Er 4102

X-band cw-EPR spectra were recorded with a Bruker Elexsys 500 X-band spectrometer equipped with a standard ER 4102 (Bruker) X-band resonator, a Bruker teslameter, an Oxford Instruments cryostat (ESR 900) and an Oxford ITC504 temperature controller. Flash illumination at room temperature was provided by a neodymium:yttriumaluminum garnet (Nd:YAG) laser (532 nm, 550 mJ, 8 ns Spectra Physics GCR-230-10).
Illumination at 198 K with visible light was done in a non-silvered Dewar filled with ethanol cooled with dry ice for approximately 5-10 seconds with a 800 W tungsten lamp filtered by water and infrared cut-off filters. Illumination at 4.2 K was done in the EPR cavity using a low-voltage halogen lamp (24V, 250W, Philips Type 13163) filtered by water and infrared cut-off filters. Near infrared (NIR) illumination was provided by a laser diode emitting at 813 nm (Coherent, diode S-81-1000C) with a power of 600-700 mW at the level of the sample.
The samples were synchronized in the S 1 -state with one pre-flash (Styring and Rutherford 1987) . Then, after a further 1 h dark-adaptation at room temperature the samples were frozen in the dark to 198 K in a dry ice ethanol bath and then transferred to 77 K in liquid N 2 . Prior to recording the spectra the samples were degassed at 198 K.

Absorption spectra during the purification of the Fds were measured with an Uvikon XL Secomam (Bioserv) spectrometer. For the redox titrations, the UV-Vis spectra were recorded in the 390 to 660 nm range, using a Cary 14 spectrophotometer. The electrochemical cell was thermostated at 15°C with a water circulation system and the sample compartment was purged with dry air to avoid water vapour condensation. The data were analysed using the integrated spectral area from 390 to 640 nm.
X-band cw-EPR spectra were recorded with a Bruker Elexsys 500 X-band spectrometer equipped with a standard ER 4102 (Bruker) X-band resonator, a Bruker teslameter, an Oxford Instruments cryostat (ESR 900) and an Oxford ITC504 temperature controller.