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Analytical Chemistry

Shedding Light on Photosynthesis

2-D method directly measures electronic coupling in a photosynthetic complex

by CELIA HENRY
April 4, 2005 | A version of this story appeared in Volume 83, Issue 14

NEW DIMENSIONS
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Credit: COURTESY OF GRAHAM FLEMING AND HARSHA VASWANI
The 2-D spectrum plots two different frequency dimensions along the x and y axes, with signal amplitude in the z direction. The dominant peaks, corresponding to the linear absorption spectrum, form a diagonal, and the off-diagonal peaks directly measure coupling between electronic energy levels.
Credit: COURTESY OF GRAHAM FLEMING AND HARSHA VASWANI
The 2-D spectrum plots two different frequency dimensions along the x and y axes, with signal amplitude in the z direction. The dominant peaks, corresponding to the linear absorption spectrum, form a diagonal, and the off-diagonal peaks directly measure coupling between electronic energy levels.

BIOPHYSICS

A new method is bringing two-dimensional spectroscopy to the visible region of the spectrum and revealing details about energy transport pathways in photosynthesis. Its applications, however, go beyond photosynthetic systems.

Using the new method, chemists Graham R. Fleming of the University of California, Berkeley, and Lawrence Berkeley National Laboratory; Minhaeng Cho of Korea University, Seoul; and coworkers have found that the excitation energy absorbed by the antenna pigments of the bacteriochlorophyll a-containing protein from green sulfur bacteria doesn't simply cascade down the energy ladder to the reaction center (Nature 2005, 434, 625). Conventional measurements tell you only that "the energy started here and finished there," Fleming says. "It doesn't tell you how, and it doesn't tell you the way the molecules communicate with each other."

Using 2-D spectroscopy to measure directly the electronic coupling between energy levels, Fleming and coworkers have identified distinct energy transport pathways that depend on the spatial properties of the excited states of the entire pigment-protein complex. "You might imagine that if you start in the highest energy level, you just rappel down the energy levels like rappelling down a stepladder until you get to the lowest rung. But that's not what happens," Fleming says.

ANTENNAE UP
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Credit: NATURE © 2005
The seven chlorophyll molecules in bacteriochlorophyll a interact with one another in patterns indicated by the shading. The red and green arrows indicate the two major energy transport pathways.
Credit: NATURE © 2005
The seven chlorophyll molecules in bacteriochlorophyll a interact with one another in patterns indicated by the shading. The red and green arrows indicate the two major energy transport pathways.

The excited states of the bacteriochlorophyll-protein complex tend to be "smeared out" over two or more molecules, according to Fleming. The system can skip energy levels on its way to the final energy level, getting from anywhere in the protein to the final acceptor level in two or three steps.

In Fleming's method, the 2-D spectrum identifies electronic interactions between molecules. Because those interactions can occur only when those molecules are close enough, the spectrum also yields spatial information, Fleming says.

The details about this relatively simple and fairly well-understood bacteriochlorophyll- protein complex are not the most exciting aspect of the work, says Robert E. Blankenship, a photosynthesis expert at Arizona State University and coauthor on the Nature paper. "You can use this same sort of method to study other systems," he says. "There are many complexes where the details of the electronic properties are not well-understood."

Sergei Savikhin, a physicist at Purdue University who studies photosynthesis, would also like to see the technique applied to more complex protein systems. "It would be interesting to see if the reported experimental method could be applied to larger systems such as photosystem I with 96 pigments or enormous chlorosomes with thousands of strongly coupled pigments," he says.

Fleming has every intention of doing just that. He's also thinking ahead to practical applications. "This analysis suggests different design strategies for synthetic light-harvesting devices," Fleming says. "It might be interesting to see if we can design synthetic systems with this characteristic of taking large energy jumps by exploiting the spatial overlap" of the electronic energy levels.

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