ENDOR AND PULSED EPR STUDIES OF PHOTOSYNTHETIC REACTION CENTERS - PROTEIN-COFACTOR INTERACTIONS

The influence of specific protein-cofactor interactions on the electro nic structure of the primary donor cation radical P-+. and the accepto r anion radicals Q(A)(-.) and Q(B)(-.) in wild type and mutant reactio n centers of photosynthetic bacteria is investigated by ENDOR and Puls ed EPR techniques. The results show that hydrogen bonds to the primary donor have a strong effect an the distribution of the unpaired electr on over the two BChl halves of the special pair, P-+.. A correlation b etween the rate of reduction of P-+. by cytochrome c(2) and the spin d ensity distribution within the dimer is found. ESEEM made it possible to detect N-14 nuclear quadrupole resonances for Q(A)(-.) and Q(B)(-.) that are assigned to nitrogen atoms of ligating amino acids (histidin es and peptide backbone). The deduced hyperfine couplings indicate sig nificant delocalization of spin density onto these residues. This indi cates an active role of these ligands in the electron transfer from Q( A) to Q(B).