DIFFERENCES IN THE BINDING OF THE PRIMARY QUINONE ACCEPTOR IN PHOTOSYSTEM-I AND REACTION CENTERS OF RHODOBACTER SPHAEROIDES-R26 STUDIED WITH TRANSIENT EPR SPECTROSCOPY

The binding of the primary quinone acceptor, Q, in Photosystem I (PS I ) and reaction centres (RC's) of Rhodobacter Sphaeroides-R26 in which, the non-heme iron has been replaced by zinc (Zn-bRC's) is studied usi ng transient EPR spectroscopy. In PS I, Q is phylloquinone (vitamin K- 1, VK1) and is referred to as A(1). In Zn-bRC's, it is ubiquinone-10 ( UQ(10)) and is called Q(A). Native samples of the two RC's as well as those in which Al and QA have been replaced by perdeuterated naphthoqu inone (NQ-d(6)) and duroquinone (DQ-d(12)) are compared. The spin pola rized K-band (24 GHz) spectra of the charge separated state p(+.)Q(-.) (P = primary chlorophyll donor) in Zn-bRC's show that substitution of Q(A), With NQ-d(6) and DQ-d(12) does not have a measurable effect on the quinone orientation in the Q(A) site. In contrast, large differenc es in the orientation of VK1, NQ-d(6) and DQ-d(12) in the A(1) site in PS I are found. In addition, all three quinones in PS I are oriented differently than Q(A) in Zn-bRC's. Further, the x and y principal valu es of the g-tensors of VK1-. , NQ(-.) and DQ(-.) in PS I are shown to be significantly larger than in frozen alcohol and Zn-bRC's. It is sug gested that the differences in the orientation and the g-values of the quinones in the two RC's arise from a weaker binding to the protein i n PS I.