Electron Laser Helps ‘See’ Proteins Move
There are tens of thousands of proteins, and scientists understand a lot about them. They know where all the atoms are and what shape the proteins take. They also have a pretty good understanding of their function. What remains a mystery is how they move, particularly in water, the key to understanding how proteins do their job.
UCSB physicist and Center for Terahertz Science and Technology Director Mark Sherwin, biophysical chemist Song-I Han and their colleagues are developing new instrumentation that will enable scientists to, in essence, make movies of a protein in motion with ultrahigh-frequency electromagnetic radiation. The W.M. Keck Foundation granted Sherwin and his collaborators $1.75 million in 2008 to create the equipment to do that.
Han and Sherwin also received a $1.26 million National Science Foundation Major Research Instrumentation grant in 2008 to build a spectrometer that uses pulsed electron spin resonance (ESR) and dynamic nuclear polarization (DNP) to “see” the proteins move.
“Ideally, what we’d like to do is make movies of the proteins as they’re performing their functions,” Sherwin says. “You can’t do that with visible light because they’re so small. An ordinary microscope won’t work.”
Also, the proteins have to be in their native environment, or something close to it, for example dissolved in solution. So Sherwin and collaborators are using two techniques to look at the proteins: ESR and absorption spectroscopy.
“For both of these techniques, which are complementary in getting at different aspects of protein motion, the interesting dynamics require using electromagnetic radiation that is in the frequency range of roughly a trillion cycles per second, or terahertz,” Sherwin says. “Just to put this in perspective, it’s kind of an electromagnetic no-man’s land.”
A terahertz is about three orders of magnitude above the frequency of a microwave oven and the clock speed on a computer. Visible light is another thousand times higher than terahertz, Sherwin says. And while higher and lower frequencies are fairly well developed, technology is not as advanced in the terahertz range.
“It is very difficult to make sources like lasers operate at terahertz frequencies,” he says.
UCSB’s gigantic free-electron lasers are very powerful and tunable over a broad range, so they will be used to enable ESR and DNP analysis with unprecedented temporal resolution and information content, allowing scientists to follow a particular protein in action and to understand how they cooperatively carry out their function with other biological components. This protein happens to be a proton pump that lives in the cell membrane of marine bacteria and photosynthesizes.
“A flash of visible light initiates the motion of the protein, and then the spectrum of the electron spins should change as the protein moves,” Sherwin says. “So essentially we flash it with visible light to start the motion and then we give the electron spins a kick to get them ‘ringing’ like tiny bells. The free electron laser is what boosts the electron spin causing them to resonate like a bell.”
Scientists will measure the movement between the kicks and get some idea of how the protein works.
Another part of the overall project is measuring the absorption of proteins dissolved in water with the free electron lasers. That effort is led by UCSB biochemist Kevin Plaxco and solid state physicist Jim Allen, who are also affiliated with the Center for Terahertz Science and Technology.
“They’ve discovered some interesting results where there’s apparently a minimum frequency at which proteins oscillate, which has to do with their size,” Sherwin says.
Sherwin and his group believe they can build a tabletop instrument to do these experiments up to 660 gigahertz and then the electron laser can take over at frequencies above that.
“A part of this project is to keep up with the state-of-the-art in terahertz and to have tools that other people on campus can use for whatever problems they have and may need at these frequencies,” he says.