Structural and Oxidation State Changes of the Photosystem II Manganese Complex in Four Transitions of the Water Oxidation Cycle (S₀ → S₁, S₁ → S₂, S₂ → S₃, and S_3,4 → S₀) Characterized by X-ray Absorption Spectroscopy at 20 K and Room Temperature

Abstract

Structural and electronic changes (oxidation states) of the Mn(4)Ca complex of photosystem II (PSII) in the water oxidation cycle are of prime interest. For all four transitions between semistable S-states (S(0) --> S(1), S(1) --> S(2), S(2) --> S(3), and S(3),(4) --> S(0)), oxidation state and structural changes of the Mn complex were investigated by X-ray absorption spectroscopy (XAS) not only at 20 K but also at room temperature (RT) where water oxidation is functional. Three distinct experimental approaches were used: (1) illumination-freeze approach (XAS at 20 K), (2) flash-and-rapid-scan approach (RT), and (3) a novel time scan/sampling-XAS method (RT) facilitating particularly direct monitoring of the spectral changes in the S-state cycle. The rate of X-ray photoreduction was quantitatively assessed, and it was thus verified that the Mn ions remained in their initial oxidation state throughout the data collection period (>90%, at 20 K and at RT, for all S-states). Analysis of the complete XANES and EXAFS data sets (20 K and RT data, S(0)-S(3), XANES and EXAFS) obtained by the three approaches leads to the following conclusions. (i) In all S-states, the gross structural and electronic features of the Mn complex are similar at 20 K and room temperature. There are no indications for significant temperature-dependent variations in structure, protonation state, or charge localization. (ii) Mn-centered oxidation likely occurs on each of the three S-state transitions, leading to the S(3) state. (iii) Significant structural changes are coupled to the S(0) --> S(1) and the S(2) --> S(3) transitions which are identified as changes in the Mn-Mn bridging mode. We propose that in the S(2) --> S(3) transition a third Mn-(mu-O)(2)-Mn unit is formed, whereas the S(0) --> S(1) transition involves deprotonation of a mu-hydroxo bridge. In light of these results, the mechanism of accumulation of four oxidation equivalents by the Mn complex and possible implications for formation of the O-O bond are considered.

Photosynthetic water oxidation at elevated dioxygen partial pressure monitored by time-resolved X-ray absorption measurements

“…2 A and B). At ambient pO 2 (0.2 bar), similar shifts of the K-edge to higher energies on flashes 1 and 2 were detected as previously (32,36). They are attributable to oxidation of the Mn complex in the S 1 3S 2 and S 2 3S 3 transitions.…”

“…A decrease in the fluorescence level after the flash indicates oxidation of the Mn complex, an increase reflects Mn reduction. [We note that the magnitude of ⌬F X-ray is S-state-dependent; for excitation at 6,552.5 eV, it is larger in S 1 3S 2 than in S 2 3S 3 (36)(37)(38).] At 0.2 bar O 2 , Mn reduction was absent on flash 1 and small on flash 2, whereas at 13 bar O 2 , significant Mn reduction occurred already on flash 2.…”

“…4). Simula- tions on the basis of Kok's S-state model (15,36) of the data in Fig. 4 were used to characterize and quantify the O 2 effects.…”

Functional models for the oxygen-evolving complex of photosystem II

“…Decades of biophysical study have provided a solid groundwork for this project. Crystal structures of photosystem II [1,2] (PSII) have revealed the location of many important components such as chlorophylls and the oxygen-evolving center (OEC) while spectroscopy such as EXAFS [3][4][5][6][7], EPR [8][9][10][11][12], and IR [13,14] have allowed the study of reactive intermediates, the composition of the OEC and the protein ligation of the metal centers. Unfortunately, while we now have a much better structural and mechanistic basis for PSII, we still are some way from the goal of a molecular mechanistic understanding of water oxidation.…”

Focusing the view on nature's water-splitting catalyst

“…Interestingly, the structure of the cluster changes dramatically upon S 3 -state formation (Liang et al 2000;Haumann et al 2005) and is also different in the S 0 state Haumann et al 2005). This structural flexibility may be key to the catalytic activity of the WOC.…”