Photosynthetic reaction centers (RCs) of Rhodobacter sphaeroides R26 were reconstituted at the QA site with ubiquinone-10, selectively 13C-enriched on positions 1, 2, 3, 4, and 3-Me (IUPAC numbering). RCs dispersed in LDAO detergent were studied with 13C CP/MAS NMR spectroscopy at temperatures between 180 and 240 K, while RCs precipitated by removal of the detergent were investigated at ambient temperature and at temperatures down to 180 K. Electrostatic charge differences in QA induced by polarization from the protein are less than 0.02 electronic equivalent for any of the labeled positions. This includes the 4-carbonyl, which is therefore not significantly polarized by an electrostatic binding interaction with the protein. The QA site is slightly heterogeneous on the scale of the NMR as the observed line widths of the labels are between 150 and 300 Hz and inhomogeneous broadening is observed for the signals of positions 1, 2, and 3 upon cooling. This contrasts with earlier MAS observations for labels in the vicinity of the special pair. The chemical shifts are 184, 144, and 137 ppm for the labels at positions 1, 2, 3, and 12 ppm for the 3-methyl 13C. For the 4-carbonyl only at sample temperatures below approximately 255 K a CP/MAS response can be observed at 183 ppm. The principal components of the chemical shift tensors for the ring labels in QA were estimated using difference spectroscopy.(ABSTRACT TRUNCATED AT 250 WORDS)