LIGHT-INDUCED STRUCTURAL-CHANGES IN PHOTOSYNTHETIC REACTION CENTERS STUDIED BY ESEEM OF SPIN-CORRELATED D(-) RADICAL PAIRS()Q(A)()

Zn-substituted Rhodobacter sphaeroides R26 reaction centres (RCs) froz en in the dark and under illumination exhibit quite different recombin ation kinetics of the D(+)Q(A)(-) radical pairs [Kleinfeld et al., Bio chemistry, 23 (1984) 5780]. We have applied electron spin echo envelop e 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 s pin-polarized EPR signal for the two preparations have been monitored by time-resolevd EPR spectroscopy. For the samples frozen under illumi nation, a slight increase in the distance, 0.4 +/- 0.7 Angstrom, has b een detected. (C) 1998 Elseviei Science B.V.