Examination of fluorescence lifetime and radical-pair decay in Photosystem II membrane fragments from spinach

Absorption changes at 830 nm, ΔA 830 , induced by repetitive (1 Hz) strong actinic 35 ps laser pulses ( λ = 532 nm; E = 4 · 10 15 photons / cm 2 per pulse) were detected in oxygen-evolving Photosystem (PS) II membrane fragments with a time resolution of about 500 ps. The following results were obtained: (a) in the presence of 2 mM K 3 (Fe(CN) 6 ) the relaxation of ΔA 830 is dominated by the multiphasic P-680 + -reduction kinetics (ns and μs range) and the fast I ⨪ reoxidation, (b) if the reaction centers are kept largely in the reduced state P-680IQ ⨪ A , the initial amplitude of ΔA 830 remains almost unaffected, but the decay mainly occurs via a kinetics of at most 500 ps, (c) if the reaction centers are transformed into the state P-680I ⨪ Q ⨪ A by strong illumination in the presence of Na 2 S 2 O 4 , then the initial amplitude of ΔA 830 becomes drastically diminished, (d) after a few cycles of strong continuous actinic light and dark recovery (4 min) ΔA 830 is dominated by a 3 ns relaxation kinetics. Comparative measurements of the fluorescence decay after excitation with weak 620 nm pulses ( E = 2 · 10 10 photons / cm 2 per pulse) exhibited the following dependencies of the kinetics of the redox state of the reaction center: up to 200 ps kinetics dominate for the states P-680IQ A and for P-680I ⨪ Q ⨪ A , whereas for the state P-680IQ ⨪ A the fluorescence relaxes predominantly via 1.3 ns kinetics. The analysis of these results led to the following conclusions. (i) The PS II reaction center provides a rather shallow trap for excitons. (ii) The fluorescence emission in the state P-680IQ ⨪ a is probably not a recombination fast delayed light emission but represents a prompt fluorescence of the antenna. (iii) If the yield of flash-induced radical pair formation in reaction centers of the state P-680IQ ⨪ A is significant (at least 25%), then the recombination reaction P-680 + I • Q • A → P-680IQ • A takes place with t 1 2 ⩽ 500 ps .