Type I reaction center from the green sulfur bacterium Chlorobium tepidum: is Chl a a primary electron acceptor?

The green sulfur bacterium Chlorobium tepidum has one of the simplest type I reaction center (RC) complexes. While its structure is still unknown, biochemical and protein sequence analyses suggest that it is similar to photosystem I (PS I), with two BChl a forming a special pair P840, four Chl a serving as pairs of accessory and primary electron acceptor (A0) pigments and 14 BChl a constituting as an immediate RC antenna. This is a dramatic simplification compared to PS I RC, where 90 Chl a antenna pigments serve as antenna and 6 additional Chl a molecules function as electron transfer cofactors. The resulting spectral congestion has prevented direct visualization of ultrafast electron transfer processes within PS I RC and even the sequence of primary electron transfer processes in PS I is still under debate. The suggested presence of two types of pigments in RC from Chlorobium tepidum removes spectral congestion and opens a way to directly visualize electron transfer steps in type I RC using ultrafast spectroscopy, since the Chl a and BChl a pigments absorb at ∼670 nm and ∼800 nm, respectively. To confirm the proposed functional role of Chl a as electron transfer cofactor we performed extensive ultrafast optical pump-probe experiments on different preparations of RC complexes from Chlorobium tepidum, revealing energy/electron transfer rates between different groups of pigments. Surprisingly, we found that ∼3 out of 4 Chl a pigments do not transfer excitation energy to the BChl a antenna or to P840, which indicates that these pigments must be >20Å away from any other BChl a pigment and thus argues against the suggested presence of 4 Chl a in the reaction center core complex.