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Electron Transfer Reactions 359

Chelated oxalate and sulfite ions have conducting II-
orbital systems that can include two t 2g orbitals on Cr,
which creates the possibility of transferring one electron
to each of those orbitals while the other ligand transfers
an electron to the third. Such activated complexes make
full use of the maximum potential available for reduction
of chromium (VI) and give Cr(III) products directly with
their most stable electron configuration. When the three- FIGURE 6 Iron–sulfur clusters for biological electron transfer.
electron process involves two different electron donors Distorted Fe 4 S 4 “cubes” (left) are found in biological systems in-
(as in the case of oxalate and 2-propanol at right above), volved in electron transfer. Fe 2 S 2 clusters (right) are active in elec-
tron transport systems in oxidases.
the process has been called cooxidation by its discoverer,
Jan Rocek. How favorable the three-electron process can
−
be is shown by the fact that H 2 PO has a rate constant
3
about eight orders of magnitude smaller than that for iso- oxide ions to give O 2 directly [see Mn(IV) gluconate, Eq.
−
electronic HSO . Apparently the phosphite has an inert (2)]. Any notions concerning actual electron transfer path-
3
proton on the phosphorus(III) atom that inhibits the two- ways in such multielectron systems remain conjectural as
electron transfer from the P(III) center, and it is incapable of this writing.
of being a facile one-electron donor like the sulfite. It The advent of supercomputers will permit the massive
is also interesting that sulfite, which has been used to calculations necessary to develop theoretical probes of
discriminate between one- and two-electron acceptors, is pathways for electron transfer in large complex systems.
oxidized equally by one and two electrons by the three- An early result of just such a study by Atsuo Kuki and
electron acceptor Cr(VI). Peter Wolynes is shown in Fig. 7, about which Wolynes
had this to say in 1988:

VIII. ELECTRON TRANSFER IN Perhaps the ultimate level of description of an electron transfer
BIOLOGICAL SYSTEMS process would be a movie showing how the electron leaves the
reductant and flies to the reactant. There are two difficulties with
this. First is the short time scale; the electron’s leap, when it oc-
Space does not permit adequate description of the enor-
curs requires only about 10 −15 second. Second is the fact that the
mous effort and progress being made in the study of elec-
electron is a quantum-mechanical particle. Because of the un-
tron transfers in biological systems. The respiratory chain certainty principle of quantum mechanics the electron does not
whose bare outline is given above has been under inten- follow a unique path, but follow simultaneously a whole fam-
sive study since the 1970s. The breakthrough that now ily of paths. With modern computers it is possible, however, to
provides detailed structural information on proteins and simulate typical paths that an electron can follow. Such a calcu-
nucleic acids has made possible serious study of macro- lation has been carried out (1985) at the University of Illinois. In
molecular mechanisms including electron transfer. Very Figure 7 is shown an electron path during the electron transfer be-
elaborate enzyme systems with interesting new chemical tween a ruthenium center bound to the surface of myoglobin and
structures are used by organisms to effect oxidation and a heme-iron center (cf. cytochrome-c) in the interior of the pro-
tein. The quantum fluctuations cause it to deviate from a straight
reduction of small molecules that are the raw materials of
line. When families of such paths are generated they give an
life—H 2 O, O 2 , CO 2 , N 2 , NH 3 , CH 4 , and so forth. In the
idea of which parts of the protein are most involved in electron
enzyme nitrogenase, for instance, several iron–sulfur clus-
transfer. Site directed mutagenesis of the protein changing its
ters (Fig. 6) in one protein may serve as an electron sink structure will allow experimental tests of these calculations.
poised to add four or six electrons to ahhhn inert N molecule
2
as it encounters a molybdenum center in a second protein
By the end of the twentieth century, the ability of
in nitrogen fixing bacteria:
research chemists to determine detailed structures of
proteins and DNA systems in which electron transfer re-
+
−
N 2 + 6e + 6H → 2NH 3 .
actions can be monitored on time scales ranging from sec-
Oxidases may effect reduction of O 2 to H 2 O 2 and/or onds down to nano-, pico-, and femtoseconds has brought
2H 2 Oinsuccessivetwo-electronstepsoronefour-electron Wolynes’ dream of a movie of an electron transfer pro-
step. Photosystem II uses chlorophyll to effect the photo- cess to the realm of possibility in biological oxidation–
chemical oxidation of H 2 O to O 2 after every fourth light reduction systems. In protein chemistry several metalloen-
3+
pulse, which suggests the possibility that the manganese zymes containing buried metal ions such as Fe in Fig. 2
centers present might be oxidized to an activated state have been modified by attaching other metal ions such as
capable of extracting four electrons from two proximate Ru(bipy) 2+ at various sites on the surface of the proteins
2