Low-temperature interactions of NO with the S-1 and S-2 states of the water-oxidizing complex of photosystem .2. A novel Mn-multiline EPR signal derived from the S-1 state

The spin-1/2-carrying NO molecule interacts with both the S-1 and S-2 states of the water oxidizing complex. The intermediates of the interaction can be resolved and trapped by NO treatment at subzero temperatures. At -30 degrees C and in the presence of approx. 500-700 mu M NO, S-1 loses the ability to yield by illumination an EPR active S-2-state with an approximate half-time of 40-60 min. At longer incubation times (t(1/2) = 4-5 h), an intense new multiline signal develops. The new signal has a hyperfine splitting similar to the S-2 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 S-1 state can act as a low-temperature electron donor yielding an EPR silent state upon illumination at 200 K. NO interacts also with the S-2 state of the water oxidizing complex rapidly at temperatures as low as -75 degrees C, to yield an EPR silent state. The rates of the latter interaction show analogies to the ammonia binding to the S-2 state. It is possible, however, that NO, unlike ammonia, destabilizes the S-2 state. On the basis of preliminary experiments with varying chloride concentrations in the range 0.1-50 mM, the S-1 multiline state is attributed to binding of NO at a chloride sensitive site on the Mn cluster. The rapid interactions with the S-2 state as well as the intermediate binding to the S-1 state are less well understood at present, but they are tentatively assigned to the chloride-insensitive site of ammonia binding in the Mn cluster.