Low-Temperature Interactions of NO with the S1 and S2 States of the Water-Oxidizing Complex of Photosystem II. A Novel Mn-Multiline EPR Signal Derived from the S1 State

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Abstract
The spin-1/2-carrying NO molecule interacts with both the S1 and S2 states of the water oxidizing complex. The intermediates of the interaction can be resolved and trapped by NO treatment at subzero temperatures. At −30 °C and in the presence of approx. 500−700 μM NO, S1 loses the ability to yield by illumination an EPR active S2-state with an approximate half-time of 40−60 min. At longer incubation times (t1/2 = 4−5 h), an intense new multiline signal develops. The new signal has a hyperfine splitting similar to the S2 multiline [Dismukes, G. C., & Siderer, Y. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 274−278], but a modified shape with intense lines on the high field side. The NO modified S1 state can act as a low-temperature electron donor yielding an EPR silent state upon illumination at 200 K. NO interacts also with the S2 state of the water oxidizing complex rapidly at temperatures as low as −75 °C, to yield an EPR silent state. The rates of the latter interaction show analogies to the ammonia binding to the S2 state. It is possible, however, that NO, unlike ammonia, destabilizes the S2 state. On the basis of preliminary experiments with varying chloride concentrations in the range 0.1−50 mM, the S1 multiline state is attributed to binding of NO at a chloride sensitive site on the Mn cluster. The rapid interactions with the S2 state as well as the intermediate binding to the S1 state are less well understood at present, but they are tentatively assigned to the chloride-insensitive site of ammonia binding in the Mn cluster.