The effects of light-induced reduction of the photosystem II reaction center.

Accumulation of reduced pheophytin a (Pheo-D1) in photosystem II reaction center (PSII RC) under illumination at low redox potential is accompanied by changes in absorbance and circular dichroism spectra. The temperature dependences of these spectral changes have the potential to distinguish between changes caused by the excitonic interaction and temperature-dependent processes. We observed a conformational change in the PSII RC protein part and changes in the spatial positions of the PSII RC pigments of the active D1 branch upon reduction of Pheo-D1 only in the case of high temperature (298 K) dynamics. The resulting absorption difference spectra of PSII RC models equilibrated at temperatures of 77 K and 298 K were highly consistent with our previous experiments in which light-induced bleaching of the PSII RC absorbance spectrum was observable only at 298 K. These results support our previous hypothesis that Pheo-D1 does not interact excitonically with the other chlorins of the PSII RC, since the reduced form of Pheo-D1 causes absorption spectra bleaching only due to temperature-dependent processes.