The . Hovewer, more recent measurements with 13 C isotope labels introduced revealed a dimeric nature of the donor in PSI . Similar experiments for PSII reported a highly asymmetrical spin-density distributions over P D1 . Substitution of axial His molecules in PSI by other uncharged polar amino acids using site-directed mutagenesis did not lead to significant changes in the spin-density pointing at rather weak interaction between Chl and His molecules in PSI. The influence of amino acid substitution on ENDOR spectra, however, was much stronger in case of the His molecule coordinated at P B than for those at P A .