ENDOR and pulsed EPR studies of photosynthetic reaction centers: Protein‐cofactor interactions

The influence of specific protein‐cofactor interactions on the electronic structure of the primary donor cation radical P +• and the acceptor anion radicals Q −• A and QB −• B in wild type and mutant reaction centers of photosynthetic bacteria is investigated by ENDOR and Pulsed EPR techniques. The results show that hydrogen bonds to the primary donor have a strong effect on the distribution of the unpaired electron over the two BChl halves of the special pair, P +• . A correlation between the rate of reduction of P +• by cytochrome C 2 and the spin density distribution within the dimer is found. ESEEM made it possible to detect 14 N nuclear quadrupole resonances for Q −• A and Q −• B that are assigned to nitrogen atoms of ligating amino acids (histidines and peptide backbone). The deduced hyperfine couplings indicate significant delocalization of spin density onto these residues. This indicates an active role of these ligands in the electron transfer from Q A to Q B .