The method of solid-state Photochemically Induced Dynamic Nuclear Polarization (Photo-CIDNP) Nuclear Magnetic Resonance (NMR) is especially attractive for studying the electron donors in photosynthetic systems as it provides an access to their electronic structures and can be applied to RCs in living cells of bacteria and plants as well as to whole plants . Using this approach, chemical shifts of photosynthetic pigments associated with the electronic ground-state can be measured . The electronic spin-density distribution within the radical-pair state is related, although in a complex fashion , to photo-CIDNP intensities. Moreover, considering electronic structures of (bacterio)chlorophyll [(B)Chl] co-factors in terms of simplified frontier orbital models, the spatial distribution of highest occupied molecular orbitals can be reconstructed , whereas electron-spin densities measured for the donor triplet state can be used to access spin-density contributions from the lowest unoccupied molecular orbital within similar model assumptions. 13 C and 15 N photo-CIDNP MAS NMR measurements carried out for the electron donor in PSII revealed a strongly asymmetric (compared to Chl a in solution ) electron spin-density distribution and led to the formulation of the so-called hinge model in Ref. . According to this model, the strong spin-density shift is caused by the coordinated histidine (His) molecule, which is deprotonated at the π-N atom (for nomenclature, see Fig. ) and, therefore, is negatively charged. Thus, the electron donor in its dark state appears to be the anion [Chl• • • His] -, which is oxidized to [Chl• • • His] • in the charge-separation event. The His moiety can slightly bend towards pyrrole ring D of the Chl a co-factor (Fig. ) increasing the π-π overlap of the conjugated systems and, as a result, inducing shifts of the spin-density into ring D and into the aromatic ring of the His residue . Additionally, it was proposed that the distance between Chl a and His can also be changing depending on the redox state of the complex. Earlier ENDOR measurements indicated that Chl a molecules of the inner pair are weakly coupled and about 82% of the spin density is localized at a single Chl a molecule. Thus, according to the hinge model of the electron donor, a part of the remaining contribution is localized at the His molecule.