PRIMARY PROCESSES AND STRUCTURE OF THE PHOTOSYSTEM-II REACTION-CENTER.4. LOW-INTENSITY FEMTOSECOND TRANSIENT ABSORPTION-SPECTRA OF D1-D2-CYT-B559 REACTION-CENTER

Low-intensity transient absorption spectroscopy has been performed on the isolated D1-D2-cyt-b559 reaction center complex of photosystem II from spinach. The excitation intensity was low enough to keep annihila tion at a relatively low level (similar to 11%) and still maintain a v ery high signal/noise ratio. The kinetics has been measured with simil ar to 200 fs resolution over two time ranges extending up to 200 ps fo r two different excitation wavelengths (680 nm, preferential primary d onor excitation, and 670 nm, preferential external chlorophyll excitat ion). Detection wavelengths both in the pheophytin Q(x) region (535-55 5 nm) and in the chlorin Q(y) region, including the stimulated emissio n range (660-760 nm) have been employed. The data for all excitation/ detection wavelength pairs and from both time ranges have been analyze d by combined global analysis. A highly complex kinetics, i.e. six lif etimes, has been found necessary for a good description of the data ov er the entire excitation/detection matrix and time range. These lifeti mes are T-1 = 2.4 +/- 0.3 ps, T-2 = 8.9 +/- 1 ps, T-3 = 19.8 +/- 3 ps, T-4 = 56 +/- 10 ps, and T-5 greater than or equal to 1 ns (long-lived , nondecaying). The fastest component (T-6) in these fits had an ca. 3 00 +/- 50 fs lifetime. Decay-associated spectra of these components ar e presented for both excitation wavelengths. In a preliminary analysis the 2.4 ps component is assigned to primary charge separation; the 8. 9 and 19.8 ps components, to slow energy transfer from external chloro phylls; and the 56 ps component, to a relaxation process among differe nt radical pairs. The ultrafast component most likely reflects the exc ited-state equilibration within the reaction center core. This assignm ent is supported by a minimal-yet incomplete-kinetic model which provi des rate constants and species-associated difference spectra for the i ntermediates. In this modeling the apparent rate constant for primary charge separation from the excited reaction center core is 120 +/- 30 ns(-1). In this minimal kinetic model the formation of the primary rad ical pair occurs exclusively with a 2.8 ps lifetime. Two radical pair states and two external chlorophylls have to be included in the minima l kinetic model for a reasonable description of the data. The data is compared with transient absorption data from other groups and with the results from our previously reported fluorescence study. The high app arent rate constant of 120 ns(-1) for charge separation found in the k inetic models as well as other data exclude the possibility that prima ry charge separation could be associated primarily with a similar to 2 1 ps lifetime.