Light-induced structural changes in photosynthetic reaction centres studied by ESEEM of spin-correlated D+QA- radical pairs

Zn-substituted Rhodobacter sphaeroides R26 reaction centres (RCs) frozen in the dark and under illumination exhibit quite different recombination kinetics of the D(+)Q(A)(-) radical pairs [Kleinfeld et al., Biochemistry, 23 (1984) 5780]. We have applied electron spin echo envelope modulation (ESEEM) of the spin-correlated D(+)Q(A)(-) radical pairs to assess a possible light-induced change in the distance between the D and Q(A) cofactors. The recombination kinetics and the field-swept spin-polarized EPR signal for the two preparations have been monitored by time-resolevd EPR spectroscopy. For the samples frozen under illumination, a slight increase in the distance, 0.4 +/- 0.7 Angstrom, has been detected. (C) 1998 Elseviei Science B.V.