II. Primary electrogenic reactions in chloroplasts probed by picosecond flash-induced dielectric polarization

Abstract

Excitation of chloroplasts by non-saturating flashes is known to generate an electric polarization of the suspension which can be measured as a photovoltage with electrodes positioned alongside the propagation axis of the light. The polarization originates from the light-gradient in the suspension and the anisotropic excitation of the primary charge separation in the reaction centers of both photosystems. By using a capacitative measuring cuvette and light pulses of 30 ps duration we could achieve a time resolution on the order of 200 ps, even at low ionic strengths. At the excitation wavelengths of 532 and 694 nm, a photovoltage developed with the rise-time of the apparatus when the reaction centers of both photosystems were open. Closing of the reaction centers of Photosystem I by oxidizing P-700 resulted in a photovoltage with the same fast rise, but of half amplitude. This shows that (i) the primary charge separation in both types of reaction centers and (ii) the mean trapping time of excitions from the antenna system by P-680 and P-700 are shorter or equal to 175 ps. When the traps of Photosystem I were closed and the primary quinone acceptor Q in the reaction centers of Photosystem II was reduced, there was almost no photovoltage response around 1 ns. The only primary reaction known under these conditions is the charge separation in Photosystem II between the primary donor P-680 and the intermediary pheophytin acceptor. Assuming this reaction, the observed lack of a photovoltage indicates that the intermediary acceptor Phe lies on the same membrane side as P-680.