TRANSIENT ABSORPTION STUDIES OF THE PRIMARY CHARGE SEPARATION IN PHOTOSYSTEM-II

Femtosecond transient absorption studies of the primary charge separat ion in photosystem II (PSII) are presented. A careful study of the dep endence of the observed signal on laser intensity demonstrates that th e multiple excitation of reaction centers produces additional fast com ponents not observed at low excitation energy. In the regime where the observed signals are linear with excitation energy, a 20 +/- 2 ps ris e of the pheophytin anion absorption, bleach of the pheophytin Q(x) ab sorption, and appearance of the chlorophyll cation absorption are obse rved. Three different protocols, involving varying exposure of the PSI I 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.