Electrostatic Effects on the Speed and Directionality of Electron Transfer in Bacterial Reaction Centers: The Special Role of Tyrosine M-208

Abstract

The solution of the crystal structures of purple bacterial reaction centers has raised several puzzling questions: First, considering the symmetry of the crystal structure, why does the special pair of bacteriochlorophylls (P) transfer an electron to the bacteriopheophytin on the “L” side of the complex (H_L) so much more rapidly than it does to the bacteriopheophytin on the “M” side (H_M)? And what roles do the accessory bacteriochlorophylls (B_L and B_M) play in the electron-transfer reaction? The answers to these questions are likely to be intertwined. One possibility is that favorable electrostatic interactions with the protein lower the energy of the P+B_L- radical-pair so that this state lies close to or below the excited dimer (P*), whereas the corresponding radical-pair on the M side (P+B_M-) lies at a significantly higher energy. This would allow B_L to act as an intermediate electron carrier betwen P* and H_L as suggested by the recent work of Holzapfel et al. [1,2]. The competing route through P+B_M- to P+H_M- would be blocked by the need for thermal activation, particularly at low temperatures. A difference between the electrostatic interactions of the protein with the two radical-pairs might suffice to explain the directionality even if P+B_L- lies above P* and facilitates the reaction only by mixing quantum-mechanically with P* and P+H_L- (superexchange) [3–8]. If P+B_M- is at a higher energy than P+B_L-, its mixing with P* would be weaker.