Abstract:Photosystem Ⅱ reaction center D1-D2-cytochrome b-559 pigment-protein complex has been isolated and purified from chloroplasts of spinach and its properties have been studied. The Isotared photosystem II reaction center contains close to six chlorophyll a per two pheophytin a molecules. Analysis of fluorescence decaying by phase modulation fluorometry suggests that the reaction center has three components of fluorescence decaying with lifetimes of 1.5 nS, 6.23 nS, 36.26 nS in terms of fractions to total fluorescence yield as 0.06, 0.67, 0.27 respectively. The ,6.25 nS fluorescence component corresponds to chlorophyll a which is energetically uncoupled from the process of charge separation. The proportion of 1.51 nS component is very low, and its source is unclear. The 36.25 nS fluorescence component is attributed to the recombination of the primary radical pair, and so represents the activity of charge separation.