Excitation of light-harvesting pigment-protein complexes is the first step in photosynthesis. The absorbed energy is transferred to reaction centers where it is used to fuel biological processes. Pump-probe and time-resolved fluorescence spectroscopy have been traditionally used to study the energy flow within these systems. However, in the past decades two-dimensional electronic spectroscopy (2DES) emerged as a powerful technique for detailed study of the ultrafast energy transfer within photosynthetic systems. 1–3 State of the art 2DES has advantages over the other methods, such as higher temporal and spectral resolution as well as higher signal-to-noise ratio. 4,5 This makes possible the production of 2D maps between excitation and emission energies at different delay times, reveling information about the dynamics of the excited states. Employing this method on the Fenna-MatthewOlson (FMO) photosynthetic protein complex has shown that, unlike previously thought, the mechanism of energy transfer is not a simple stepwise series of decay between electronic states. Instead, preferred pathways that depend on the spatio-energetic distribution of chromophores seems to exist.1 Further developments allowed the in situ study of the photosynthetic apparatus of the green sulfur bacteria.6 The authors were able to obtain long-sought evidence of the FMO serving as a mediator between the chlorosome and the reaction center.
(1) Brixner, T.; Stenger, J.; Vaswani, H. M.; Cho, M.; Blankenship, R. E.; Fleming, G. R. Two-dimensional spectroscopy of electronic couplings in photosynthesis. Nature 2005, 434, 625–628.
(2) Zigmantas, D.; Read, E. L.; Mancal, T.; Brixner, T.; Gardiner, A. T.; Cogdell, R. J.; Fleming, G. R. Two-dimensional electronic spectroscopy of the B800-B820 lightharvesting complex. Proceedings of the National Academy of Sciences 2006, 103, 12672– 12677.
(3) Dostál, J.; Vácha, F.; Pšenčík, J.; Zigmantas, D. 2D Electronic Spectroscopy Reveals Excitonic Structure in the Baseplate of a Chlorosome. The Journal of Physical Chemistry Letters 2014, 5, 1743–1747.
(4) Gelzinis, A.; Augulis, R.; Butkus, V.; Robert, B.; Valkunas, L. Two-dimensional spectroscopy for non-specialists. Biochimica et Biophysica Acta (BBA) - Bioenergetics 2019, 1860, 271–285.
(5) Schlau-Cohen, G. S.; Dawlaty, J. M.; Fleming, G. R. Ultrafast Multidimensional Spectroscopy: Principles and Applications to Photosynthetic Systems. IEEE Journal of Selected Topics in Quantum Electronics 2012, 18, 283–295.
(6) Dostál, J.; Pšenčík, J.; Zigmantas, D. In situ mapping of the energy flow through the entire photosynthetic apparatus. Nature Chemistry 2016, 8, 705–710.