REACTION CNTR CIDNP · Origin of the asymmetric electron transfer in photosynthesis explored by photo-CIDNP MAS NMR

Origin of the asymmetric electron transfer in photosynthesis explored by photo-CIDNP MAS NMR

Photosynthetic reaction centres, such as the reaction centre from purple bacteria, or photosystem I and photosystem II of plants, have their cofactors along which charge separation proceeds arranged in a nearly twofold symmetry. Despite the symmetrical arrangement, the electron pathway is entirely unidirectional occurring along one branch in the bacterial reaction centre. However, it has been shown that both electron transfer branches are equally active in photosystem I. On the other hand, a symmetry break between the two electron-transfer branches is required photosystem II, since a quinone-alpha at the asymmetric acceptor side has to be reduced selectively. It appears that the understanding of the very basic principles of the directionality of light-induced electron transfer is lacking. Here, we have pursued a new approach to solve this question by combining laser-flash photochemically induced dynamic nuclear polarisation (photo-CIDNP) magic angle spinning (MAS) nuclear magnetic resonance (NMR), providing electronic structure information of the highest occupied molecular orbital (HOMO) and the donor triplet at atomic resolution. This experimental data in combination with theoretical calculations, allows reconstructing the lowest unoccupied molecular orbital (LUMO) from which the electron is transferred. Our approach is based on the assumption that the structure of the LUMO is responsible for the directionality of the light-induced electron transfer and that therefore the reconstruction of the LUMO of the electron donor is the key for understanding. Within the project the relevant photosynthetic reaction centres have been prepared and selective 13C labelling could be employed. In addition, time-resolved photo-CIDNP MAS NMR spectra could be recorded. In combination with spin dynamic calculations the importance of precise computational models has been identified with respect to the reaction operator employed and with respect to the treatment of crystal structure coordinates. Very recently, we could show that the triplet state can indeed be reconstructed from these experiments, which paves the way for the reconstruction of the LUMO. A full understanding of natural photosynthesis might indeed be of highest importance with respect to renewable energy, carbon fixation and the global food chain.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз