Electron-microscopic studies have shown the presence of a highly
invaginated surface. Biochemical studies have shown that the invagina-
tions are packed with the light-harvesting complexes. To optimize the
efficiency of light capture and energy conversion, the cells contain many
light-harvesting II complexes that surround the light-harvesting I reac-
tion center. AFM has revealed in fine detail the packing of groups of the
light-harvesting II complex surrounding the light-harvesting complex I
and reaction center. The arrangement of the rings observed in the AFM
images fits extremely well with the known structural models. Such studies
demonstrated the utility of AFM and how it can be used to delineate the
organization of protein complexes in cell membranes.
Research direction: electron transfer II: distance dependence
Electron transfer plays a key role in many metabolic processes includ-
ing respiration (Chapter 9) and photosynthesis (Chapter 20). In some
cases, electron transfer occurs as a second-order reaction, for example,
when the electron donor belongs to one protein that must dock to a
second protein that has the electron acceptor. Such cases are found
when proteins are part of a metabolic pathway, such as respiration,
where cytochrome serves as an electron carrier between two complexes.
For these electron-transfer reactions, the overall rate will be dictated
by the diffusion of the carrier as the electron transfer does not occur
until a complex is formed. In other cases, a protein has more than one
cofactor as both the electron donor and acceptor are part of a large com-
plex. The theory presented is applicable to either case: a protein with a
bound donor and acceptor or a protein–protein complex that has formed
transiently.
Electron transfer occurs between an electron donor, D, and electron
acceptor, A, that can be separated by relatively large distances of up to
25 Å:
DA →D+A− (10.51)
In Marcus theory (Marcus & Sutin 1985), the transfer is an activated pro-
cess and the rate ketcan be divided into two components, the electronic
coupling Vand the Franck–Condon term that described the energetics:
(10.52)
In Chapter 7 the energetics of electron transfer were discussed. In this
section the factors that influence the coupling will be discussed. In a
k
h
et= V()Franck–Condon
2 π 2