The anoxygenic Type II reaction center in purple bacteria has 4 bacteriochlorophyll pigments, 2 bacteriopheophytin pigments, while the reaction center in green non-sulfur bacteria has 3 bacteriochlorophyll and 3 bacteriopheophytin pigments. The third one is located at the position of the monomeric bacteriochlorophyll on the M branch (BchlM).
In purple bacteria, the BchlM is in contact with a carotenoid, which has a protective role. Could the "third bacteriopheophytin" (ΦM) have a similar role? I feel like it could not, but I'm not sure... perhaps in a different way.
Apparently, in a mutant of the purple bacteria where a bacteriopheophytin is located at the position of BchlM charge separation can occur on the M branch. Some experiments by Yakovlev et al., 2009 (Biochemistry Moscow) using femtosecond studies, determined that extremely fast charge separation to ΦM occurred reversibly, but then the normal charge separation on the L branch would be the stable charge separation route with full yield.
Do you know what is the mechanistic significance of this adaptation in the green non-sulfur bacteria? Or alternatively, do you have any speculations as to what the role of ΦM could be?
Following sits may be helpful:
https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/pheophytin
Article Discovery of pheophytin function in the photosynthetic energ...
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