PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Photochemically induced dynamic nuclear polarization in Photosystem I observed by ¹³C magic-angle spinning NMR. Esha Roy^*,1, Alia¹, Peter Gast², Hans van Gorkom², Huub de Groot¹, Gunnar Jeschke³, Jörg Matysik¹, ¹ Leiden Institute of Chemistry, Leiden, The Netherlands² Department of Biophysics, Leiden, The Netherlands³ Max-Planck-Institut für Polymerforschung, Mainz, Germany

ABSTRACT- Photochemically induced dynamic nuclear polarisation (photo-CIDNP) has been observed in photosystem I of spinach by ¹³C magic angle spinning solid-state NMR under continuous illumination with white light. An almost complete set of chemical shifts of the aromatic ring carbons of a single Chl a molecule has been obtained. On that basis, the electronic ground state of P700 can be mapped with atomic resolution. The intensity of the photo-CIDNP signals of PSI is very strong compared to other RCs. All the light induced ¹³C NMR signals appear to be emissive indicating the possibility of a predominance of the three-spin mixing mechanism (TSM) over the differential decay (DD) mechanism [For recent discussion of the mechanism of photo-CIDNP in solids, see: Jeschke and Matysik, Chem. Phys. 2003, NOV (3): 239-255]. This predominance may be the reason for the strong intensities of the photo-CIDNP signals relative to the dark background. The origin of the strong TSM contribution will be discussed. A significant chemical shift difference compared to P680 is observed on the carbon C-14. In contrast to PSII, the C13¹ resonance appears in the spectrum at the same position as expected from model Chl a molecules. The narrow linewidth of approximately 60 Hz provides evidence for a rather rigid, ordered as well as structurally and electrostatically stable donor site without structural heterogeneities. It appears to be a general property of RCs to have a rigid donor side, keeping reorganisation energies of electron transfer low. The photo-CIDNP signals can be assigned to a single Chl a molecule, which appears to be the P2 cofactor of the primary electron donor P700.

KEY WORDS: photosystem I, plant photosynthesis, photo-CIDNP, reaction center