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The roi.. of tryptophan (W) in the quinone reduction in the reaction centers of R. viridis and Rb. sphueroides is studied by a quantum chemical calculation of the electronic coupling matrix elements within the bacteriopheophytin-W-quinone subsystem, which is based on the X-ray structural data. The results favor the superexchange mechanism via W over the direct coupling. Uncertainties in the available X-ray coordinates and structural relaxation accompanying the formation of the P+H-state result in large changes in the relative contributions of the electronic couplings due to electron and hole superexchange. Aromatic amino acid residues may serve as essential functional components in electron transfer.

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