Spectroscopic properties of the intermediary electron carrier in the reaction center of Rhodopseudomonas viridis evidence for its interaction with the primary acceptor

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The spectroscopic properties of the intermediary electron carrier (I), which functions between the bacteriochlorophyll dimer, (BChl) 2, and the primary acceptor quinone · iron, QFe, have been characterized in Rhodopseudomonas viridis. Optically the reduction of I is accompanied by a bleaching of bands at 545 and 790 nm and a broad absorbance increase around 680 nm which we attribute to the reduction of a bacteriopheophytin, together with apparent blue shifts of the bacteriochlorophyll bands at 830 and possibly at 960 nm. Low temperature electron paramagnetic resonance analysis also reveals complicated changes accompanying the reduction of I. In chromatophores I⨪ is revealed as a broad split signal centered close to g 2.003, which is consistent with I⨪ interacting, via exchange coupling and dipolar effects, with the primary acceptor Q⨪Fe. This is supported by experiments with reaction centers prepared with sodium dodecyl sulfate, which lack the Q⨪Fe g 1.82 signal, and also lack the broad split I⨪ signal; instead, I⨪ is revealed as an approximately 13 gauss wide free radical centered close to g 2.003. Reaction centers prepared using lauryl dimethylamine N-oxide retain most of their Q⨪Fe g 1.82 signal, and in this case I⨪ occurs as a mixture of the two EPR signals described above. However, the optical changes accompanying the reduction of I⨪ are very similar in the two reaction center preparations, so we conclude that there is no direct correlation between the two optical and the two EPR signals of I⨪. Perhaps the simplest explanation of the results is that the two EPR signals reflect the reduced bacteriopheophytin either interacting, or not interacting, with Q⨪Fe, while the optical changes reflect the reduction of bacteriophenophytin, together with secondary, perhaps electrochromic effects on the bacteriochlorophylls of the reaction center. However, we are unable to eliminate completely the possibility that there is also some electron sharing between the reduced bacteriopheophytin and bacteriochlorophyll.