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
PDF ArticleMore Like This
David M. Jonas, Matthew J. Lang, Yutaka Nagasawa, and Graham R. Fleming
TuD.5 International Conference on Ultrafast Phenomena (UP) 1996
W. Zinth, T. Arlt, and J. Wachtveitl
QThA3 European Quantum Electronics Conference (EQEC) 1996
J. Breton, J.-L. Martin, A. Migus, A. Antonetti, and A. Orszag
WA3 International Conference on Ultrafast Phenomena (UP) 1986