Photosynthetic bacteria special pair BChl

In Rhodobacter sphaeroides and Rhodospirillum rubrum, a new type of antenna Bchl-form whose energy level is lower than that of the special pair Bchl of the reaction center (RC) has been reported (4-6). Such components should be surveyed in other photosynthetic bacteria to account for the functional role and structural basis for the specific energy level. 

Fig. 1. Schematic representation of the reaction center of purple photosynthetic bacteria (A) and a stereogram of the BChl molecules Pa and Pb of the primary electron donor, or "special pair (B). Fig. (B) source Huber (1988) A structural basis of light energy and electron transfer in biology [Nobel lecture]. Bloscience Reports 9 643.

The photophysical and electron transfer properties of bacteriochlorophylls (Bchl) and bacteriopheophytins (Bpheo) found in the reaction centers of photosynthetic bacteria have been directly associated with the mechanism of charge separation which underlies photosynthesis [1]. The appearance of the Bpheo anion (Bpheo ) within 3-5 ps after excitation of the special pair of Bchl (P) is well documented from transient absorption spectroscopy [2-4]. The 200 ps lifetime of Bpheo which is primarily determined by the electron transfer process to a quinone also has been established by picosecond changes in absorption [5,6], Thus, the general kinetic time scale for the primary processes in bacterial photosynthesis has been determined by the transient differences in electronic state properties. 

The primary electron donor D in reaction centers (RCs) of photosynthetic bacteria is a dimer ( special pair ) of Bacteriochlorophyll (BChl) molecules (Fig. 1). The structure and spectral (EPR, ENDOR) characteristics of the dimer have been investigated extensively in RCs of Rhodopseu-domonas viridis [1-4] and Rhodobacter sphaeraides [5-9]. 

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