The influence of the oxidation state of the oxygen-evolving complex of Photosystem II on the spin-lattice relaxation time of Signal II as determined by electron spin-echo spectroscopy

Abstract The spin-lattice relaxation time of Signal II, which arises from two plastoquinol cation radicals, D + and Z + , has been measured with electron spin-echo spectroscopy in Photosystem II preparations with inactivated and with intact oxygen-evolving complex. In Tris- and subsequently EDTA-washed Photosystem II preparations the spin-lattice relaxation times of D + and Z + are equal and remain unchanged if the pH is increased from 6.0 to 8.3. In preparations in which the oxygen-evolving complex is not inactivated by the Tris washing, the spin-lattice relaxation times of D + and Z + are affected by the redox state of the oxygen-evolving complex. At pH 6.0 the spin-lattice relaxation time decreases with higher redox state of the manganese cluster. The relaxation behavior at pH 8.3 indicates that at this pH the stability of the S 1 state is decreased such that in dark-adapted samples nearly 100% of the systems is in the S 0 state. This was confirmed by optical experiments where the period-four oscillation in the absorption at 350 nm was monitored as a function of the flash number.