Location of chlorophyllZ in photosystem II.

Saturation-recovery and progressive microwave power saturation EPR spectroscopies have been used to probe the location of the chlorophyllZ+ (ChlZ+) radical species in Mn-depleted photosystems II (PSII). The spin-lattice relaxation transients of ChlZ+ were non-single-exponential due to a dipole-dipole interaction with one of the other paramagnetic centers in PSII. Measurements on CN(-)-treated, Mn-depleted PSII membrane samples, in which the non-heme Fe(II) is converted into its low-spin, diamagnetic form, confirmed that the non-heme Fe(II) caused the dipolar relaxation enhancement of ChlZ+. The saturation-recovery EPR data were fit to a dipolar model [Hirsh, D. J., Beck, W. F., Innes, J. B., & Brudvig, G. W. (1992) Biochemistry 31, 532] which takes into account the isotropic (scalar) and orientation-dependent (dipolar) contributions to the spin-lattice relaxation of the radical. The temperature dependence of the dipolar rate constants of ChlZ+ was identical to the temperature dependencies recently observed for the stable tyrosine radical, YD., and the special pair bacteriochlorophyll radical, (BChla)2+, in PSII and in reaction centers from Rhodobacter sphaeroides, respectively. Because the non-heme Fe(II) is known to cause a dipolar relaxation enhancement of the radicals in both of the latter cases, this result provides further evidence that the non-heme Fe(II) causes the dipolar relaxation enhancement of ChlZ+ and, moreover, demonstrates that the magnetic properties of the non-heme Fe(II) in PSII and in reaction centers from Rhodobacter sphaeroides are very similar. By using the known Fe(II)-(BChla)2+ distance for calibration, we estimate the Fe(II)-ChlZ+ distance to be 39.5 +/- 2.5 A.(ABSTRACT TRUNCATED AT 250 WORDS)