Photochemically induced dynamic nuclear polarization observed by solid-state NMR in a uniformly 13C-isotope labeled photosynthetic reaction center

Abstract

A sample of solubilized and quinone-depleted reaction centers (RC) from the purple bacterium Rhodobacter (R.) sphaeroides wild-type (WT) has been prepared entirely 13C and 15N isotope labelled at all positions of the protein as well as of the cofactors. In this sample, the occurrence of the solid-state photo-CIDNP (photochemically induced dynamic nuclear polarization) effect has been probed by 13C solid-state magic-angle spinning (MAS) NMR under illumination. Under continuous illumination, signal intensities are modified by the three-spin mixing (TSM) mechanism. Time-resolved illumination experiments reveal the occurrence of light-induced nuclear polarization on the time-scale of hundreds of microseconds, initially dominated by the transient polarization of the singlet branch of the radical-pair mechanism (RPM). A first kinetic analysis shows that the lifetime of the polarization from the singlet branch, indicated by the enhanced absorptive intensities of the signals from aliphatic carbons, is significantly extended. Upon arrival of the polarization from the triplet decay branch, emissive polarization caused by the three-spin mixing mechanism (TSM) is observed. Also this arrival is significantly delayed. The decay of TSM polarization occurs in two steps, assigned to intra- and intermolecular spin diffusion.