Effects of Ammonia on the Structure of the Oxygen-Evolving Complex in Photosystem II As Revealed by Light-Induced FTIR Difference Spectroscopy

NH3 is a structural analogue of substrate H2O and an inhibitor to the water oxidation reaction in photosystem II. To test whether or not NH3 is able to replace substrate water molecules on the oxygen-evolving complex in photosystem II, we studied the effects of NH3 on the high-frequency region (3750-3550 cm(-1)) of the S(2)Q(A)(-)/S(1)Q(A) FTIR difference spectra (pH 7.5 at 250 K), where OH stretch modes of weak hydrogen-bonded active water molecules occur. Our results showed that NH3 did not replace the active water molecule on the oxygen-evolving complex that gave rise to the SI mode at similar to 3586 cm(-1) and the S-2 mode at similar to 3613 cm(-1) in the S(2)Q(A)(-)/S(1)Q(A) FTIR difference spectrum of PSII. In addition, our mid-frequency FTIR results showed a clear difference between pH 6.5 and 7.5 on the concentration dependence of the NH4Cl-induced upshift of the S-2 state carboxylate mode at 1365 cm(-1) in the S(2)Q(A)(-)/S(1)Q(A) spectra of NH4Cl-treated PSII samples. Our results provided strong evidence that NH3 induced this upshift in the spectra of NH4Cl-treated PSII samples at 250 K. Moreover, our low-frequency FTIR results showed that the Mn-O-Mn cluster vibrational mode at 606 cm(-1) in the S(2)Q(A)(-)/S(1)Q(A) spectrum of the NaCl control PSII sample was diminished in those samples treated with NH4Cl. Our results suggest that NH3 induced a significant alteration on the core structure of the Mn4CaO5 cluster in PSII. The implication of our findings on the structure of the NH3-binding site on the OEC in PSII will be discussed.