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Abstract

Membrane preparations of two species of the green sulfur bacteria Chlorobium have been studied by EPR. Three signals were detected which were attributed to iron-sulfur centers acting as electron acceptors in the photosynthetic reaction center. (1) A signal from a center designated F_B, (g_z=2.07, g_y=1.91, g_x=1.86) was photoinduced at 4K. (2) A similar signal, F_A (g_z=2.05, g_y=1.94, g_x=1.88), was photoinduced in addition to the F_B signal upon a short period of illumination at 200K. (3) Further illumination at 200K resulted in the appearance of abroad feature at g=1.78. This is attributed to the g_x component of an iron-sulfur center designated F. The designations of these signals as F_B, F_A, and F_x are based on their spectroscopic similarities to signals in photosystem I (PSI). The orientation dependence of these EPR signals in ordered Chlorobium membrane multilayers is remarkably similar to that of their PSI homologues. A magnetic interaction between there duced forms of FB and FA occurs, which is also very similar to that seen in PSI. However, in contrast to the situation in PSI, F_A and F_B cannot be chemically reduced by sodium dithionite at pH 11. This indicates redox potentials for F_A and F_B which are lower by at least 150 mV than their PSI counterparts. The triplet state of P₈₄₀, the primary electron donor, could be photoinduced at 4 K in samples which had been preincubated with sodium dithionite and methyl viologen and then pre illuminated at 200K. The preillumination reduces their ion-sulfur centers while the preincubation is thought to result in the inactivation of an earlier electron acceptor, possibly the double reduction of aquinone which could occur at potentials higher than those associated with its functional one-electron couple. Orientation studies of the triplet signal in ordered multilayers indicate that the bacteriochlorophylls which act as the primary electron donor in Chlorobium are arranged with a structural geometry almost identical with that of the special pair in purple bacteria. The Chlorobium reaction center appears to be similar in some respects to both PSI and to the purple bacterial reaction center. This is discussed with regard to the evolution of the different types of reaction centers from a common ancestor. This has significance to the current understanding of the structure of the PSI reaction center.