LIGHT-INDUCED FOURIER-TRANSFORM INFRARED-SPECTRUM OF THE CATION-RADICAL P680+

The structure of the primary electron donor of photosystem II, P680, i s still under debate. It is not decided if it is composed of a chlorop hyll (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 pre paration. It is demonstrated that illumination of the above in the pre sence of artificial electron accepters results in a light-minus-dark a bsorbance change typical of the formation of P680(+). The light-minus- dark difference FTIR spectrum obtained under similar conditions is cha racterized 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 latt er band being indicative of a strongly associated group. These vibrati ons are shifted to 1714 and 1676 cm(-1), respectively, in the positive features of the difference spectrum attributed to P680(+). The occurr ence of two pairs of bands attributed to 9-keto groups is discussed in terms of P680 being formed of a Chl dimer.