Basic Research Needs for Solar Energy Utilization

HOW DO PHOTOSYNTHETIC ORGANISMS CAPTURE LIGHT AND SEPARATE CHARGE?

B800-B850

1.2 ps

B800-B800

LH2-LH1 500 fs

3-5 ps

LH1-RC

35 ps

B850-B850

100-200 fs

Energy Flow within Bacterial Antenna Proteins

and Funneling to the Reaction Center

B800-B850

1.2 ps

B800-B800

LH2-LH1 500 fs

3-5 ps

LH1-RC

35 ps

B850-B850

100-200 fs

Energy Flow within Bacterial Antenna Proteins

and Funneling to the Reaction Center

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.

3.5 ps

0.9 ps

+.

-. 200 ps

BChla 2

(P865)

Car

BChla

BPha

QB QA

BChla

BPha

B Side A Side

200 μs

3.5 ps3.5 ps

0.9 ps0.9 ps

+.

-. 200 ps

+.

-.-. 200 ps200 ps

BChla 2

(P865)

Car

BChla

BPha

QB QA

BChla

BPha

B Side A Side

200 μs

Photoinduced Charge Separation in a

Bacterial Reaction Center

Membrane edge

Membrane edge

Periplasm

Cytoplasm

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 a 2 ) 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

a 2. 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.