Nanosecond-Flash 15N Photo-CIDNP MAS NMR on Reaction Centers of Rhodobacter sphaeroides R26

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed with time-resolved 15N magic-angle spinning (MAS) nuclear magnetic resonance (NMR) in uniformly 15N-labeled reaction centers (RCs) of the purple bacterium Rhodobacter (Rb.) sphaeroides R26 under illumination with nanosecond laser flashes. The 15N NMR signals enhanced by the solid-state photo-CIDNP effect are related to electron spin densities in the primary radical pair and allow for reconstruction of the electronic structure at atomic scale. New assignments for the photochemically active nitrogens are proposed based on simulations of the intensities. The ratio of electron spin densities between cofactors PL and PM is observed to be about 4:1. The origin of the high asymmetry is discussed.