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Multiple Pathways for Ultrafast Transduction of Light Energy in the Photosynthetic Reaction Center of Rhodobacter sphaeroides

Marion E. van Brederode, Frank van Mourik, Ivo H. M. van Stokkum, Michael R. Jones and Rienk van Grondelle
Proceedings of the National Academy of Sciences of the United States of America
Vol. 96, No. 5 (Mar. 2, 1999), pp. 2054-2059
Stable URL: http://www.jstor.org/stable/47012
Page Count: 6
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Multiple Pathways for Ultrafast Transduction of Light Energy in the Photosynthetic Reaction Center of Rhodobacter sphaeroides
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Abstract

A pathway of electron transfer is described that operates in the wild-type reaction center (RC) of the photosynthetic bacterium Rhodobacter sphaeroides. The pathway does not involve the excited state of the special pair dimer of bacteriochlorophylls (P*), but instead is driven by the excited state of the monomeric bacteriochlorophyll (BA *) present in the active branch of pigments along which electron transfer occurs. Pump-probe experiments were performed at 77 K on membrane-bound RCs by using different excitation wavelengths, to investigate the formation of the charge separated state P+HA -. In experiments in which P or BA was selectively excited at 880 nm or 796 nm, respectively, the formation of P+HA - was associated with similar time constants of 1.5 ps and 1.7 ps. However, the spectral changes associated with the two time constants are very different. Global analysis of the transient spectra shows that a mixture of P+BA - and P* is formed in parallel from BA * on a subpicosecond time scale. In contrast, excitation of the inactive branch monomeric bacteriochlorophyll (BB) and the high exciton component of P (P+) resulted in electron transfer only after relaxation to P*. The multiple pathways for primary electron transfer in the bacterial RC are discussed with regard to the mechanism of charge separation in the RC of photo-system II from higher plants.

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