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Biomedical Optics & Medical Imaging

2-D Spectroscopy Yields Information on How Energy Moves between Molecules

Eye On Technology: Spectroscopy

From oemagazine June/July 2005
31 May 2005, SPIE Newsroom. DOI: 10.1117/2.5200506.0002

A new spectroscopy technique developed to study photosynthesis could also help scientists gain greater understanding of energy transfer among molecules.

The technique, called 2-D electronic spectroscopy, was developed by Graham Fleming and his colleagues. Fleming is deputy director of Lawrence Berkeley National Laboratory (Berkeley, CA) and a professor of chemistry at the University of California, Berkeley.

Previous spectroscopy techniques allowed researchers to measure the distribution of energy levels in a system but gave no information about how the levels were spatially related. Fleming's approach lets researchers see how the molecules interact.

Two pathways for energy transport in a protein, revealed through 2-D spectroscopy, show the process is not just a stepwise energy decrease but depends on the spatial relationship between molecules.

Using three 50-fs pulses of 800-nm light from an amplified Ti:Sapphire laser that they built themselves, the researchers excited the molecules in a Fenna-Matthews-Olson photosynthetic light-harvesting protein. That protein is a molecular complex made up of seven molecules. It is important in helping green sulfur bacteria collect photons and convert their energy to chemical energy.

A separate laser provided interferometric information to measure the field of the signal. The researchers recorded the response of the system to the pulses as a function of the time between the pulses. A Fourier transform of the results yields a plot of the energy levels. If there are interactions between the molecules, the plot will show cross peaks, where the energy levels of two molecules mix on the spectrum.

"What these cross peaks tell you is who's talking to whom," Fleming says. "It tells me that one given molecule is contributing to more than one frequency in the spectrum."

Unexpected Finding

The results in this particular experiment were surprising. With seven molecules and seven excited states, you might imagine that the energy levels would decrease in seven steps, like climbing down a ladder, Fleming says. "That's not how the system works, actually."

It turns out that most of the energy states extend over two or three molecules, so the energy can get from anywhere in the system to anywhere else in three steps at most. That's much more efficient, Fleming says. The information provides a clearer understanding of how photosynthesis works and might lead to a way of artificially replicating the process. "It might suggest a new way of synthesizing light-harvesting systems," Fleming says.

Sergei Savikhan, an assistant professor of physics at Purdue University (West Lafeyette, IN), says the technique certainly sheds light on the details of energy transfer in photosynthesis. It also does what other methods cannot, replacing theoretical modeling of energy interactions with direct observation. "It certainly opens new dimensions for direct experimental observation and facilitates better understanding of energy flow," Savikhan says.

In a sense, the technique is not new, Fleming says. It has been used for a long time with nuclear magnetic resonance imaging and, more recently, at infrared wavelengths. As wavelengths get shorter, however, the measurements become more difficult. "The only way you can do it at optical frequencies is through interferometry," he says.

For the interferometry to work, the optical paths have to be stable to fractions of a wavelength. Fleming's team achieved stability by using a transmission grating to control the phase of the pulses. An optical parametric amplifier allowed them to try the experiment at different wavelengths. There is no problem in performing the technique at any visible wavelength, Fleming says, however, experimenting in the UV will be more difficult. That might be useful, though, for people who wish to look at the electronic states of proteins and DNA molecules using that portion of the spectrum.

Fleming's technique should also prove useful to those developing computer models of energy transfer systems. "Because it produces information at a much richer level than conventional methods, it's going to have a big impact on theoretical work," he predicts.