PULSED EPR STUDY OF THE S'(3) SIGNAL IN THE CA2-DEPLETED PHOTOSYSTEM-II()

The Ca2+-depleted photosystem II (PS IT) was studied by pulsed EPR spe ctroscopy. A short (5 s) illumination of the PS II in the S'(2) state resulted in formation of the doublet S'(3) signal at g approximate to 2.02 with a splitting of about 15 mT concomitant with the decrease of the modified Mn multiline signal by about 40%. Further illumination up to three minutes led to the formation of another S'(3) signal with a singlet-like feature at g approximate to 1.98 and to the complete disa ppearance of the modified Mn multiline signal. Computer simulations of the shape of the doublet S'(3) signal and the dependence of its elect ron spin echo amplitude on the microwave field strength suggest that d ipole and exchange interactions between two organic radicals are respo nsible for the doublet S'(3) signal. The pulsed ENDOR-induced EPR meas urements indicate that the ENDOR spectrum of the tyrosine radical Y-Z( +) is associated with the doublet S'(3) signal only. (C) 1997 Elsevier Science B.V.