Does the structure of the water-oxidizing Photosystem II-manganese complexat room temperature differ from its low-temperature structure? A comparative X-ray absorption study

Detailed information on room-temperature structure and oxidation state of t he Photosystem II (PS II) manganese complex is needed to put mechanistic co nsiderations on solid grounds. Because previously this information had not been available, the tetranuclear manganese complex was investigated by X-ra y absorption spectroscopy (XAS) on PS II membrane particles at 290 K. Due t o methodical progress (collection of XAS spectra within 10 s or less), sign ificant X-ray radiation damage can be avoided; room-temperature XAS investi gations on the PS II in its native membrane environment become feasible. Th us, the ambiguity with respect to the mechanistic relevance of low-temperat ure XAS results is avoidable. At 290 K as well as at 18 K, the manganese co mplex in its dark-stable state (S-1-state) seemingly is a Mn(III)(2)Mn(IV)( 2) complex comprising two di-mu(2)-oxo bridged binuclear manganese units ch aracterized by the same Mn-Mn distance of 2.71-2.72 Angstrom at both temper atures. Most likely, manganese oxidation states and the protonation state o f the bridging oxides are fully temperature independent. Remarkably, at roo m-temperature manganese-ligand distances of 3.10 and 3.65 Angstrom are clea rly discernible in the EXAFS spectra. The type of bridging assumed to resul t in Mn-Mn or Mn-Ca distances around 3.1 Angstrom is, possibly, temperature -dependent as suggested by distance lengthening upon cooling by 0.13 Angstr om. However, mechanistic proposals on photosynthetic water oxidation, which involve the dimer-of-dimers model [Yachandra, V. K., ct al. (1993) Science 260, 675-679] are not invalidated by the presented results.