Reduced Derivatives of the Mn Cluster in the Oxygen-Evolving Complex of Photosystem II: An EXAFS Study

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Abstract

X-ray absorption spectroscopy (XAS) has been used to characterize the local structural environment of the Mn in the resting (S1) state and two different reduced derivatives of the photosynthetic oxygen-evolving complex (OEC). Short-term incubation with NH2OH gives a state with minimal structural rearrangement relative to the S1 state, consistent with the small shift in X-ray absorption edge energy for the NH2OH reduced sample. In contrast, hydroquinone reduced samples show significant structural rearrangements, including the appearance of a new Mn−O shell at 2.17 Å and a decrease in the amplitude of the 2.7 Å Mn···Mn interaction. These changes are consistent with hydroquinone producing a reduced state consisting of ca. 2 Mn(II) and a single, oxidized Mn2(μ-O)2 core. The interaction assigned to Mn···Mn or Mn···Ca scattering at ca. 3.3 Å is not present in the hydroquinone reduced sample, but is present in the EXAFS of the NH2OH reduced sample. The effects of both NH2OH and hydroquinone are reversed by illumination and dark adaptation, indicating that the reductant induced changes are not the result of sample decomposition. Long-term incubation with NH2OH and short-term incubation with higher concentrations of NH2OH both result in greater reduction, more extensive structural change, and loss of activity. There is a linear correlation between the activity of these highly reduced samples and the Mn content of the samples. However, the activity per Mn atom remains constant, demonstrating that all of the Mn detected by XAS is present in active OEC centers. This demonstrates that highly reduced centers can be produced not only by hydroquinone but also by NH2OH. However, when NH2OH is used to produce highly reduced centers, Mn loss competes with reduction. Based on differences in the reactivity of these reduced states and on their very different structural properties, a refined mechanism for reduction of Mn in the OEC is proposed.

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