Studies on the reaction mechanism of tetraphenylboron at the Photosystem II donor side in isolated spinach chloroplasts

Abstract

In the present study the effect of low tetraphenylboron (TPB −) concentrations on the flash-induced oxygen yield pattern in dark-adapted spinach chloroplasts has been analysed. The following was found. (1) TPB − at a concentration of less than ten molecules per Photosystem II strongly damps the characteristic oscillation pattern. (2) TPB − induces an accelerated decay of S 2 and S 3 that is concentration dependent. In contrast to the acceleration of the deactivation reactions of the water-splitting enzyme system Y agent (ADRY agent) 2-(3-chloro-4-trifluoromethyl)-anilino-3,5-dinitrothiophene (ANT 2p) the acceleration by TPB − disappears after preflashing. (3) TPB − does not induce a bivalent reduction of and S 3 of the type S i+2 + TPB − → S 1 + oxidation products. (4) The effect of TPB − on the decay kinetics of S 2 and S 3 can be quantitatively described by a mobile-type mechanism as discussed recently for ANT 2p (Hanssum B., Dohnt G. and Renger, G. (1985) Biochim. Biophys. Acta 806, 210–220) if one additionally accounts for the irreversible oxidative consumption of TPB −. (5) Cyclic voltammetry in buffer solution reveals that TPB − becomes irreversibly oxidized with redox potential of the order of +0.7 and +1.0 V, respectively. Similar measurements with different 2-anilino-3,5-dinitrothiophene derivatives support the idea that also ANT 2p becomes irreversibly oxidized. Based on these data the mechanism of TPB − and ADRY agents on the donor side of system II is discussed.