Electrons produced by Chl-aII in a short flash are finally transfered to Chl-ai after having passed several intermediates. The reaction between Chl-aI and its primary electron donators PD (probably Cyt-f and PC) has been studied by means of high time resolution flash photometry in isolated spinach chloroplasts with the following results:
1. When PD are in the oxidized state, the reduction of Chl-aI⊕ takes place with the rate limiting reaction time of the electron transport chain (≈ 20 ms).
2. When PD are in the reduced state, the reduction of Chl-aI⊕ takes place with the reaction times between the PD and Chl-ai. We found two half life times: one of about 200 μs and one of about 10 μs.
The reduced state of PD is realized when weak monitoring light or hvII-preillumination is used.
The two reaction times of ≈ 200 μs and of ≈ 10 µs are interpreted to represent the electron transfers between Cyt-f and Chl-aI and between PC and Chl-aI resp.
Equilibrium constants of the electron transfer from PD to Chl-aI and the arrangement of the intermediates in the electron transport chain are discussed.