The Asymmetry of Primary Electron Transfer

One of the most intriguing features of the bacterial reaction centre is the asymmetry of trans-membrane electron transfer. Although the BChl, BPhe and UQjq cofactors are arranged in two approximately symmetrical trans-membrane branches, only the so-called A-branch is used for transmembrane electron transfer. The factors determining this functional asymmetry continue to be the subject of great interest, as the reaction centre presents a chain of cofactors that catalyses electron transfer with great efficiency, and a similar chain of cofactors that is much less effective. This functional asymmetry is due to small but crucial differences in the structure of the proteinxofactor system along the two branches. 

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The photochemical activity of PS II reaction centers associated with primary charge separation has been documented by direct measurements of electron transfer by substrate donors (including water) or acceptors as well as by spectroscopic methods involving optical absorption, fluorescence and EPR. On the picosecond time scale it appears that the sequence of events and even the kinetics associated with the earliest steps are very similar between PS II and the purple bacterial reaction centers (15). Nevertheless, other aspects of this similarity remain to be demonstrated whether the primary electron donor of PS II consists of a special pair of chlorophylls, whether the PS II reaction center possesses a structural two-fold symmetry together with a functional asymmetry and whether there is a portion of the PS II complex that corresponds to the H polypeptide. 

The observed asymmetry of the spin density distribution in favor of Dl has been explained using a model that assumes an energetic difference between the dimer halves Dl and Dj of a magnitude comparable to that of the interdimer interaction energy. Significant shifts of spin density between Dl and Df have been observed, when RC s of different native bacteria and mutants were compared, indicating differences in the energetics of the primary donors in the different bacteria. The orbital asymmetry of the primary donor is obviously a common feature in many bacterial photosystems and may play a functional role for the unidirectional electron transfer in bacterial photosynthesis. 

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