Photosynthetic complexes can be damaged by reactive oxygen species generated from photoexcitation inthe presence of oxygen. The Fenna Matthews olson complex of <i>Chlorobacter tepidum</i> is known to show fluorescence that is sensitive to redox potential (the "dithionite effect")  Blankenship and coworkers recently assigned this behavior to a cysteine-mediated mechanism that quenches excitations on the picosecond to nanosecond timescale. Yet, the effects of oxidative stress on ultrafast energy transfer remain unclear. Here, we vary the redox conditions and employ mutagenesis to investigate the ultrafast dynamics under oxidative conditions. Using femtosecond 2D electronic spectroscopy, we observe more efficient energy transfer through the wild-type complex under reductive conditions. The differences in the ultrafast dynamics result from changes to the system-bath coupling around different chromophores. Performing the same measurements on a cysteine-lacking C353A/C49A mutant, we observe dramatically different dynamics under oxidative or reductive conditions implicating another redox sensitive effect apart from the cysteine mechanism. We identify in the crystal structure of FMO a highly-conserved tyrosine/tryptophan near the chromophores that show the largest change in ultrafast dynamics under different redox conditions. We discuss how the mechanism of tyrosine/tryptophan chain oxidation may contribute to these differences in ultrafast dynamics, thereby protecting photosynthetic protein-pigment complexes from ROS.

Ultrafast dynamics of photosynthetic energy transfer regulated by intrinsic redox reactivity of amino acid side chains in binding pocket

255th National Meeting (2018)

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