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 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.