A '2+1' pulse sequence electron spin echo (ESE) method was applied to measure the dipole interactions between the tyrosine YD+ and QA- in Photosystem II (PS II). In a CN--treated PS II, QA- EPR signal was observed at g=2.0045 position, because the non-heme Fe(II) was converted into a low-spin (S=0) state. The radical pair of YD+QA- was trapped by illumination for 8 min at 273 K, followed by dark adaptation for 3 min and freezing into 77 K. By using a proton matrix ENDOR, these trapped radicals were confirmed to be YD+ and QA-, respectively. The distance between the radical pair was estimated from the dipole interaction constant fitted to the observed '2+1' ESE time profile. The distance of YD+-QA- is determined to be 38.8+/-1.1 A. The magnetic dipole interaction between YD+ and ChlZ+ was determined in a Tris-treated PS II in which ChlZ+ was generated by illumination at 200 K for 10 min. The YD+-ChlZ+ distance was estimated to be 29.4+/-0.5 A. Copyright 1998 Elsevier Science B.V.

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