These experimental measurements predicted 82% of the unpaired spin to be localized at the P D1 monomer, while the 18% left were assigned to P D2 . The authors of Ref. conducted a 13 C photo-CIDNP MAS NMR experiment assigning most of the measured signals to a single Chl a co-factor. The observed spin-density distribution showed a pronounced asymmetry (shift towards rings III and V) within Chl a compared to the results obtained in solution (largest contribution at ring II) . This asymmetry was later confirmed in a subsequent experiment . Finally, 15 N photo-CIDNP MAS NMR measurements carried out in Ref. detected signals originating from an axial histidine molecule leading to the formulation of the hinge-type model for the primary electron donor of PSII. Therefore, according to the experimental results the inner pair of PSII is a weakly-coupled pair with the largest spin-density contribution localized at the P D1 co-factor. The remaining spin-density is probably localized at the axial His molecule or shared between His and P D2 .