High-field/high-frequency EPR/ENDOR — A powerful new tool in photosynthesis research

Abstract

In photosynthesis research the elucidation of the spatial and electronic structures of the electron donor and acceptor ion radicals is very important for an understanding of the light-induced electron transfer process. Recent 3 mm (W-band, 95 GHz) high-field EPR and ENDOR studies on the primary donor cation radicals P+. (bacteriochlorophyll dimer), the acceptor anion radicals Q+. (quinones), and the charge-separated radical pair (P+.Q−.) in photosynthetic bacteria and biomimetic model systems are presented. From single crystals of, for instance,Rb. sphaeroides R-26 reaction centers both the hyperfine tensors of various protons and theg-tensor of P865 +. have been determined and compared with calculated tensor values based on recent X-ray structure data. The results consistently reveal a breaking of the local C2 symmetry of the electronic structure at the primary donor side of the reaction center. This is of particular interest since it might be relevant for the vectorial electron transfer along the protein complex. Among the quinone radical anions studied are frozen solutions of the electron acceptors of bacterial and plant reaction centers (ubiquinone and plastoquinone, respectively). The increased electron Zeeman interaction in high-field EPR leads to almost completely resolvedg-tensor components even in disordered samples. Theg-tensors and component linewidths are sensitive probes for specific anisotropic interactions with the environment. In the case of the transient correlated coupled radical pair P865 +.-QA −. ofRb. sphaeroides (Fe replaced by Zn) the spin-polarized high-field EPR spectra allow an unambiguous determination of the relative orientation of theg-tensors of the donor and acceptor parts. Thereby high-precision structure information is obtained on the electron transfer pigments after light-induced charge separation.