Probing subtle coordination changes in the iron-quinone complex of photosystem II during charge separation, by the use of NO

The terminal electron acceptor of Photosystem II PSII, is a linear complex consisting of a primary quinone, a non-heme iron(II), and a secondary quinone, Q(A)Fe(2+)Q(B). The complex is a sensitive site of PSII, where electron transfer is modulated by environmental factors and notably by bicarbonate. Earlier studies showed that NO and other small molecules (CN-, F-, carboxylate anions) bind reversibly on the non-heme iron in competition with bicarbonate. In the present study, we report on an unusual new mode of transient binding of NO, which is favored in the light-reduced state (Q(A)(-)Fe(2+)Q(B)) of the complex. The related observations are summarized as follows: (i) Incubation with NO at -30 degreesC, following light-induced charge separation, results in the evolution of a new EPR signal at g = 2.016. The signal correlates with the reduced state Q(A)(-)Fe(2+) of the iron-quinone complex. (ii) Cyanide, at low concentrations, converts the signal to a more rhombic form with g values at 2.027 (peak) and 1.976 (valley), while at high concentrations it inhibits formation of the signals. (iii) Electron spin-echo envelope modulation (ESEEM) experiments show the existence of two protein N-14 nuclei coupled to electron spin. These two nitrogens have been detected consistently in the environment of the semiquinone Q(A)(-) in a number of PSII preparations. (iv) NO does not directly contribute to the signals, as indicated by the absence of a detectable isotopic effect ((NO)-N-15 vs (NO)-N-14) in cw EPR. (v) A third signal with g values (2.05, 2.03, 2.01) identical to those of an Fe(NO)2(imidazole) synthetic complex develops slowly in the dark, or faster following illumination. (vi) In comparison with the untreated Q(A)(-)Fe(2+) complex, the present signals not only are confined to a narrow spectral region but also saturate at low microwave power. At I I K the g = 2.016 signal saturates with a P-1/2 of 110 muW and the g = 2.027/1.976 signal with a P-1/2 of 10 muW. (vii) The spectral shape and spin concentration of these signals is successfully reproduced, assuming a weak magnetic interaction (J values in the range 0.025-0.05 cm(-1)) between an iron-NO complex with total spin of (1)/(2) and the spin, 1/2, of the semiquinone, Q(A)(-). The different modes of binding of NO to the non-heme iron are examined in the context of a molecular model. An important aspect of the model is a trans influence of Q(A) reduction on the bicarbonate ligation to the iron, transmitted via H-bonding Of Q(A) with an imidazole ligand to the iron.