REDUCED DERIVATIVES OF THE MN CLUSTER IN THE OXYGEN-EVOLVING COMPLEX OF PHOTOSYSTEM-II - AN EXAFS STUDY

X-ray absorption spectroscopy (XAS) has been used to characterize the local structural environment of the Mn in the resting (S-1) state and two different reduced derivatives of the photosynthetic oxygen-evolvin g complex (OEC). Short-term incubation with NH2OH gives a state with m inimal structural rearrangement relative to the S-1 state, consistent with the small shift in X-ray absorption edge energy for the NH2OH red uced sample. In contrast, hydroquinone reduced samples show significan t structural rearrangements, including the appearance of a new Mn-O sh ell at 2.17 Angstrom and a decrease in the amplitude of the 2.7 Angstr om Mn ... Mn interaction. These changes are consistent with hydroquino ne producing a reduced state consisting of ca. 2 Mn(II) and a single, oxidized Mn-2(mu-O)(2) core. The interaction assigned to Mn ... Mn or Mn ... Ca scattering at ca. 3.3 Angstrom is not present in the hydroqu inone reduced sample, but is present in the EXAFS of the NH2OH reduced sample. The effects of both NH2OH and hydroquinone are reversed by il lumination and dark adaptation, indicating that the reductant induced changes are not the result of sample decomposition. Long-term incubati on with NH2OH and short-term incubation with higher concentrations of NH2OH both result in greater reduction, more extensive structural chan ge, and loss of activity. There is a linear correlation between the ac tivity of these highly reduced samples and the Mn content of the sampl es. However, the activity per Mn atom remains constant, demonstrating that all of the Mn detected by XAS is present in active OEC centers. T his demonstrates that highly reduced centers can be produced not only by hydroquinone but also by NH2OH. However, when NH2OH is used to prod uce highly reduced centers, Mn loss competes with reduction. Based on differences in the reactivity of these reduced states and on their ver y different structural properties, a refined mechanism for reduction o f Mn in the OEC is proposed.