Dual-Mode EPR Detects the Initial Intermediate in Photoassembly of the Photosystem II Mn Cluster: The Influence of Amino Acid Residue 170 of the D1 Polypeptide on Mn Coordination

We report the first parallel polarization EPR signal from the Mn(III) ion formed by photooxidation of Mn(II) bound at the high affinity Mn-binding site of photosystem II (PSII). This species corresponds to the first photoactivation intermediate formed on the pathway to assembly of the water-splitting Mn cluster. The parallel mode EPR spectrum of the photooxidation product of 1.2/1 stoichiometry Mn(II)/Mn-depleted wild-type Synechocystis sp. PCC 6803 PSII particles consists of six well-resolved transitions split by a relatively small 55Mn hyperfine coupling (44 G). This spectral signature is absent in photooxidized Mn apoPSII complexes prepared from D1-Asp170Glu and D1-Asp170His mutants, providing direct spectral evidence for a role for this specific D1-Asp170 residue in the initial photoactivation chemistry. Temperature-dependence measurements and spectral simulations performed on the Mn(III) parallel mode EPR signal of the wild-type sample give an axial zero-field splitting value of D ≈ −2.5 cm-1 and a r...