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Photosystem I
Electron Transfer Pathways as Studied by Multifrequency Time-Resolved
Electron Paramagnetic Resonance Techniques
Thurnauer, Marion1
and Kothe, Gerd2
Argonne National Laboratory1
University of Freiburg2
Abstract-
EPR spectroscopy remains a major tool for identifying the intermediates
in photosynthetic charge separation. Moreover, time-resolved, multifrequency
EPR provides important structural and mechanistic information on the
electron transfer pathway in Photosystem I (PSI). Recent advances in
very high frequency EPR extends the power of these techniques, as illustrated
in our study of the structure of the secondary radical pair P+700A-1
in PSI. The geometry of P+700A-1
from the deuterated cyanobacterium Synechococcus lividis has
been determined by analyses of the quantum beats in the transverse magnetization
monitored at short times after the laser pulse (obtained at three microwave
frequencies). The two-dimensional Q-band experiment provided the orientation
of the various magnetic tensors of P+700A-1,
while the X-band spectra allowed evaluation of the orientation of the
cofactors, and W-band spectroscopy revealed the arrangement of the secondary
radical pair in the photosynthetic membrane. Preliminary room temperature
studies, employing X-band quantum beat oscillations, indicate a different
orientation of A-1 in its binding pocket. Also,
multifrequency time-resolved EPR has been applied to the study of quinone
exchange at the A1 site in Photosystem I. This approach allows
precise measurement of the exchange degree, which opens the way for
the detailed study of the kinetics and mechanism of the exchange reaction.
Work at Argonne National Laboratory was supported by the U.S. Department
of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences,
contract W-31-109-ENG-38.
Keywords: Photosystem
I, time-resolved EPR, high-frequency EPR
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