Abstract
During photosynthesis, sunlight is efficiently captured by light-harvesting complexes, and the excitation energy is then funneled towards the reaction centre. These photosynthetic excitation energy transfer (EET) pathways are complex and proceed in a multistep fashion. Ultrafast two-dimensional electronic spectroscopy (2DES) is an important tool to study EET processes in photosynthetic complexes. However, the multistep EET processes can only be indirectly inferred by correlating different cross peaks from a series of 2DES spectra. Here we directly observe multistep EET processes in LHCII using ultrafast fifth-order three-dimensional electronic spectroscopy (3DES). We measure cross peaks in 3DES spectra of LHCII that directly indicate energy transfer from excitons in the chlorophyll b (Chl b) manifold to the low-energy level chlorophyll a (Chl a) via mid-level Chl a energy states. This new spectroscopic technique allows scientists to move a step towards mapping the complete complex EET processes in photosynthetic systems.
Highlights
During photosynthesis, sunlight is efficiently captured by light-harvesting complexes, and the excitation energy is funneled towards the reaction centre
Two-dimensional electronic spectroscopy (2DES)[1,2,3] measures a two-point correlation plot, and is an excellent tool to measure single-step excitation energy transfer (EET) processes as the appearance and evolution of offdiagonal cross peaks provide a direct observation of electronic coupling between the donor and acceptor pigment molecules, by reading off the corresponding frequencies on the excitation and emission axes, respectively[4,5,6,7]
Since it is clear that complex multistep EET network is present in most light-harvesting complexes[9,10,11,12], higher-order multidimensional electronic spectroscopy will be needed to directly measure multistep EET processes
Summary
Sunlight is efficiently captured by light-harvesting complexes, and the excitation energy is funneled towards the reaction centre. We measure cross peaks in 3DES spectra of LHCII that directly indicate energy transfer from excitons in the chlorophyll b (Chl b) manifold to the low-energy level chlorophyll a (Chl a) via mid-level Chl a energy states This new spectroscopic technique allows scientists to move a step towards mapping the complete complex EET processes in photosynthetic systems. In addition to the two controllable population periods, fifth-order 3D optical spectroscopy has three coherence periods that give rise to three frequency dimensions This gives the three-point frequency– frequency correlation as a function of two population times S(5)(o1, o3, o5; t2, t4). Another kind of fifth-order optical experiment, 2D Raman spectroscopy was plagued with spurious signal arising from third-order cascading processes[26]. If for some systems or conditions that the cascading signals become significant, procedures are available to isolate the contaminant signals[28]
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