Understanding the biological design principles of efficient and robust photosynthetic systems opens the door to new developments in functional materials for light-driven processes. To maximize efficient use of solar energy, most photosynthetic organisms rely on a series of pigment-protein antenna complexes to absorb and shuttle solar energy to active sites of reaction center proteins. The process of absorption and energy transfer to the reaction center happens on the scale of femtoseconds (fs; 10-15 s) to picoseconds (ps; 10-15 s) and with high quantum efficiency, but under high light conditions this process can be quenched to prevent oxidative damage caused by overexcitation. Recent work has shown that the Fenna Matthews Olson complex in green sulfur bacteria is capable of tuning its fluorescence yield (and presumably transfer efficiency) in response to oxidative conditions due to changes in the oxidation state of two specific cysteine residues in the complex. While there is clear evidence for the existence of such a mechanism, the details of its impact on the electronically-coherent energy transfer through the FMO complex and the broader implications of this regulation as a design principle in coherent energy transfer systems have not yet been explored. To illustrate these regulatory effects on ultrafast energy transfer, here we present two-dimensional electronic spectra of wild type and mutant (C49A, C353A, and C49A/C353A) FMO complexes isolated from C. tepidum. The spectra are collected using a single-shot 2D spectrometer (UVA-GRAPES), which allows for extensive sampling in the waiting time leading to high signal-to-noise observations of exciton dynamics on the scale of fs to ns. This range permits observation of both ultrafast coherent interactions and fluorescence changes due to mutation or sample conditions. We observe quantum mechanical coherences between excitons in complexes under both oxidative and reductive conditions. These results will be discussed in the context of the functioning of the photoprotective mechanism, and implications for the design of redox-regulated coherent energy transport will be explored.

Ultrafast spectroscopic observation of cysteine-based redox regulation of coherent energy transfer in a photosynthetic antenna complex

253rd National Meeting (2017)

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