Vibronic coupling to electron transfer and the structure of the R. Viridis reaction center

This paper points out that the orientations of the porphyrins, bacteriochlorophyll and bacteriopheophytin, in the reaction centers of Rhodopseudomonas viridis, as shown by the new X-ray determined structure, have a peculiar orientation towards each other: electron donors are broadside toward the acceptors and acceptors are edgeon toward donors. Vibronic coupling which is the mechanism of converting free-energy loss in electron transport to vibrational energy is examined as a possible explanation. Preliminary calculations do not support this as an explanation of the orientations but suggest strongly that the non-heme iron atom has the function of promoting vibronic coupling in the electron transfer from bacteriopheophytin to menaquinone. It is further suggested that the system of electron transport from the special pair of bacteriochlorophyll to the bacteriopheophytin is arranged to keep virbonic coupling to a minimum to match the very small electronic free-energy loss in this region.