is being addressed with the development of new theoretical and numerical techniques whose range of applicability goes well beyond quantum biology.

12:10-12:50 - Susana Huelga (Ulm Univ., Germany): "Vibrational structures, electronic coherence and efficient transport in photosynthetic complexes" - Recent observations of beating signals in the excitation energy transfer dynamics of photosynthetic complexes have been interpreted as evidence for sustained coherences that are sufficiently long-lived for energy transport and coherence to coexist. The microscopic origin of these long-lived coherences, however, remains to be uncovered. Here we present such a mechanism and verify it by numerically exact simulations of system-environment dynamics. Crucially, the non-trivial spectral structures of the environmental fluctuations and particularly discrete vibrational modes can lead to the generation and sustenance of both oscillatory energy transport and electronic coherence on timescales that are comparable to excitation energy transport. This suggests that the vibrational structure of protein environments plays a significant role for coherence in biological processes. The resulting non-trivial interplay between quantum coherence and dissipative environment-driven dynamics is likely to be fundamental for efficient energy transport in photosynthetic pigment-protein complexes. Here we identify a specific design principle - the phonon antenna - that demonstrates how inter-pigment coherence is able to modify and optimize the way that excitations spectrally sample their local environmental fluctuations. We place this principle into a broader context and furthermore we provide evidence that the Fenna-Matthews-Olson complex of green sulphur bacteria has an excitonic structure that is close to such an optimal operating point, and suggest that this general design principle might well be exploited in other biomolecular systems.

15:00-15:40 - Elisabet Romero (VU Amsterdam Univ., Netherlands): "Photosynthesis exploits quantum coherence for efficient solar energy conversion" - Photosynthesis has found an ultrafast and highly efficient way of converting the energy of the sun into electrochemical energy. The solar energy is collected by the Light-Harvesting complexes and then transferred to the Reaction Center (RC) where the excitation energy is converted into a charge separated state with almost 100% efficiency. That separation of charges creates an electrochemical gradient across the photosynthetic membrane which ultimately powers the photosynthetic organism. The understanding of the molecular mechanisms of charge separation will provide a template for the design of efficient artificial solar energy conversion systems. Upon excitation of the reaction center, the energy is delocalized over several cofactors creating collective excited states (excitons) with charge transfer (CT) character (exciton-CT states) which provide ultrafast channels for exciton relaxation and charge transfer. However, the Reaction Center has to cope with a counter effect: disorder. The slow protein motions (static disorder) produce slightly different conformations which, in turn, modulate the energy of the exciton-CT states. In this scenario, in some of the Reaction Center complexes within the sample ensemble the energy could be trapped in some unproductive states leading to unacceptable energy losses. In order to overcome the consequences of a highly disordered energy landscape, the Reaction Center has developed several strategies. In this talk, these strategies will be presented with a focus on the utilization of quantum coherence, specially on the role of specific vibrations in maintaining electronic coherences between exciton-CT states. The central question regarding the role of quantum coherence in determining the speed and efficiency of the energy conversion process in photosynthesis will also be addressed.

15:40-16:20 - Tomas Mancal (Prague Univ., Czech Republich): "Introduction to non-linear spectroscopy: What is in it, and what is not" - In this lecture, an introduction into theoretical description of non-linear spectroscopy, and an overview of its modern methods will be provided. Various spectroscopic methods and the information that can be obtained by an analysis of their spectra will be discussed. We will point out the connection between non-linear spectroscopy and modern problems of biophysics and quantum mechanics.

17:30-18:10 - Niek van Hulst (Ulm Univ., Germany): "Quantum coherence explored at the level of individual light-harvesting complexes" - Quantum mechanical effects in biological processes, such as natural photosynthesis, are intriguing and lively debated issues. The initial steps of photosynthesis comprise the absorption of sunlight by pigment-protein complexes as well as rapid and remarkably efficient funnelling of excitation energy to a reaction centre. In these energy transfer processes oscillatory signatures of surprisingly long-lived coherences have been found by 2-dimensional spectroscopy on ensembles of various light-harvesting complexes. These data have been modelled in terms of environmentally assisted quantum transport with a careful balance between coherence, dissipation, and dephasing. This precarious equilibrium is influenced by temporal, spatial and spectral inter-complex variations on a nanoscopic level, caused by the highly dynamic environments and broad conformational diversity in functioning bio-systems. Unfortunately, ensemble experiments fail to resolve this. Hence, to unravel the nature of energy transfer in light-harvesting and to uncover the possible biological role of long-lived quantum coherences in the energy transfer dynamics, it is crucial to probe the ultrafast response of antenna proteins beyond the ensemble average and to test the robustness of coherences against perturbations on the level of individual complexes. Here we demonstrate ultrafast quantum coherent energy transfer within single light-harvesting complexes of a purple bacterium under physiological conditions. We find that quantum coherences between electronically coupled energy eigenstates persist at least 400 fs, significantly longer than previously reported, and that distinct energy transfer pathways can be identified in each complex. Strikingly, also changing transfer pathways in individual complexes on time scales of seconds are revealed. This is attributed to structural rearrangements of the pigment molecules and the surrounding protein scaffold caused by ubiquitous thermal disorder at elevated temperatures. Our data indicate that long-lived quantum coherence indeed plays a biological role as it renders energy transfer robust in the presence of disorder.

For further informations, please contact Dr. Filippo Caruso or.