Ammonia-induced structural changes of the oxygen-evolving complex in photosystem II as revealed by light-induced FTIR difference spectroscopy

Light-induced Fourier transform infrared difference spectroscopy has been applied to studies of ammonia effects on the oxygen-evolving complex (OEC) of photosystem 11 (PSII). We found that NH3 induced characteristic spectral changes in the region of the symmetric carboxylate stretching modes (1450-1300 cm(-1)) of the S(2)Q(A)(-)/S(1)Q(A) FTIR difference spectra of PSII. The S-2 state carboxylate mode at 1365 cm(-1) in the S(2)Q(A)(-)/S(1)Q(A) spectrum of the controlled samples was very likely upshifted to 1379 cm(-1) in that of NH3-treated samples; however, the frequency of the corresponding S, carboxylate mode at 1402 cm(-1) in the same spectrum was not significantly affected. These two carboxylate modes have been assigned to a Mn-ligating carboxylate whose coordination mode changes from bridging or chelating to unidentate ligation during the S-1 to S-2 transition [Noguchi, T., Ono, T., and Inoue, Y. (1995) Biochim. Biophys. Acta 1228, 189-200; Kimura, Y., and Ono, T.-A. (2001) Biochemistry 40, 14061-14068]. Therefore, our results show that NH3 induced significant structural changes of the OEC in the S-2 state. In addition, our results also indicated that the NH3-induced spectral changes of the S(2)Q(A)(-)/S(1)Q(A) spectrum of PSII are dependent on the temperature of the FTIR measurement. Among the temperatures we measured, the strongest effect was seen at 250 K, a lesser effect was seen at 225 K, and little or no effect was seen at 200 K. Furthermore, our results also showed that the NH3 effects on the S(2)Q(A)(-)/S(1)Q(A) spectrum of PSII are dependent on the concentrations of NH4Cl. The NH3-induced upshift of the 1365 cm(-1) mode is apparent at 5 mM NH4CI and is completely saturated at 100 mM NH4Cl concentration. Finally, we found that CH3NH2 has a small but clear effect on the spectral change of the S(2)Q(A)(-)/S(1)Q(A) FTIR difference spectrum of PSII. The effects of amines on the S(2)Q(A)(-)/S(1)Q(A) FTIR difference spectra (NH3 > CH3NH2 > AEPD and Tris) are inverse proportional to their size (Tris similar to AEPD > CH3NH2 > NH3). Therefore, our results showed that the effects of amines on the S(2)Q(A)(-)/S(1)Q(A) spectrum of PSII are sterically selective for small amines. On the basis of the correlations between the conditions (dependences on the excitation temperature and NH3 concentration and the steric requirement for the amine effects) that give rise to the NH3-induced upshift of the 1365 cm(-1) mode in the S(2)Q(A)(-)/S(1)Q(A) Spectrum of PSII and the conditions that give rise to the altered S-2 state multiline EPR signal, we propose that the NH3-induced upshift of the 1365 cm(-1) mode is caused by the binding of NH3 to the site on the Mn cluster that gives rise to the altered S-2 state multiline EPR signal. In addition, we found no significant NH3-induced change in the S(2)Q(A)(-)/S(1)Q(A) FTIR difference spectrum 1 EPR signal and a suppressed g = 2 multiline EPR signal. Our results suggest that the structural difference of the OEC between the normal g = 2 multiline form and the NH3-stabilized g = 4.1 form is small.