Basic Research Needs for Solar Energy Utilization - Office of ...

LH1-RC
35 ps
HOW DO PHOTOSYNTHETIC ORGANISMS CAPTURE LIGHT AND SEPARATE CHARGE?
Energy Flow within Bacterial Antenna Proteins
and Funneling to the Reaction Center
B850-B850
100-200 fs
LH2-LH1
3-5 ps
B800-B850
1.2 ps
B800-B800
500 fs
38
The image to the left shows the detailed molecular
structures of the two light-harvesting proteins, LH1
and LH2, and the reaction center (RC) from a
specific species of purple photosynthetic bacteria.
The view is looking down onto the plane of the
membrane in which these proteins reside. Green
plant photosynthesis uses a larger number of
proteins, as well as greater numbers of energy and
electron transfer cofactors. The bacterial system is
illustrative because the issues and questions
concerning how energy and electrons flow within and
between proteins are similar for all photosynthetic
organisms.
The purpose of LH1 and LH2 is to increase the number of solar photons captured and to funnel them into the RC. The closely spaced
bacteriochlorophyll molecules shown in green (above) transfer energy within LH1 and LH2 very rapidly, as indicated; this transfer is
followed by somewhat slower transfer to the RC. Rapid energy transfer results in efficient utilization of the photon energy.
Periplasm
Membrane edge
Car
Membrane edge
Cytoplasm
Photoinduced Charge Separation in a
Bacterial Reaction Center
B Side
BChl a
BChl a 2
(P865)
+.
0.9 ps
BPh a BPh a
Q B
200 μs
3.5 ps
BChl a
-. 200 ps
Q A
A Side
The image to the left shows a side-on view of the RC
in the photosynthetic membrane. Only the cofactors
responsible for photo-induced charge separation
across the membrane are shown. Excitation of the
bacteriochlorophyll dimer (BChl a2) results in rapid
electron transfer to an adjacent BChl a acceptor
followed by thermal electron transfer to a
bacteriopheophytin acceptor (a magnesium-free
BChl a that is a better electron acceptor than BChl
a). Two more thermal electron transfer events to
quinone molecules, QA and QB, continue to move the
electron further from the hole that remains on BChl
a2. The result is separation of a single electron-hole
pair across a 40-Å membrane with nearly 100%
quantum efficiency.
The high quantum efficiency of photosynthetic charge separation within the RC results principally from two important features of the
structures of the protein and the electron donor-acceptor cofactors. First, the energetics for each electron transfer step are optimized to
give the fastest forward rate and the slowest back reaction rate. Second, the electron and hole are moved further away from one
another with each electron transfer step, resulting in progressively weaker interactions between them. These factors combine to yield a
very long-lived charge separation.