Pulsed EPR study of the S′3 signal in the Ca2+-depleted photosystem II

The Ca2+-depleted photosystem II (PS II) was studied by pulsed EPR spectroscopy. 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 ≈ 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 ≈ 1.98 and to the complete disappearance of the modified Mn multiline signal. Computer simulations of the shape of the doublet S′3 signal and the dependence of its electron spin echo amplitude on the microwave field strength suggest that dipole and exchange interactions between two organic radicals are responsible for the doublet S′3 signal. The pulsed ENDOR-induced EPR measurements indicate that the ENDOR spectrum of the tyrosine radical YZ+ is associated with the doublet S′3 signal only.