LOW-TEMPERATURE INTERACTIONS OF NO WITH THE S-1 AND S-2 STATES OF THEWATER-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 intera ction can be resolved and trapped by NO treatment at subzero temperatu res. 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 time s (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 [Dismuke s, G. C., & Siderer, Y. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 274-2 78], but a modified shape with intense lines on the high field side. T he NO modified S-1 state can act as a low-temperature electron donor y ielding an EPR silent state upon illumination at 200 K. NO interacts a lso with the S-2 state of the water oxidizing complex rapidly at tempe ratures as low as -75 degrees C, to yield an EPR silent state. The rat es of the latter interaction show analogies to the ammonia binding to the S-2 state. It is possible, however, that NO, unlike ammonia, desta bilizes the S-2 state. On the basis of preliminary experiments with va rying chloride concentrations in the range 0.1-50 mM, the S-1 multilin e 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.