The action of tetraphenylboron as a system II electron donor and its effect on the decay of the electrical field across the thylakoid membrane.

The effect of tetraphenylboron on the O2 evolution, the electron transport in normal and in Tris-washed chloroplasts, and on the decay of the electrical field across the thylakoid membrane has been investigated in spinach chloroplasts. It was found that: (1) The average O2 yield per flash as a function of the tetraphenylboron concentration is dependent on the number of excitation flashes. With increasing flash number, the tetraphenylboron concentration which is required for 50% suppression of the average O2 yield per flash shifts toward higher values. (2) After the irreversible consumption of tetraphenylboron by System II oxidizing equivalents, the O2 evolution reappears. (3) The electron flow from System II to System I, as indicated by the amplitude of the reduction kinetics of the 703 nm absorption change, remains unaffected in the tetraphenylboron concentration range and at flash numbers where the O2 evolution is totally suppressed. (4) Tetraphenylboron restores electron transport in Tris-washed chloroplasts. This tetraphenylboron-mediated electron transport is completely blocked by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). (5) Preliminary titration experiments favor the assumption that tetraphenylboron acts in chloroplasts rather as a 1-electron donor than as a 2-electron donor. (6) Tetraphenylboron accelerates the decay of the electrical field across the thylakoid membrane. Because of its ability to act as a System II electron donor, the accelerating effect on the electrical field is transitory if System II is active. By contrast, in DCMU-blocked chloroplasts with active System I electron transport [mediated by the 2,6-dichlorophenolindophenol (DCIP) cycle], a permanent accelerating effect on the electrical field is observed. From these results it has been concluded, that tetraphenylboron acts as an efficient 1-electron donor for System II. The functional integrity of the water-splitting enzyme system Y is not required for this donation. Tetraphenylboron itself does not destroy the O2 evolution capability of chloroplasts, it rather acts as a competitive electron donor. However, an inhibitory effect arises probably from the oxidation products of tetraphenylboron. The possible modes of action of tetraphenylboron are discussed.