A MN(II)-MN(III) EPR SIGNAL ARISES FROM THE INTERACTION OF NO WITH THE S-1 STATE OF THE WATER-OXIDIZING COMPLEX OF PHOTOSYSTEM-II

It was shown recently [Goussias, C., Ioannidis, N., and Petrouleas, V. (1997) Biochemistry 36, 9261-9266] that incubation of photosystem II preparations with NO at -30 degrees C in the dark results in the forma tion of a new intermediate of the water-oxidizing complex. This is cha racterized by an EPR signal centered at g = 2 with prominent manganese hyperfine structure. We have examined the detailed structure of the s ignal using difference EPR spectroscopy. This is facilitated by the ob servations that NO can be completely removed without decrease or modif ication of the signal, and illumination at 0 degrees C eliminates the signal. The signal spans 1600 G and is characterized by sharp hyperfin e structure. (NO)-N-14 and (NO)-N-15 cw EPR combined with pulsed ENDOR and ESEEM studies show no detectable contributions of the nitrogen nu cleus to the spectrum. The spectrum bears similarities to the experime ntal spectrum of the Mn(II)-Mn(III) catalase [Zheng, M., Khangulov, S. V., Dismukes, G. C., and Barynin, V. V. (1994) Inorg. Chem. 33, 382-3 87]. Simulations allowing small variations in the catalase-tensor valu es result in an almost accurate reproduction of the NO-induced signal. This presents strong evidence for the assignment of the latter to a m agnetically isolated Mn(II)-Mn(III) dimer. Since the starting oxidatio n states of Mn are higher than II, we deduce that NO acts effectively as a reductant, e.g., Mn(III)-Mn(III) + NO --> Mn(II)-Mn(III) + NO+. T he temperature dependence of the nonsaturated EPR-signal intensity in the range 2-20 K indicates that the signal results from a ground state . The cw microwave power saturation data in the range 4-8 K can be int erpreted assuming an Orbach relaxation mechanism with an excited state at Delta = 42 K. Assuming antiferromagnetic coupling, -2/S-1 . S-2 be tween the two manganese ions, assigned to a magnetically isolated Mn(I I)-Mn(III) dimer bears important consequences in interpreting the stru cture of the Mn cluster. Although the signal is not currently assigned to a particular S state, it arises from a state lower than S-1, possi bly lower than S-0, too.