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

The light-induced Fourier transform infrared difference (FT-IR) spectrum originating from the donor side of O-2-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). GO-depleted PSII preparations showed a quite different spectrum. The main positive and negative bands disappeared after depiction 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 lon er frequency to u(s) (COO-). C-13-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. (C) 1998 Federation of European Biochemical Societies.