Identification of the QY Excitation of the Primary Electron Acceptor of Photosystem II: CD Determination of Its Coupling Environment

Abstract

Low-temperature absorption and CD spectra, measured simultaneously, are reported from Photosystem II (PS II) reduced with sodium dithionite. Spectra were obtained using PS II core complexes before and after photoaccumulation of PheoD1-, the anion of the primary acceptor. For plant PS II, PheoD1- was generated under conditions in which the primary plastoquinone was present as an anion (QA-) and as a modified species taken to be the neutral doubly reduced hydroquinone (QAH2). The bleaches observed upon PheoD1- formation in the presence of QA- are shifted to the blue compared those in the presence of QAH2- This is attributed to the influence of the charge on QA-, and this effect mirrors the well-known electrochromic effect of QA- on the neutral pigments. The absorption bleaches induced by PheoD1 reduction are species dependent. Structured changes of the CD in the 680-690 nm spectral region are seen upon photoaccumulation of PheoD1- in PS II from plant, Synechocystis and Thermosynechococcus vulcanus. These CD changes are shown to be consistent with the overall electronic assignments of Raszewski et al. [Raszewski et al. Biophys. J. 2008, 95, 105], which place the dominant PheoD1 excitation near 672 nm. CD changes associated with PheoD1 reduction are modeled to arise from the shift and intensity changes of two CD features: one predominately of ChlD1 character, the other predominately PheoD2 in character. The assignments are also shown to account for the Qy absorption changes in samples where the quinone is its charged (QA-) and neutral (QAH2 states.