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ABSTRACT: Time-resolved cw EPR measurements of the Rhodobacter (Rb) sphaeroides R-26 reaction center primary donor triplet state excited with plane-polarised light are reported. The pigment composition of the reaction center was chemically modified, so that the bacteriopheophytin molecule in the cofactor branch which is inactive towards electron transfer was replaced by plant pheophytin a. This enabled selective excitation of the bacteriopheophytin and pheophytin molecules, and provided conditions for a high-quality magnetophotoselection study. For the first time, orientation of the Qy optical transition dipole moment relative to the molecular frame of the bacteriopheophytin in the active cofactor branch is determined. Of the four orientations allowed by magnetophotoselection, one was chosen as the most plausible. The corresponding Qy vector is tilted from the bacteriopheophytin tetrapyrrole plane by 15°, and projects onto this plane almost on the y-molecular axis. It is suggested that the deviation of the vector from the molecular plane results from an interaction of bacteriopheophytin with the neighbouring molecule of accessory bacteriochlorophyll.
Chemical Physics. 01/2003; 294(3):451-458.