524 Biochimica et Biophysica Acta, 503 (1978) 524--544 © Elsevier/North-Holland Biomedical Press BBA 47543 STRUCTURAL ORGANIZATION OF THE CHROMATIUM VINOSUM REACTION CENTER ASSOCIATED c-CYTOCHROMES DAVID M. TIEDE, JOHN S. LEIGH and P. LESLIE DUTTON Johnson Research Foundation and Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pa. 19104 (U.S.A.) (Received January 31st, 1978) Summary Magnetic interactions operating between the Chromatium vinosum reaction center associated c-cytochromes and the electron carriers of the reaction center have been assayed by comparing the magnetic properties of these components alone, and in various combinations with paramagnetic forms of the reaction center electron carriers. These studies have yielded the following results. 1. The oxidized paramagnetic forms of the high potential cytochromes c-555 produce no discernable alterationof the light-induced (BChl):'*signal. 2. Similarly, analysis of the lineshape of the light-induced (BChl)2"* signal shows that a magnetic interaction with the oxidized low potential cytochromes c-553 is likely to produce less than a 1 gauss splitting_ of the (BChl)2"*signal, which corresponds to a minimum separation of 25 ± 3/~ between the unpaired spins if the heme and (BChl)2 are orientated in a coplanar arrangement, suggest- ing a minimum separation of 15 t 3 ~ between the heine edge and the (BChl): edge. 3. A prominent magnetic interaction is observed to operate between the cytochrome c-553 and c-555, which results in a 30--35 gauss splittingof these spectra, and suggests an iron to iron separation of about 8/~. 4. Magnetic interactions are not observed between the c-cytochromes and the reaction center "primary acceptor" (the iron •quinone complex) nor with the reaction center intermediate electron carrier (which involves bacterio- pheophytin) suggesting separations greater than 10/~. 5. Magnetic interactions are not discerned between the two cytochrome c-553 hemes, nor between the two cytochrome c-555 hemes, implying that the distance between the cytochromes of the same pair is greater than I0/~. 6. EPR studies of oriented chromatophores have demonstrated that the cytochrome c-553 and c-555 heroes are perpendicular to each other, and Abbreviation: BChl, bacteriochlorophYll.