Ammonia-Induced Structural Changes of the Oxygen-Evolving Complex in Photosystem II Diminished at 277 K as Revealed by Light-Induced FTIR Difference Spectroscopy

Abstract

We have applied light-induced FTIR difference spectroscopy to study NH 3 -induced structural changes of the oxygen-evolving com-plex in photosystem II. We found that NH 3 -induced spectral changes in the flash-induced FTIR differ-ence spectrum of PSII were gradually diminished when the temperature of FTIR measurement was increased from 250 to 277 K. Our FTIR results suggest that at 277 K, NH 3 is only able to bind to a small fraction of the S 2 state OEC in PSII samples containing 100 mM NH 4 Cl. In contrast, at 250 K, NH 3 is able to bind to all the S 2 state OEC in PSII samples containing 100 mM NH 4 Cl. Our FTIR result provides a new interpretation for the previous EPR result that the oscillations with flash number of the amplitude of the NH 3 -modified S 2 state multiline EPR signal were unaffected at room-temperature (Boussac et al. 1990). Our result supports that the NH 3 -binding site on the S 2 state OEC that gives rise to the modified S 2 state multiline EPR signal is a substrate site.