TRANSIENT AND PULSED EPR SPECTROSCOPY ON THE RADICAL PAIR STATE P(865)(-DOT)Q(A)(-CENTER-DOT) TO STUDY LIGHT-INDUCED-CHANGES IN BACTERIAL REACTION CENTERS(CENTER)

The radical pair state P(865)(+.)Q(A)(-.) (P-865: primary donor, Q(A): quinone acceptor) in Zn-substituted bacterial reaction centers is inv estigated using transient and pulsed EPR spectroscopy. For photoexcite d 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 t ransient EPR spectra revealed neither a different orientation of the q uinone acceptor anion nor a change of its g-anisotropy in the sample f rozen in the charge separated state as compared with that frozen in th e dark. The latter finding indicates a similar hydrogen bonding situat ion for Q(A)(-.) in both samples. Changes observed in the transient EP R spectra are interpreted as result of contributions from spin-polariz ed Q(A)(-.) which was generated in part of the sample while freezing u nder illumination. From the out-of-phase echo modulation pattern obser ved in the pulsed EPR measurements, it follows that the distance betwe en P-865(+.) and Q(A)(-.) is the same in dark frozen samples and in th ose frozen under continuous illuminaton. This is in contrast to the mo del suggested by Kleinfeld D., Okamura M.Y., Feher G.: Biochemistry 23 , 5780 (1984), in which an increased distance and a larger distributio n of distances was suggested for samples frozen under illumination. Th e prolonged lifetime of the radical pair P(865)(+.)Q(A)(-.) is discuss ed in terms of differences in the relaxation behavior of the protein.