Triplet-State Dynamics of Bacteriochlorophyll <i>a</i> in the Fenna–Matthews–Olson (FMO) Complex and Its Modulation by PscB, a Subunit in the Reaction Center of <i>Chlorobaculum tepidum</i>

In this study, the triplet-state properties of BChl <i>a</i> in the Fenna-Matthews-Olson (FMO) light-harvesting complex were interrogated in the absence and presence of PscB, a subunit of the <i>Cba. tepidum</i> reaction center (RC), at room temperature and at 77 K. Application of nanosecond time-resolved transient absorption spectroscopy supports a model in which the pathway of the triplet excitation decay within FMO has two phases, with a fast lifetime of 2.58 μs (0.57 μs at 77 K) and a slow lifetime of 44.8 μs (44.1 μs at 77 K) in the FMO-only sample. Reconstitution of PscB and FMO, however, alters the spectral signatures of BChl <i>a</i> excitons uniquely at 815 nm in the steady-state spectrum at 77 K. Additionally, the triplet-state lifetime of BChl <i>a</i> in the FMO-PscB complex shortens by almost 40% to 28.1 μs at 77 K. The two FMO trimers asymmetrically interfacing with the homodimeric RC wire excitation energy from the chlorosome to the latter. Our data supports that the single central PscB, besides its redox active roles as the electron mediators to ferredoxin, is highly plausibly involved in fine-tuning the asymmetric excitation energy transfer from two branches of FMO to the RC in green sulfur bacteria.