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360 Electron Transfer Reactions

FIGURE 7 Computer-generated pathways (dashed lines) for the electron transfer from ruthenium(II) bound to the
surface of myoglobin (lower right) to a heme iron center in the interior. Solid line segments outline the protein structure.
Heavy shading for the surface structure and pathways fades to light shading in the interior.

and studying electron transfer processes induced by pho- what researchers are learning and seeking to learn about
toexcitation of the attached metal ion as shown in Fig. 7. these very complicated reaction systems. We shall offer
Many natural proteins whose function includes electron only a brief introduction to what has been learned and is
transfer have structures with electron donor sites close being learned about the initial processes involved in nat-
enough to allow tunneling to holes, or acceptor sites for ural photosynthesis.
electrons. Space does not permit a detailed description of
the elaborate structures, electron transfer pathways, and
A. Electron Transfer in Photosynthesis
mechanisms occurring in biological systems. Interested
readers are referred to the Bibliography, especially the ar- Protein structures have evolved in photosynthetic organ-
ticles dated from 1992 to 1999. The titles give an idea of isms with an extraordinary ability to capture light and