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

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Abstract
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 uas (COO−) and the lower frequency to us (COO−). 13C‐Labeling FT‐IR measurements confirmed these findings and also suggested that the negative band at 1560 cm−1 can be ascribed to uas (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.

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