Ref. also discussed the possiblity to assign some of the measured 13 C photo-CIDNP signals in the Rhodobacter sphaeroides wild type (WT) to either axial histidine atoms or bacteriopheophytin (BPhe). In this case, it was not possible to draw a definite conclusion without further experimental data with selective isotope labeling or oriented samples. More recent photo-CIDNP MAS NMR measurements of 15 N-labeled Rhodobacter sphaeroides WT reported in Ref. showed that all signals originate from the special pair co-factors and BPhe. Nowadays it is generally accepted that the axial His molecule in bacterial RC does not carry any spin-density contributions. KS-DFT calculations presented in Ref. showed a minor influence of axial ligation to the spin density of a single BChl a molecule. According to the results in Ref. , axial His molecules are protonated at π-N positions. All four axial His molecules coordinated at inner and accessory BChls were also found to be protonated in Ref. based on results of cross-polarization-magicangle spinning (CP-MAS) 15 N for Rhodobacter sphaeroides R26. The properties of one of the His residues were, however, reported to be different from the other three due to formation of a strong X-H-π-N hydrogen bond. Based on these experimental findings, we assume in the following that i) the D A +• cation radical is coordinated by a protonated His molecule and ii) the spin density is fully localized at the D A +• co-factor (when D B is not included into the model).