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 (A_1,iso≈ 300 MHz; A_{2, iso}, A_{3, iso}, A_{4, 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′ (Mn_A, Mn_B, Mn_C: IV, Mn_D: III). The isotropic hyperfine values of the individual Mn ions, especially A_{2, iso} and A_{4 iso}, do change upon Ca²⁺ depletion, indicating that, nonetheless, the electronic spin coupling scheme of the cluster is affected by Ca²⁺ removal.