Optical Spectra of Synechocystis and Spinach Photosystem II Preparations at 1.7 K:  Identification of the D1-Pheophytin Energies and Stark Shifts

We report and compare highly resolved, simultaneously recorded absorption and CD spectra of active Photosystem II (PSII) samples in the range 440−750 nm. From an appropriately scaled comparison of spinach membrane fragment (BBY) and PSII core spectra, we show that key features of the core spectrum are quantitatively represented in the BBY data. PSII from the cyanobacterium Synechocystis 6803 display spectral features in the Q<i><sub>y</sub></i> region of comparable width (50−70 cm<sup>-1</sup> fwhm) to those seen in plant PSII but the energies of the resolved features are distinctly different. A comparison of spectra taken of PSII poised in the S<sub>1</sub>Q<sub>A</sub> and S<sub>2</sub>Q<sub>A</sub><sup>-</sup> redox states reveals electrochromic shifts largely attributable to the influence of Q<sub>A</sub><sup>-</sup> on Pheo<sub>D1</sub>. This allows accurate determinations of the Pheo<sub>D1</sub> Q<i><sub>y</sub></i> absorption positions to be at 685.0 nm for spinach cores, 685.8 nm for BBY particles, and 683.0 nm for Synechocystis. These are discussed in terms of earlier reports of the Pheo<sub>D1</sub> energies in PSII. The Q<i><sub>x</sub></i> transition of Pheo<sub>D1</sub> undergoes a blue shift upon Q<sub>A</sub> reduction, and we place a lower limit of 80 cm<sup>-1</sup> on this shift in plant material. By comparing the magnitude of the Stark shifts of the Q<i><sub>x</sub></i> and Q<i><sub>y</sub></i> bands of Pheo<sub>D1</sub>, the directions of the transition-induced dipole moment changes, Δμ<i><sub>x</sub></i> and Δμ<i><sub>y</sub></i>, for this functionally important pigment could be determined, assuming normal magnitudes of the Δμ's. Consequently, Δμ<sub><i>x</i></sub> and Δμ<sub><i>y</i></sub> are determined to be approximately orthogonal to the directions expected for these transitions. Low-fluence illumination experiments at 1.7 K resulted in very efficient formation of Q<sub>A</sub><sup>-</sup>. This was accompanied by cyt <i>b</i><sub>559</sub> oxidation in BBYs and carotenoid oxidation in cores. No chlorophyll oxidation was observed. Our data allow us to estimate the quantum efficiency of PSII at this temperature to be of the order 0.1−1. No Stark shift associated with the S<sub>1</sub>-to-S<sub>2</sub> transition of the Mn cluster is evident in our samples. The similarity of Stark data in plants and Synechocystis points to minimal interactions of Pheo<sub>D1</sub> with nearby chloropyll pigments in <i>active</i> PSII preparations. This appears to be at variance with interpretations of experiments performed with <i>inactive</i> solubilized reaction-center preparations.