Reduced derivatives of the manganese cluster in the photosynthetic oxygen-evolving complex

X-ray spectroscopy of the photosynthetic oxygen-evolving complex

“…Depleting Ca from intact PS II, which results in a decrease in the EXAFS amplitude at this distance, provided confirmation of this conclusion [76] as well as evidence that scattering by Mn-Mn interactions also contributes to Peak III [34]. Nevertheless, the close association of Ca (Sr) with the Mn cluster was disputed at the time by similar Mn EXAFS studies involving Ca replacement by heavier atoms [77].…”

Absence of Mn-Centered Oxidation in the S2→ S3Transition: Implications for the Mechanism of Photosynthetic Water Oxidation

“…S 2 -state EPR multiline signal simulations by Hasegawa et al 96,97 and 55 Mn ENDOR spectroscopy on the S 2 state by Britt and co-workers 98,99 100,101 the 2nd derivative of the XANES spectrum for the S 1 state shows that its edge shape is unlike the edge shape observed for Mn(III) complexes; when the 2nd-derivative XANES spectrum for the S 1 state is fit using Mn(III) and Mn(IV) model compounds, it cannot be fit well using only Mn(III) model compounds. 18,102 Furthermore, inspection of the only available set of tetranuclear Mn complexes with all O ligation, four distorted Mn 4 (III 3 ,IV) cubanes, shows XANES IPE values at ~6551 eV for these complexes, 20 consistent with the S 0 -state XANES IPE of 6550.8 eV and lower than the S 1 -state XANES IPE of 6552.9 eV. Thus, this is also consistent with the conclusion that the oxidation states of Mn in the S 1 state are Mn 4 (III 2 ,IV 2 ), not Mn 4 (III 4 ).…”

The PSII calcium site revisited

“…Extraction of the PsbP and PsbQ subunits exposes the Mn cluster to reduction and loss of Mn(II) catalyzed by hydroquinone and NH 2 OH (Ghanotakis et al 1984c), and it was later shown Yocum 1991, 1992) that Ca 2+ added to PSII in the absence of the PsbP and PsbQ subunits could stabilize the Mn cluster in reduced states. The S -1 state formed by hydroquinone reduction was extensively characterized by XAFS and XANES spectroscopy and shown to consist of a 2 Mn(II)/2 Mn(IV) oxidation state (Riggs et al 1992) that was reversed to the S 1 oxidation state by illumination (Riggs-Gelasco et al 1996). The hydroquinone reduced samples retained about 80% of their control activities, and it was proposed that Ca 2+ functioned to stabilize the Mn ligand environment, resulting in the retention of activity even after Mn(II) formation.…”