Photo-CIDNP MAS NMR in intact cells of Rhodobacter sphaeroides R26:: Molecular and atomic resolution at nanomolar concentration

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed in photosynthetic reaction centers of the carotenoid-less strain R26 of the purple bacterium Rhodobacter sphaeroides by C-13 solid-state NMR at three different magnetic fields (4.7, 9.4, and 17.6 T). The signals of the donor appear enhanced absorptive (positive) and of the acceptor emissive (negative). This spectral feature is in contrast to photo-CIDNP data of reactions centers of Rhodobacter sphaeroides wildtype reported previously (Prakash, S.; Alia; Gast, P.; de Groot, H. J. M.; Jeschke, G.; Matysik, J. J. Am. Chem. Soc. 2005, 127, 14290-14298) in which all signals appear emissive. The difference is due to an additional mechanism occurring in RCs of R26 in the long-living triplet state of the donor, allowing for spectral editing by different enhancement mechanisms. The overall shape of the spectra remains independent of the magnetic field. The strongest enhancement is observed at 4.7 T, enabling the observation of photo-CIDNP enhanced NMR signals from reaction center cofactors in entire bacterial cells allowing for detection of subtle changes in the electronic structure at nanomolar concentration of the donor cofactor. Therefore, we establish in this paper photo-CIDNP MAS NMR as a method to study the electronic structure of photosynthetic cofactors at the molecular and atomic resolution as well as at cellular concentrations.