According to this work, the spin density is localized at the P B co-factor. Later, theoretical calculations by Plato et al. confirmed the asymmetric spin-density distribution in the inner pair of PSI. The ratio was found to be 75%/25% in favor of P B . It was also proposed that this asymmetry is not caused by the structural difference of isolated P B (Chl a) and P A (Chl a ), but by the interaction of the co-factors with the protein environment. This assumption was verified by calculating molecular models including a different number of amino acid residues from the surrounding protein environment. This showed an increasing degree of the spin-density asymmetry for larger molecular models. In case of photo-CIDNP MAS NMR experiments, measured 13 For the BRC of Rhodobacter viridis, an asymmetric spin-density distribution of about 66%/33% in favor of the D A co-factor was reported in Ref. . In case of Rhodobacter sphaeroides, fully-assigned EPR and ENDOR spectra were not presented for a long time.