Bicarbonate binding to the water‐oxidizing complex in the photosystem II. A Fourier transform infrared spectroscopy study¹

The light-induced Fourier transform infrared difference (FT-IR) spectrum originating from the donor side of O2-evolving photosystem (PS) II was obtained in non-depleted and CO2-depleted PSII membrane preparations. The observed spectrum free of contributions from the acceptor side signals was achieved by employing 2 mM/18 mM ferri-/ferrocyanide as a redox couple. This spectrum showed main positive bands at 1589 and 1365 cm(-1) and negative bands at 1560, 1541, 1522 and 1507 cm(-1). CO-depleted PSII preparations showed a quite different spectrum. The main positive and negative bands disappeared after depletion of bicarbonate. The addition of bicarbonate partially restored those bands again. Comparison between difference FT-IR spectra of untreated and bicarbonate-depleted PSII membranes indicated that the positive bands at 1589 and 1365 cm(-1) can be assigned to COO- stretching modes from bicarbonate. The higher frequency corresponds to u[as] (COO-) and the lower frequency to u[s] (COO-). 13C-Labeling FT-IR measurements confirmed these findings and also suggested that the negative band at 1560 cm(-1) can be ascribed to u[as] (COO-). The data are discussed in the framework of the suggestion that bicarbonate can be a ligand to the Mn-containing water-oxidizing complex of PSII.