Transient and pulsed EPR spectroscopy on the radical pair state P 865 +. Q A −. to study light-induced changes in bacterial reaction centers

Abstract

The radical pair state P 865 +. Q A −. (P865: primary donor, QA: quinone acceptor) in Zn-substituted bacterial reaction centers is investigated using transient and pulsed EPR spectroscopy. For photoexcited samples not frozen in the dark but under continuous illumination, a prolonged lifetime of this radical pair state is observed in agreement with previous studies using time resolved optical spectroscopy. The transient EPR spectra revealed neither a different orientation of the quinone acceptor anion nor a change of itsg-anisotropy in the sample frozen in the charge separated state as compared with that frozen in the dark. The latter finding indicates a similar hydrogen bonding situation for Q A −. in both samples. Changes observed in the transient EPR spectra are interpreted as result of contributions from spin-polarized Q A −. which was generated in part of the sample while freezing under illumination. From the out-of-phase echo modulation pattern observed in the pulsed EPR measurements, it follows that the distance between P 865 +. and Q A −. is the same in dark frozen samples and in those frozen under continuous illuminaton. This is in contrast to the model suggested by Kleinfeld D., Okamura M.Y., Feher G.: Biochemistry23, 5780 (1984), in which an increased distance and a larger distribution of distances was suggested for samples frozen under illumination. The prolonged lifetime of the radical pair P 865 +. Q A −. is discussed in terms of differences in the relaxation behaviour of the protein.