Differences in the binding of the primary quinone receptor in Photosystem I and reaction centres of Rhodobacter sphaeroides-R26 studied with transient EPR spectroscopy Original Research Article

The binding of the primary quinone acceptor, Q, in Photosystem I (PS I) and reaction centres (RCʹs) of Rhodobacter Sphaeroide-R26 in which, the non-heme iron has been replaced by zinc (Zn-bRCʹs) is studied using transient EPR spectroscopy. In PS I, Q is phylloquinone (vitamin K1, VK1) and is referred to as A1. In Zn-bRCʹs, it is ubiquinone-10 (UQ10) and called QA. Native samples of the two RCʹs as well as those in which A1 and QA have been replaced by perdeuterated napthoquinone (NQ-d6) and duroquinone (DQ-d12) are compared. The spin polarized K-band (24 GHz) spectra of the charge separated state P+.Q−. (P = primary chlorophyll donor) in Zn-bRCʹs show that substitution of QA, with NQ-d6 and DQ-d12 does not have a measurable effect on the quinone orientation in the QA site. In contrast, large differences in the orientation of VK1, NQ-d6 and DQ-d12 in the A1 site in PS I are found. In addition, all three quinones in PS I are oriented differently than QA in Zn-bRCʹs. Further, the x and y principal values of the g-tensors of VK1−., NQ−. and DQ−. in PS I are shown to be significantly larger than in frozen alcohol and Zn-bRCʹs. It is suggested that the differences in the orientation and a g-values of the quinones in the two RCʹs arise from a weaker binding to the protein in PS I.