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Femtosecond Infrared Studies of Photosynthetic Reaction Centers: New Charge Transfer Bands and Ultrafast Energy Redistribution

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Abstract

An accurate description of the mechanism of the primary electron transfer process in Photosynthetic Reaction Centers requires a full elucidation of the electronic structure of the cofactors involved in the process. In order to characterize low-lying excited states of reaction centers we performed ultrafast visible pump/ IR probe experiments. The mid- to near-IR difference spectra of optically pumped and unpumped Reaction Centers were recorded over a time range circa 0.1 to 20 ps (Wynne et al., 1996), allowing us to isolate kinetically the transient electronic spectra of the excited special pair P* and its oxidized form P+H-. The spectrum of P* between 1 and 5.2 μm (Figure 1A) exhibits two main bands, one at 5,300 cm-1 and another at 2,710 cm-1 while P+H- (Figure 1B) exhibits two bands, one at 2,600 cm-1 and another at 8,000 cm-1. These new states do not appear as transitions of unexcited Reaction Centers at E(P*)+5,300 cm-1 or E(P*)+2,710 cm-1 (although the former is close to the Qx transition region of the special pair) suggesting they derive almost exclusively from charge transfer (PL+PM- or PL-PM+) states. This conclusion is strengthened by the anisotropies obtained by pumping P* and probing these transitions with polarized light. The transition of P+H- at 2,600 cm-1 corresponds to the known hole transfer transition (Bréton et al., 1992) and this assignment is confirmed by kinetic and anisotropy measurements: The transition dipole is along the direction of charge transfer between PL and PM. The anisotropy for the transition at 8,000 cm-1 is consistent with a transition dipole along the y-axis of PM, in agreement with theoretical predictions for a trip-doublet transition. The results constitute the first direct measurement of the charge transfer bands of the Special Pair.

© 1996 Optical Society of America

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