A new technique developed at the University of Toronto has captured — with femtosecond accuracy and atomic resolution — real-time images of molecules undergoing structural transitions. (A femtosecond is one quadrillionth of a second.) The breakthrough makes it possible to probe the ultra-fast motions that are involved in overcoming activation energy barriers to drive chemical reaction, including complex ones like protein folding.

Four flat ethylenedioxytetrathiafulvalene
(EDO-TTF) ions and two phosphorus hexafluoride (PF6) counter-ions make up a unit cell of the organic salt (EDO-TTF)2PF6. A new electron diffraction technique allows the bending and shifting of these ions — a process that takes mere picoseconds — to be directly imaged for the first time. The system could unlock the secrets of more complex motions, like protein folding.

For decades, scientists have captured still images of molecules by shining X-rays on them and observing the pattern of diffracted light. However, in order to image moving­ molecules, the light source needs to be very bright and capable of emitting extremely short pulses. Modern X-ray technology has allowed some light sources to approach these conditions, but only in the largest and most expensive facilities. “A state-of-the-art, free-electron laser for X-ray diffraction can be the size of a football field,” says Ray Gao, a PhD candidate in physics at the University of Toronto.

Gao is part of a research group run by Dwayne Miller, who is jointly appointed to the departments of Chemistry and Physics at U of T and the Max Planck Research Group for Structural Dynamics at the University of Hamburg in Germany. Instead of X-rays, Miller’s team uses electrons, which also have a wavelength comparable to atomic distances. However, because electrons repel one another and spread out, the team had to invent a device known as a re-buncher cavity. As a stream of electrons enters the cavity, an electric field slows down the electrons at the front, but is then quickly reversed, accelerating the electrons at the back. “The better temporal resolution achieved by re-bunching, combined with the fact that electrons are very bright, is what allows us to see what we see,” says Gao.

In a paper published in Nature, the team tested the process on the organic salt (EDO-TTF)2PF6, which changes from an insulating phase at low temperature to a conducting phase at high temperature. (EDO-TTF stands for ethylenedioxytetrathiafulvalene.) By comparing the actual diffraction patterns to those predicted by computer models, the team could track the order and timing of the ultra-fast molecular movements involved in the phase change. In the future, the team hopes to refine the technique and apply it to other systems, such as directly imaging proteins at work. “Our goal is to look at how small, fast motions drive larger, slower motions,” says Gao. “What happens on the femtosecond or picosecond time scales is currently unknown.”