Comparative study of effects of artificial electron donors on the A(T)-band of photosystem II thermoluminescence

Extraction of the Mn-cluster from photosystem II (PS II) inhibits the main bands of thermoluminescence and induces a new A(T)-band at -20 degreesC. Th is band is attributed to the charge recombination between acceptor Q(A)(-) and a redox-active histidine residue on the donor side of PS II. The effect of Mn(II) and Fe(II) cations as well as the artificial donors diphenylcarb azide and hydroxylamine on the A(T)-band of thermoluminescence was studied to elucidate the role of the redox-active His residue in binding to the Mn( II) and Fe(II). At the Mn/PS II reaction center (RC ratio of 90: 1 and Fe/P S II RC ratio of 120: 1, treatment with Mn(II) and Fe(II) causes only 60% i nhibition of the AT-band. Preliminary exposure of Mn- depleted PS II prepar ations to light in the presence of Mn(II) and Fe(II) causes binding of the cations to the high-affinity Mn-binding site, thereby inhibiting oxidation of the His residue involved in the A(T)-band formation. The efficiency of t he A(T)-band quenching induced by diphenylcarbazide and hydroxylamine is al most an order of magnitude higher than the quenching efficiency of Mn(II) a nd Fe(II). Our results suggest that the redox-active His is not a ligand of the high-affinity site and does not participate in the electron transport from Mn(II) and Fe(II) to Y-Z. The concentration dependences of the A(T)-ba nd inhibition by Mn(II) and Fe(II) coincide with each other, thereby implyi ng specific interaction of Fe(II) with the donor side of PS II.