Light-induced Fourier transform infrared spectrum of the cation radical P680⁺

The structure of the primary electron donor of photosystem II, P680, is still under debate. It is not decided if it is composed of a chlorophyll (Chl) monomer or dimer. In this study, Fourier transform infrared (FTIR) spectroscopy was used to analyze the changes in the vibration modes occurring upon photooxidation of P680 in a Mn-depleted PS II preparation. It is demonstrated that illumination of the above in the presence of artificial electron acceptors results in a light-minus-dark absorbance change typical of the formation ofP680⁺. The light-minus-dark difference FTIR spectrum obtained under similar conditions is characterized by two negative peaks located at 1694 and 1652 or 1626 cm^-1 that can be assigned to the 9-keto groups of the P680 Chl, the latter band being indicative of a strongly associated group. These vibrations are shifted to 1714 and 1676 cm^-1, respectively, in the positive features of the difference spectrum attributed to P680⁺. The occurrence of two pairs of bands attributed to 9-keto groups is discussed in terms of P680 being formed of a Chl dimer.