Identification of the Q_Y 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 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(A)H(2)). 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(A)H(2). 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(A)H(2)) states.

Primary Photophysical Processes in Photosystem II: Bridging the Gap between Crystal Structure and Optical Spectra

“…The present set of site energies was confirmed by calculations of linear optical spectra and Stark spectra and comparison with experimental data by Novoderezhkin et al [37] Further support was reported by Cox et al [38] from an analysis of difference circular dichroism spectra involving photoaccumulated Pheo D1 À .…”

Site Energies of Active and Inactive Pheophytins in the Reaction Center of Photosystem II from Chlamydomonas reinhardtii

“…13 The latter suggests the presence of Q A ; therefore, an electrochromic shift in the Q x -region of the pheophytins is possibly due to the formation of the P + Q A  state, as previously observed in bacterial RCs 60,77,78 and PSII cores. 14,58,59 As expected, there was no bleach or shift in the 530-560 nm region in the D2-L209H mutant, since there is no formation of a P + Q A  state, even if Q A is present, due to the lack of active Pheo D1 . This proves that our isolated RCs from C. reinhardtti are to a large extent intact with a bleach at 672.7 nm being due to the oxidation of P (mostlikely P D1 ), or, expressed more properly and as discussed by Schlodder et al in the case of PSII core from Synechocystis 13 , due to the bleaching of an exciton state dominated by contributions from P D1 .…”

“…As expected, the more intact sample (frames A, C, and E) has red-shifted peaks of Q y -and Q x -absorption bands, as well as the position of the lowest-energy bands, in agreement with spectra obtained for RC from intact PSII core samples. 14,58,59 25,26,61 ) and has a smaller fractional depth of 9.8%. Note the shape of this transient hole is exactly the same as previously observed in spinach PSII RCs 25,26,61 with a bleach near 680/681 nm and a weak shoulder near 683/684 nm 26 , which we assigned previously to P680 and P684 RCs, respectively 4 .…”

“…The recent X-ray structure of the Photosystem II (PSII) reaction center (RC) at 1.9 Å resolution 1 has confirmed that some of the general features of the PSII RC are homologous to those of the bacterial RC (BRC). 2 The S 1 (Q y )-electronic structure, excitation energy transfer (EET), and primary charge separation (CS) processes in the D 1 -D 2 -Cytb 559 RC complex of PSII (PSII RC) of plants, [3][4][5][6][7][8][9] cyanobacteria, [10][11][12][13][14] and algae [15][16][17][18] have been the subjects of many timedomain [18][19][20][21][22] and frequency-domain studies [23][24][25][26][27][28] . Although in recent years consistent progress has been made in our understanding of the electronic structure and CS dynamics of the isolated PSII RC, 10,[29][30][31] an adequate global understanding has yet to be achieved.…”

Energetic insights into two electron transfer pathways in light-driven energy-converting enzymes

“…In addition, the longest wavelength pigment is thought to be Chl D1 in PSII. 50 , 52 Given that Chl D1 is the primary electron donor ( i.e. , Chl a , where excitation occurs due to the lowest site-energy) in PSII ( e.g.…”