PULSED EPR STUDY OF SPIN-COUPLED RADICAL PAIRS IN PHOTOSYNTHETIC REACTION CENTERS - MEASUREMENT OF THE DISTANCE BETWEEN P-700(CENTER-DOT-DOT-) IN PHOTOSYSTEM-I AND BETWEEN P-865(CENTER-DOT+) AND Q(A)(CENTER-DOT-) IN BACTERIAL REACTION CENTERS() AND A(1)(CENTER)

The spin-polarized radical pairs P(865)(.+)Q(A)(.-) in protonated and deuterated Zn-substituted reaction centers from two different mutants of the photosynthetic bacteria Rhodobacter sphaeroides and P(700)(.+)A (1)(.-) in plant photosystem I from Synechococcus elongatus are invest igated by pulsed EPR spectroscopy. Spin-polarized radical pairs give r ise to a characteristic out-of-phase electron spin echo. This echo sho ws a deep envelope modulation with a frequency governed by the spin-sp in interaction. The known distance dependence of the magnetic dipolar interaction allows the determination of the distance between the cofac tors carrying the unpaired electron spins. For the bacterial reaction centers this distance is known for the electronic ground state from cr ystal structures and is compared here with the distance of the radical pair spins, i.e. the charge-separated state. In photosystem I the loc ation of the acceptor A(1) is not known yet. A distance of 25.4 +/- 0. 3 Angstrom between P-700(.+) and A(1)(.-) is obtained here and gives n ew structural information on photosystem I.