Photo-CIDNP studies on reaction centers of rhodobacter sphaeroides

Abstract

The reaction centers (RCs) of the photosynthetic purple bacterium Rhodobacter (Rb.) sphaeroides have been studied with photochemically induced dynamic nuclear polarization (photo-CIDNP) MAS NMR. The magnetic-field dependence of photo-CIDNP at field strength of 17.6, 9.4 and 4.7 T has been studied. The strongest enhancement of NMR intensities is observed at 4.7 T, which is by a factor of 10000, leading to the observation of bacteriochlorophyll molecules at nanomolar concentrations inside intact bacterial cells. All the photo-CIDNP signals from Rb. sphaeroides WT appear to be enhanced emissive (negative). The simulations, considering the electron-electron-nuclear three spin mixing (TSM) and differential decay (DD) mechanisms, show good agreement with the experimental photo-CIDNP spectrum. On the other hand, in the photo-CIDNP spectrum of R26, the signals from the donor are enhanced absorptive (positive) and of the acceptor emissive (negative). This difference to WT is explained by considering an additional mechanism, the differential relaxation (DR) mechanism. In addition, the strong enhancement in photo-CIDNP MAS NMR spectrum at 4.7 T has been applied to achieve a comprehensive map of the electronic ground-state structure of the photochemically active cofactors. In this context, 13C-13C dipolar correlation photo-CIDNP MAS NMR studies have been performed on selectively isotope labelled RCs of Rhodobacter sphaeroides WT.