What is it about?
This paper models how a multi-pigment type-I photosynthetic reaction center from Chloracidobacterium thermophilum (CabRC) harvests both visible and far-red light. CabRC contains three chlorophyll species—16 bacteriochlorophyll a (BChl a), 10 chlorophyll a (Chl a), and a Zn-bacteriochlorophyll a′ special pair (P840)—letting it absorb around 670 nm (Chl a) and ~812 nm (BChl a). Using a structure-based exciton Hamiltonian and modified Redfield theory, we simulate excitation energy transfer (EET) after virtual 670 nm and 780 nm excitation. We find the rate-limiting step is energy transfer between the Chl a exciton band and the BChl a exciton band (slower than transfer within each band). Including intramolecular vibrational modes (exciton-phonon coupling) strongly enhances this cross-band transfer; without it, Chl→BChl transfer is much less direct. Overall, the heterogeneous pigment layout appears optimized to funnel energy to P840 while enabling efficient use of a broad light spectrum.
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Why is it important?
It’s important because it explains how photosynthetic systems can efficiently use far-red light (low-energy photons) and visible light by mixing different chlorophyll types in one reaction center, which is key for understanding adaptation to diverse light environments. CabRC uniquely contains Chl a + BChl a + Zn-BChl a′; the study shows the main bottleneck is energy transfer between the Chl-a exciton band (visible) and the BChl-a exciton band (far-red), and that intramolecular vibrational coupling is indispensable to make this cross-band transfer happen on the needed ps timescale (otherwise it becomes ineffective). These insights help clarify design principles and evolution of reaction centers across species, and may guide discovery/engineering of light-harvesting systems that work in unusual or low-light/far-red environments.
Perspectives
Our perspective is to extract general design principles of type-I RCs by comparative, structure-based analysis across diverse pigment compositions. Building on the unique Chl a/BChl a hybrid architecture of CabRC, we aim to clarify functional and evolutionary relationships among RCs and to facilitate the discovery of new RC systems adapted to yet-unidentified light environments.
Akihiro Kimura
Nagoya University
Read the Original
This page is a summary of: Light harvesting mechanism of the multi-pigment reaction center with three chlorophyll species adapted to both visible and far-red light, Photosynthesis Research, January 2026, Springer Science + Business Media,
DOI: 10.1007/s11120-026-01198-1.
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