Single molecule studies on purple bacterial antenna complexes

Lead Research Organisation: University of Glasgow
Department Name: Institute of Biomedical & Life Sciences


Photosynthesis begins with the absorption of solar energy by antenna pigments. This absorbed energy is then rapidly and efficiently transferred to reaction centres where it is trapped and used to drive photosynthesis. Light-harvesting is therefore and essential and fundamental part of this very important biological process that ultimately provides all the food we eat, the oxygen we breathe and cleans the air by removing carbon dioxide. This proposal aims to use a combination of X-ray crystallography and single molecule spectroscopy to study both the physical and electronic structure of a group on light-harvesting antenna complexes obtained from purple photosynthetic bacteria. These bacteria have proved to be excellent model systems in which to study the fundamental reactions going on in the earliest reaction of photosynthesis. Normal spectroscopy looks at the properties of large populations of molecules, so-called ensemble spectroscopies. However because this averages over all the molecules present and because each molecule is not identical many important details are averaged out and lost. Looking at individual molecules allows these details to be observed and they are, in this case with the antenna complexes, essential if we wish to understand how antenna complexes function. In this regard single molecule spectroscopy is a uniquely important tool.

Technical Summary

This research aims to use a combination of low temp. single molecule spectroscopy and X-ray crystallography to compare the electronic structure and the physical structure of the Bchl molecules in a range of purple bacterial antenna complexes. This data is absolutely required in order to undertsand the detailed molecular mechanisms of energy transfer that are at the heart of photosynthetic light-harvesting. The single molecule approach allows the detailed information that is typically lost in ensemble spectroscopy to be measured. This is essential information if the molecular details of light-harvesting are to be really, deeply understood. In particular we will be determining if there are gaps in the electronic structure of the LH1/RC core complexes that could correlate with physical gaps through which quinones could move at part of the cyclic electron transfer process. We shall also be using the combination of X-ray crystallography and single molecule spectroscopy to study some LH2 complexes that have unsual absorption spectra. The aim here is to be able to understand how these different spectroscopic form can be produced and to get a deeper understanding of how the pigment-binding apoproteins can modulate the photochemical properties of the Bchl molecules, in order to tailor them for optimum fuction.


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Description Single molecule spectroscopy of our light harvesting complexes has revealed details of their structure and function that could never have been anticipated by more normal ensemble spectroscopic techniques. We have seen that our proteins show conformational memory that may explain how conformational control of protein function evolved. We have clearly demonstrated that some antenna complexes show apoprotein heterogeneity within single LH2 rings. Single LH2 complexes show unexpected quantum coherence effects.
Exploitation Route All our findings have been published or presented at international conferences. Though our findings are largely important in an academic sense the finding of the quantum effects has become a subject of intense international interest. I have been on the radio in both the UK and Germany to discuss the importance of this finding. It is also now being looked for in such areas as dye sensitized solar cells to see if it presents a new way to optimize function.
Sectors Energy,Environment

Description We have communicated our results through radio broadcasts. These have helped to arose interest in so called Quantum Biology. This has featured in a recent Solvay Conference in Brussels and at a post conference question and answer session that was open to the general public and broadcast in Belgium.
First Year Of Impact 2010
Sector Energy,Environment
Impact Types Societal

Description Quantum Coherent Energy Transfer over Varying Pathways in Single Light-Harvesting Complexes 
Organisation University of Bayreuth
Country Germany 
Sector Academic/University 
PI Contribution As a result of the primary collaboratation with Professor Koehler, this extra collaboration was arranged. The initial steps of photosynthesis comprise the absorption of sunlight by pigment-protein antenna complexes followed by rapid and highly efficient funneling of excitation energy to a reaction centre. In these transport processes, signatures of unexpectedly long-lived coherences have emerged in two-dimensional ensemble spectra of various light-harvesting complexes. Here, we demonstrate ultrafast quantum coherent energy transfer within individual antenna complexes of a purple bacterium under physiological conditions. We find that quantum coherences between electronically coupled energy eigenstates persist at least 400 femtoseconds and that distinct energy-transfer pathways that change with time can be identified in each complex. Our data suggest that long-lived quantum coherence renders energy transfer in photosynthetic systems robust in the presence of disorder, which is a prerequisite for efficient light harvesting.
Start Year 2012
Description Single-molecule spectroscopy reveals photosynthetic LH2 complexes switch between emissive states 
Organisation University of Bayreuth
Country Germany 
Sector Academic/University 
PI Contribution As a result of the primary collaboratation with Professor Koehler, this extra collaboration was arranged. Photosynthetic organisms flourish under low light intensities by converting photoenergy to chemical energy with near unity quantum efficiency and under high light intensities by safely dissipating excess photoenergy and deleterious photoproducts. The molecular mechanisms balancing these two functions remain incompletely described. One critical barrier to characterizing the mechanisms responsible for these processes is that they occur within proteins whose excited-state properties vary drastically among individual proteins and even within a single protein over time. In ensemble measurements, these excited-state properties appear only as the average value. To overcome this averaging, we investigate the purple bacterial antenna protein light harvesting complex 2 (LH2) from Rhodopseudomonas acidophila at the single-protein level. We use a room-temperature, single-molecule technique, the anti-Brownian electrokinetic trap, to study LH2 in a solution-phase (nonperturbative) environment. By performing simultaneous meas-urements of fluorescence intensity, lifetime, and spectra of single LH2 complexes, we identify three distinct states and observe transitions occurring among them on a timescale of seconds. Our results reveal that LH2 complexes undergo photoactivated switching to a quenched state, likely by a conformational change, and thermally revert to the ground state. This is a previously unobserved, reversible quenching pathway, and is one mechanism through which photosynthetic organisms can adapt to changes in light intensities.
Start Year 2012