UV-visible absorption spectra of the Rieske cluster in crystals and the mixed solution. The absorption spectra of crystals of the resting state [Oxy: Fdrest, (1) in Figure 1, reduced state [Oxy: Fdred, (2)], and oxygen-bound state [Oxy: FdO2, (3’)] under a cryo-stream of nitrogen at 100 K are shown in curves 1, 2, and 3, respectively, in the main panel. In the inset, curve 1 in the small panel shows the non-reduced spectrum of the mixed solution of purified CARDO-O and CARDO-F, which were used for crystallization. Curve 2 in the inset represents the spectrum after the addition of sodium dithionite to the same mixed solution. (Curves 1 and 2 from both crystal and solution are referred from our previous study) .