Transient Absorption Studies of the Primary Charge Separation in Photosystem II

Femtosecond transient absorption studies of the primary charge separation in photosystem II (PSII) are presented. A careful study of the dependence of the observed signal on laser intensity demonstrates that the multiple excitation of reaction centers produces additional fast components not observed at low excitation energy. In the regime where the observed signals are linear with excitation energy, a 20 ± 2 ps rise of the pheophytin anion absorption, bleach of the pheophytin Q x absorption, and appearance of the chlorophyll cation absorption are observed. Three different protocols, involving varying exposure of the PSII complex to the detergent Triton X−100, are used to prepare D1-D2-Cyt b 559 complexes from spinach. The kinetic signals are independent of the method of sample preparation.