Intermediates in assembly by photoactivation after thermally accelerated disassembly of the manganese complex of photosynthetic water oxidation

The Mn4Ca complex bound to photosystem II ( PSII) is the active site of photosynthetic water oxidation. Its assembly involves binding and light-driven oxidation of manganese, a process denoted as photoactivation. The disassembly of the Mn complex is a thermally activated process involving distinct intermediates. Starting from intermediate states of the disassembly, which was initiated by a temperature jump to 47 degrees C, we photoactivated PSII membrane particles and monitored the activity recovery by O-2 polarography and delayed chlorophyll fluorescence measurements. Oxidation state and structural features of the formed intermediates of the Mn complex were assayed by X-ray absorption spectroscopy at the Mn K-edge. The photoactivation time courses, which exhibit a lag phase characteristic of intermediate formation only when starting with the apo-PSII, suggest that within similar to 5 min of photoactivation of apo-PSII, a binuclear Mn complex is formed. It is proposed that a Mn-2(III)(di-mu-oxo) complex is a key intermediate both in the disassembly and in the assembly reaction paths.