Electron paramagnetic resonance signals from the S-3 state of the oxygen-evolving complex. A broadened radical signal induced by low-temperature near-infrared light illumination
N. Ioannidis et V. Petrouleas, Electron paramagnetic resonance signals from the S-3 state of the oxygen-evolving complex. A broadened radical signal induced by low-temperature near-infrared light illumination, BIOCHEM, 39(18), 2000, pp. 5246-5254
The tetranuclear manganese cluster responsible for the oxidation of water i
n photosystem II cycles through five redox states denoted S-i (i = 0, 1, 2,
3, 4). Progress has been made recently in the detection of weak low-field
EPR absorptions in both the perpendicular and parallel modes, associated wi
th the integer spin state S-3 [Matsukawa, T., Mine, H., Yoneda, D., and Kaw
amori, A. (1999) Biochemistry 38, 4072-4077]. We confirm observation of the
se signals and have obtained them in high yield by illumination of photosys
tem II membranes, in which the non-heme iron was chemically preoxidized. It
is shown that a split g = 4 signal accompanies the S-3 State signals. The
signals diminish in the presence of ethanol and vanish in the presence of m
ethanol. This effect is similar to that exerted by these alcohols to the hi
gh-spin component (g = 4.1) of the S-2 State and suggests that the latter s
pin configuration is the precursor of the Sg State low-field signals. The S
g State shows similar sensitivity to infrared illumination as has been obse
rved previously in the S-2 State [Boussac, A., Un, S., Horner, O., and Ruth
erford, A. W. (1998) Biochemistry 37, 4001-4007]. Illumination of the Sg St
ate with near-infrared light (700-900 nm), at temperatures around 50 K, res
ults in the modification of the low-field signals and most notably to the a
ppearance of a broad (Delta H similar to 200 G) radical-type signal centere
d at g = 2. The signal is tentatively assigned to the interaction of the Mn
cluster in a modified S-2 State with a radical.