Figure 3.

(a) A schematic of the detection scheme. Mn and Fe Kα₁ and Kα₂. The multi-crystal monochromator with approximately 1 eV resolution is tuned to the Kα₁ peak (red). The fluorescence peaks broadened by the Ge-detector with 150–200 eV resolution are shown below (blue). Right: The PSII Mn K-edge EXAFS spectrum from the S₁ state sample obtained with a traditional energy-discriminating Ge-detector (blue) compared with that collected using the high-resolution crystal monochromator (red). Fe present in PSII does not pose a problem with the high-resolution detector (the Fe edge is marked by a green line). (b) Fourier transform (FT) of Mn K-edge EXAFS spectra from isotropic solution and oriented PSII membrane samples in the S₁, S₂ and oriented S₁ state obtained with a high-resolution spectrometer (range-extended EXAFS). Angles indicate orientation of the membrane normal with respect to the X-ray e-field vector. The k range was 3.5–15.2 Å⁻¹. (c) Polar plot of the X-ray absorption linear dichroism of PSII samples in the S₁ state. The N_app values (solid circles) for oriented membranes are plotted at their respective detection angles (θ). The best fits of N_app versus θ to the equation in the text are shown for the various Mn–Mn vectors as solid lines that take into account the experimentally determined mosaic spread of Ω=20°.