Modulation of singlet excited state dynamics of bacteriochlorophyll a in Fenna-Matthews-Olson (FMO) complex by PscB, a subunit with an intrinsically disordered region in Chlorobaculum tepidum reaction center.

Modulation of the chlorophyll singlet state by protein environment is a key aspect of photosynthesis and light-harvesting in biological systems. This modulation affects excited-state dynamics, energy transfer efficiency, and photochemical reactivity. The PscB subunit of the reaction center (RC) of green sulfur bacterium Cba. tepidum contains an iron-sulfur cluster domain that is involved in light-driven electron transfer and a domain of intrinsically disordered region. The latter seemingly coil around one of the two trimeric FMO in the FMO-RC, acting as a molecular clamp. In this work, spectroscopic comparative studies of FMO-only and FMO-PscB complexes were performed. Our study reveals that reconstitution of the PscB with FMO protein alters the spectral line shape of the excitonic band of BChl a manifold and the properties of its singlet excited state as state lifetime. Though not substantial, the observable altered 815-nm excitonic band suggests that clamping of PscB around trimeric FMO shell slightly affects the overall pigment packing network. Further application of time-resolved fluorescence and absorption suggested that reconstitution of FMO trimers with PscB at the excess molecular ratio of the latter one likely leads to spontaneous oligomerization of the pigmented FMO with enhanced quenching capabilities which are essentially required during PscB's recruiting of FMO trimers and sandwiching them between chlorosome and the membrane-embedded RCs.