Photosynthetic pigment-protein complexes can capture and transport solar energy with high efficiency while maintaining the capability of preventing excitons from damaging the cell. It has been shown that the Fenna-Matthews-Olson (FMO) complex can quench excess excitations in oxidizing redox environments through a cysteine-mediated mechanism to prevent the excitation from generating reactive oxygen species. In this study, we show that FMO steers excitations toward quenching sites in oxidizing conditions by modulating the vibronic coupling of its chromophores. We use two-dimensional electronic spectroscopy (2DES) to study the excited state dynamics of wild-type and cysteine-mutated (C49A/C353A) FMO complexes in oxidizing and reducing conditions. We use a new analysis method that leverages symmetries between the time evolution of different peaks in the two-dimensional spectra to extract precise time constants for energy transfer. The change in time constants between the FMO samples reveals that the redox condition influences which of the competing excitonic pathways is favorable. We use a Redfield model to show that under oxidizing conditions, cysteines shift the site energies of the chromophores to detune the resonant coupling with a vibrational mode in bacteriochlorophyll-<i>a</i>, promoting an energy transfer pathway where there is a higher overlap with the quenching cysteine residue. These results show that photosynthetic complexes use the redox environment as a switch to modulate the vibronic coupling of their chromophores and move excitations through different spatial regions of the complex. The findings indicate that photosynthesis has evolved the ability to utilize the coupling of electronic and nuclear degrees of freedom to enhance its photoprotective capabilities.

Photosynthetic pigment-protein complexes steer excitons toward quenching sites using redox-dependent vibronic coupling

ACS Spring Meeting 2021

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