Analysis of electron donors in photosystems in oxygenic photosynthesis by photo-CIDNP MAS NMR

• Solid-state photo-CIDNP effect allows for dramatic enhancement of the NMR signals.
• The method can be applied to various levels of sample isolation.
• Important insight into the electronic structures of PSI and PSII donor is obtained.
• Electron spin density distribution on the oxidized PSII donor is highly asymmetric.

Both photosystem I and photosystem II are considerably similar in molecular architecture but they operate at very different electrochemical potentials. The origin of the different redox properties of these RCs is not yet clear. In recent years, insight was gained into the electronic structure of photosynthetic cofactors through the application of photochemically induced dynamic nuclear polarization (photo-CIDNP) with magic-angle spinning NMR (MAS NMR). Non-Boltzmann populated nuclear spin states of the radical pair lead to strongly enhanced signal intensities that allow one to observe the solid-state photo-CIDNP effect from both photosystem I and II from isolated reaction center of spinach (Spinacia oleracea) and duckweed (Spirodela oligorrhiza) and from the intact cells of the cyanobacterium Synechocystis by 13C and 15N MAS NMR. This review provides an overview on the photo-CIDNP MAS NMR studies performed on PSI and PSII that provide important ingredients toward reconstruction of the electronic structures of the donors in PSI and PSII.