15N photo-CIDNP MAS NMR analysis of a bacterial photosynthetic reaction center of Rhodobacter sphaeroides wildtype

The solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) effect has been studied in a quinone-depleted uniformly (u-)C-13,N-15-labeled photosynthetic reaction center (RC) protein from purple bacterium Rhodobacter (R.) sphaeroides wild type (WT). As a method for investigation, solid-state (15)NNMRunder magic-angle spinning (MAS) is applied under both continuous illumination (steady state) and nanosecond-laser flashes (time-resolved). While all previous N-15 photo-CIDNP MAS NMR studies on the purple bacterial RC used the carotenoid-less mutant R26, this is the first using WT samples. The absence of further photo-CIDNP mechanisms (compared to R26) and various couplings (compared to C-13 NMR experiments on C-13-labeled samples) allows the simplification of the spin-system. We report N-15 signals of the three cofactors forming the spin-correlated radical pair (SCRP) and, based on density-functional theory calculations, their assignment. The simulation of photo-CIDNP intensities and time-resolved N-15 photo-CIDNP MAS NMR data matches well to the frame of the mechanistic interpretation. Three spin-chemical processes, namely, radical pair mechanism, three spin mixing, and differential decay, generate emissive (negative) N-15 polarization in the singlet decay channel and absorptive (positive) polarization in the triplet decay channel of the SCRP. The absorptive N-15 polarization of the triplet decay channel is transiently obscured during the lifetime of the triplet state of the carotenoid ((3)Car); therefore, the observed N-15 signals are strongly emissive. Upon decay of (3)Car, the transiently obscured polarization becomes visible by reducing the excess of emissive polarization. After the decline of (3)Car, the remaining nuclear hyperpolarization decays with nuclear T-1 relaxation kinetics. Published under license by AIP Publishing.