Photochemically induced dynamic nuclear polarization (photo-CIDNP) is non-Boltzmann nuclear magnetization which can be observed by magic angle spinning NMR spectroscopy as enhanced absorptive or emissive signals. In solids, photo-CIDNP has been observed since its discovery in 1994 in various photosynthetic reaction centers. In natural photosynthetic charge separation, electron-electron interactions are fine-tuned to lead to highly efficient electron transfer. Nanosecond laser-flash photo-CIDNP magic-angle spinning NMR allows for determination of the nuclear polarizations and hyperfine interactions with atomic selectivity and with a resolution of a few microseconds. The build-up of nuclear polarization in reaction centers of Rhodobacter sphaeroides is found to depend on the presence and lifetimes of the molecular triplet states of the donor and carotenoid. Time-resolved 13C photo-CIDNP MAS NMR spectroscopy is used to map the electronic structure of the donor. In the dark state, maximum electron density is localized in the center of the special pair. In contrast, in the light state, the maximum of the electron spin density is localized at the periphery of the two cofactors. The balance of electron spin density between the two bacteriochlorophyll cofactors is shifted in favor of the L branch of the protein by the ratio of 7:3. We show that the asymmetry is induced by both geometric differences between the two cofactors and non-covalent interactions with the protein.EMBO, NWO, Spectraphysics-Newport, Bruker BiospinUBL - phd migration 201
