To analyze a possible correlation between the extent of QA-* reoxidation and protein dynamics, fluorometric and Mössbauer spectroscopic measurements were performed in photosystem II membrane fragments from spinach. Numerical evaluation of the flash-induced change of the normalized fluorescence quantum yield revealed that the extent of reoxidation starts to decrease below 275 K and is almost completely suppressed at 230 K. Detailed analyses of Mössbauer spectra measured at different temperatures in 57Fe-enriched material indicate that the onset of fluctuations between conformational substates of the protein matrix occurs also at around 230 K. Based on this correspondence, protein flexibility is inferred to play a key role for QA-* reoxidation in photosystem II. Taking into account the striking similarities with purple bacteria and the latest structural information on these reaction centers [Stowell, M. H. B., McPhillips, T. M., Rees, D. C., Soltis, S. M., Abresch, E., and Feher, G. (1997) Science 276, 812-816], it appears most plausible that also the headgroup of plastoquinone-9 bound to the QB-site in PSII requires a structural reorientation for its reduction to the semiquinone.

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Fe enriched Dl/D2/cyt 6559 preparations were isolated from spinach grown hydroponically on a 7 Fe containing medium. In terms of polypeptide and pigment composition these samples are of high purity and functional integrity of P680. Mössbauer spectra measured in Dl/D2/cyt A559 complexes revealed that these preparations are completely deprived of the non-heme iron center. Possible implications of this finding are discussed for the electron transfer from Pheo ' to exogenous electron acceptors.

Mössbauer spectroscopy was applied to study the properties of cytochrome b559 (Cyt b559) in isolated D1/D2/Cyt b559 preparations (from spinach) that are completely deprived of non-heme iron. In these samples, all Cyt b559 exists as low-potential form(s) with the iron center attaining the low-spin ferric state in the absence of a strong reductant. The Mössbauer spectra were analyzed using isomer shift and quadrupole splitting parameters below 100 K, gathered from an extrapolation of the temperature dependence of experimental data of photosystem II membrane fragments from spinach. The calculations, based on the Griffith model, lead to the conclusion that the crystal field around the heme iron of Cyt b559 is characterized by a strong rhombic distortion. The g-values obtained are in agreement with recently published EPR results. The use of an extended theoretical approach permits the description of the relaxation changes of the Mössbauer spectra in the temperature region from 5 K to 60 K. It shows that the environment of the heme iron in D1/D2/Cyt b559 is not homogeneous but most likely reflects the existence of two different forms. We assume that factors other than changes of the first coordination sphere are responsible for the drastic negative shift in the redox potential of Cyt b559 that takes place during the isolation procedure of D1/D2/Cyt b559 complexes. Possible implications of these findings are discussed.

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