CHARACTERIZATION BY FTIR SPECTROSCOPY OF THE PHOTOREDUCTION OF THE PRIMARY QUINONE ACCEPTOR QA IN PHOTOSYSTEM-II

Molecular changes associated with the photoreduction of the primary quinone acceptor Qa of photosystem II have been characterized by Fourier transform infrared spectroscopy. This reaction was light-induced at room temperature on photosystem II membranes in the presence of hydroxylamine and diuron. A positive signal at 1478 cm-1 is assigned to the C⋯O stretching mode of the semiquinone anion, and can be correlated to the negative CO mode(s) of the neutral QA at 1645 cm-1 and/or 1630 cm-1. Analogies with bacterial reaction center are found in the amide I absorption range at 1672 cm-1, 1653 cm-1 and 1630 cm-1. The stabilization of QA- does not result from a large protein conformation change, but involves perturbations of several amino acid vibrations. At 1658 cm-1, a negative feature sensitive to 1H-2H exchange is tentatively assigned to a δNH2 histidine mode, while tryptophan D2252 could contribute to the signal at 1560 1550 cm-1. © 1990.