An EPR and ENDOR Spectroscopic Investigation of the Ca2+-Depleted Oxygen-Evolving Complex of Photosystem II

Multifrequency EPR and 55Mn ENDOR spectroscopy were used to characterize the S2 ′ state of the Ca2+- depleted oxygen-evolving complex. The 55Mn ENDOR spectrum of the S2 ′ state is broader than that of the native S2 state. Simulations of the data were performed using the spin Hamiltonian formalism. It was observed that the magnitudes of the four 55Mn hyperfine tensors (A1,iso≈ 300 MHz; A2, iso, A3, iso, A4, iso≈200MHz) are approximately the same as in the native S2 state. In addition, the geometries of the anisotropic hyperfine tensors are not changed. Thus, the same oxidation states are assigned to the Mn ions both in S2 and S2′ (MnA, MnB, MnC: IV, MnD: III). The isotropic hyperfine values of the individual Mn ions, especially A2, iso and A4 iso, do change upon Ca2+ depletion, indicating that, nonetheless, the electronic spin coupling scheme of the cluster is affected by Ca2+ removal.