How efficient is excitation energy transfer in photosynthetic complexes and what leads to this efficiency?

Photosynthetic light harvesting consists in the absorption of light by pigment–protein complexes (PPCs), which then transfer its energy to a precise location (the reaction center), where a charge separation event occurs, in turn triggering a series of chemical reactions that lead to the production of chemical energy. The complicated mechanisms in which these complexes in plants, algae and bacteria perform light harvesting are driven by their design principles. Two-dimensional spectroscopy is able to unravel these mechanisms by mapping out how the light energy moves through the system.

The efficiency and directionality of this energy flow are due to the sophisticated arrangements of many chromophores into these PPCs. Being able to determine how the molecular construction is related to the function is of great importance to obtain insight into the design principles behind photosynthesis. Independent control over the polarization of the pulses in two-dimensional electronic spectroscopy allows us to investigate excited state geometries and relate this to the molecular composition, and to discern the possible presence of different conformations within the same complex.

Helpful Background Reading:

Two-dimensional Electronic Spectroscopy and Photosynthesis: Fundamentals and Applications to Photosynthetic Light-Harvesting, G. S. Schlau-Cohen, A. Ishizaki, G. R. Fleming, J. Chem. Phys. 2011, in press.

Visualization of excitonic structure in the Fenna-Matthews-Olson photosynthetic complex by polarization-dependent two-dimensional electronic spectroscopy, E. L. Read, G. S. Schlau-Cohen, G. S. Engel, J. Z. Wen, R. E. Blankenshi, G. R. Fleming, Biophys. J., 95, 847-856, (2008).  

Pathways of Energy Flow in LHCII from Two-Dimensional Electronic Spectroscopy,
G. S. Schlau-Cohen, T. R. Calhoun, N. S. Ginsberg, E. L. Read, M. Ballottari, R. Bassi, R. van Grondelle, G. R. Fleming, J. Chem. Phys. B, 113,15352-15363 (2009).

Spectroscopic elucidation of uncoupled transition energies in the major photosynthetic light-harvesting complex, LHCII, G.S. Schlau-Cohen, T.R. Calhoun, N.S. Ginsberg, M. Ballottari, R. Bassi, G.R. Fleming, P NATL ACAD SCI USA,107, 13276-13281 (2010).

Solving structure in the CP29 light harvesting complex with polarization-phased 2D electronic spectroscopy, N. S. Ginsberg, J. A. Davis, M. Ballottari, Y.-C. Cheng, R. Bassi, G. R. Fleming, P NATL ACAD SCI USA, 108, 3848-3853 (2011).