Identification of the Qy excitation of the primary electron acceptor of photosystem II: CD determination of its coupling environment

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 Pheo_D1^-, the anion of the primary acceptor. For plant PS II, Pheo_D1^- was generated under conditions in which the primary plastoquinone was present as an anion (Q_A^-) and as a modified species taken to be the neutral doubly reduced hydroquinone (Q _AH₂). The bleaches observed upon Pheo_D1 ^- formation in the presence of Q_A^- are shifted to the blue compared those in the presence of Q_AH₂- This is attributed to the influence of the charge on Q_A^-, and this effect mirrors the well-known electrochromic effect of Q_A ^- on the neutral pigments. The absorption bleaches induced by Pheo_D1 reduction are species dependent. Structured changes of the CD in the 680-690 nm spectral region are seen upon photoaccumulation of Pheo _D1^- 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 Pheo_D1 excitation near 672 nm. CD changes associated with Pheo_D1 reduction are modeled to arise from the shift and intensity changes of two CD features: one predominately of Chl_D1 character, the other predominately Pheo_D2 in character. The assignments are also shown to account for the Q_y absorption changes in samples where the quinone is its charged (Q_A^-) and neutral (Q_AH₂ states.