The Basic Properties of the Electronic Structure of the Oxygen-evolving Complex of Photosystem II Are Not Perturbed by Ca2+ Removal

Proton Matrix ENDOR Studies on Ca2+-depleted and Sr2+-substituted Manganese Cluster in Photosystem II

“…The present results showed that the electronic structure was not significantly different between Ca 2ϩ -containing and Ca 2ϩ -depleted PSII, consistent with the 55 Mn ENDOR results reported previously (36). Therefore, Ca 2ϩ is hardly involved in the control of electronic structure (case I) and redox potential (case II), which is supported by the observation of the multiline signals in the Ca 2ϩ -depleted PSII (19,35) and recent QM/MM calculation results (49).…”

“…Another role of Ca 2ϩ would therefore be to mediate the electron transfer effectively from the manganese cluster to Y Z through W4. The back reaction would also be suppressed by the lack of the electron transfer path, consistent with the results that spin properties of the manganese cluster in Ca 2ϩ -depleted PSII are very similar to those in normal PSII (36), and the S 2 state is remarkably dark stable in Ca 2ϩ -depleted PSII (19). Recently, Yang et al (53) showed that the highest occupied molecular orbital contribution on Mn1 in the S 2 state decreased with Ca 2ϩ depletion, where Mn1 is expected to be oxidized in the transition from S 2 to S 3 state.…”

“…structure and 55 Mn-ENDOR studies, which have suggested similar geometry and electronic structure among Ca 2ϩ -containing and depleted or Sr 2ϩ -substituted PSII (22,27,36). Previously we assigned the ddЈ peaks to the proton of a water molecule W4 connecting Ca 2ϩ and Y Z based on theoretical studies (37) and the S 1 state structure and spin distribution on manganese ions (30).…”

“…There are different techniques to extract information regarding the position and number of the nuclei, such as pulsed ENDOR, ESEEM, and ELDOR. Previous studies using Q-band pulsed 55 Mn ENDOR spectra indicated that 55 Mn hyperfine couplings of untreated PSII and Ca 2ϩ -depleted (citrate-treated) PSII are similar (36), even though the latter was in the "modified multiline" state. The spectral resolution of CW-ENDOR is generally higher than pulsed ENDOR/ ELDOR; however, the measurement conditions are restricted because of the dependence of the relaxation properties of the electron and nuclei spins (37).…”

Reduction potentials of heterometallic manganese–oxido cubane complexes modulated by redox-inactive metals

“…The calcium center has been proposed to coordinate water or hydroxide ligands that participate in O-O bond formation (5)(6)(7). Other proposed functions include affecting proton-coupled electron transfer and the redox properties of the cluster, possibly via interactions with neighboring tyrosine residues (8)(9)(10)(11).…”