PARENT SESSION

Symposium S6A Type II reaction centres: Excited state dynamics and donor side
Thursday September 2nd, 2004 10:20 AM-12:20 PM Room 511D
Chair: Bill Rutherford
Co-Chair: Marilyn Gunner

The electronics structure of the primary donor of photosystem II investigated by photo-CIDNP solid-state NMR. Anna Diller¹, A Alia^*,1, Peter Gast², Hans van Gorkom², Huub de Groot¹, Clemens Glaubitz³, Joerg Matysik¹, ¹ Leiden Institute of Chemistry, Gorlaeus Laboratoria, Leiden, The Netherlands² Department of Biophysics, Huygens Laboratorium, Leiden, The Netherlands³ Institute of Biophysical Chemistry, Johann-Wolfgang-Goethe-Universität Frankfurt, Frankfurt, Germany

ABSTRACT- P680 is the strongest oxidizing agent known in living nature (approximately 1.2V). The reason for this high redox power is still under debate. Based on spectroscopic data obtained with photo-CIDNP MAS NMR, we have recently developed a hypothesis on the electronic structure of P680 and the origin of its high redox power (J. Matysik, Alia, P. Gast, H. J. van Gorkom, A. J. Hoff and H. J. M. de Groot, Proc. Natl. Acad. Sci. USA, 2000, 97, 9865). Here we present new photo-CIDNP MAS NMR data of significantly improved quality, strengthening the former hypothesis. Purified frozen PSII RC complexes (D1-D2-cytb559 with about 6 attached chlorophylls and 2 pheophytins) are studied by 13C MAS NMR under continuous illumination with white light at 9.4 Tesla (400 MHz) and a spinning frequency up to 9 kHz. The chemical shifts obtained are related to the ground state electronic structure after charge separation and recombination. The photo-CIDNP intensities are related to the hf-anisotropies and therefore approximate to local electron spin densities. The new data allow for the first time the assignment of all methine carbons. These assignments provide further evidence for a shift of electron-spin density in the macrocycle towards rings III and V. The improved spectral quality allows to assign the maximum of electron spin density to the C-15 methine carbon, while the methine carbons C-5 and C-20 carry only minor electron spin density. This more detailed view suggests a local electrostatic field pulling electron charge towards the C13-1-carbonyl, decreasing the energy of both frontier molecular orbitals and causing the increase of the redox power. The observation of a carbonyl resonance at 172.2 and the absence of a signal at the frequency expected for the C13-1-carbonyl will be discussed.

KEY WORDS: Photosystem II, photo-CIDNP, reaction centre, plant photosynthesis