Photosynthetic species evolved to protect their light-harvesting apparatus from photoxidative damage driven by intracellular redox conditions or environmental conditions. The Fenna–Matthews–Olson (FMO) pigment–protein complex from green sulfur bacteria exhibits redox-dependent quenching behavior partially due to two internal cysteine residues. Here, we show evidence that a photosynthetic complex exploits the quantum mechanics of vibronic mixing to activate an oxidative photoprotective mechanism. We use two-dimensional electronic spectroscopy (2DES) to capture energy transfer dynamics in wild-type and cysteine-deficient FMO mutant proteins under both reducing and oxidizing conditions. Under reducing conditions, we find equal energy transfer through the exciton 4–1 and 4–2-1 pathways because the exciton 4–1 energy gap is vibronically coupled with a bacteriochlorophyll-a vibrational mode. Under oxidizing conditions, however, the resonance of the exciton 4–1 energy gap is detuned from the vibrational mode, causing excitons to preferentially steer through the indirect 4–2-1 pathway to increase the likelihood of exciton quenching. We use a Redfield model to show that the complex achieves this effect by tuning the site III energy via the redox state of its internal cysteine residues. This result shows how pigment–protein complexes exploit the quantum mechanics of vibronic coupling to steer energy transfer.
Vibronic coupling between pigment molecules is believed to prolong coherences in photosynthetic pigment-protein complexes. Reproducing long-lived coherences using vibronically coupled chromophores in synthetic DNA constructs presents a biomimetic route to...
Light-harvesting
complexes in photosynthetic organisms display
fast and efficient energy transfer dynamics, which depend critically
on the electronic structure of the coupled chromophores within the
complexes and their interactions with their environment. We present
ultrafast anisotropy dynamics, resolved in both time and frequency,
of the transmembrane light-harvesting complex LH2 from Rhodobacter
sphaeroides in its native membrane environment using polarization-controlled
two-dimensional electronic spectroscopy. Time-dependent anisotropy
obtained from both experiment and modified Redfield simulation reveals
an orientational preference for excited state absorption and an ultrafast
equilibration within the B850 band in LH2. This ultrafast equilibration
is favorable for subsequent energy transfer toward the reaction center.
Our results also show a dynamic difference in excited state absorption
anisotropy between the directly excited B850 population and the population
that is initially excited at 800 nm, suggesting absorption from B850
states to higher-lying excited states following energy transfer from
B850*. These results give insight into the ultrafast dynamics of bacterial
light harvesting and the excited state energy landscape of LH2 in
the native membrane environment.
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