SEOUL, October 01 (AJP) - Researchers in South Korea used ultrafast X-rays to film a light-driven molecular switch as it flipped, capturing a motion that chemists have argued over for nearly 50 years.
The molecule, azobenzene, changes shape by moving its central hinge like a pair of bicycle pedals while its two bulky rings stay nearly still, a team from KAIST and the Institute for Basic Science reported Wednesday in the journal Nature. The researchers resolved the atomic structures of two in-between forms that last only picoseconds, or trillionths of a second.
Azobenzene is one of the best-known molecular switches in chemistry. It consists of two benzene rings, each a ring of six carbon atoms, linked by a bridge of two nitrogen atoms.
In its resting form, the rings sit on opposite sides of the bridge and the molecule is stretched out. When it absorbs light, the rings can swing to the same side and the molecule bends, without gaining or losing a single atom.
That behavior has made azobenzene a standard building block in photopharmacology, a field that aims to turn drugs on and off with light. It has also been studied for materials that change shape under light and for machines built at the scale of single molecules.
Chemists have long known the molecule's shape before and after the switch. What happens in between was far harder to establish, because the intermediate structures vanish almost as soon as they form.
Competing explanations filled the gap. Some researchers argued that the rings rotate around the nitrogen bridge, others that the molecule bends within its own plane in a motion called inversion, and others that several parts twist together in what is known as a hula twist.
Spectroscopy experiments, which track how molecules absorb light, and quantum chemistry calculations pointed in different directions.
The team, led by Ihee Hyotcherl, a KAIST chemistry professor who directs the Institute for Basic Science (IBS) Center for Advanced Reaction Dynamics, ran the experiment at the X-ray free-electron laser of the Pohang Accelerator Laboratory. The machine produces extremely short, intense X-ray pulses that can freeze a molecule's motion at a single instant.
The researchers dissolved azobenzene in methanol, a simple alcohol, and set off the switch with a laser pulse. They then hit the solution with X-ray pulses at precise delays and recorded how the X-rays scattered, a technique known as time-resolved X-ray liquidography.
The difficulty lay in finding the molecule's signal at all. Azobenzene is made only of carbon, nitrogen and hydrogen, light atoms that scatter X-rays weakly, and the surrounding methanol produced a much stronger signal that buried it.
The team applied an analysis method that separated the faint molecular signal from the solvent background, the way a listener picks out a quiet voice in a noisy room. From those measurements, it reconstructed the molecule's motion as a sequence of structures.
The reconstruction showed the switch unfolding in stages. A twist at the bond between carbon and nitrogen started the motion. Rotation around the central nitrogen-nitrogen bond then took over, and inversion played a significant role in the final stage.
When the researchers traced the early changes using the smallest possible atomic movements, the two rings barely moved. Instead, the two sides of the nitrogen hinge moved in step with each other, the way bicycle pedals turn together.
The team said the clear structures of the intermediates end the long debate over the mechanism and capture early motions that earlier spectroscopy studies had missed.
The pedal motion also answers a long-standing question. Rotation plays a major part in the switch, yet the reaction runs at nearly the same speed even when the surrounding liquid becomes thicker and more resistant to motion.
Swinging the two large rings would mean pushing aside a great deal of liquid, much like turning a large piece of furniture in a narrow hallway. By the team's analysis, the pedal-like motion reduces the volume of solvent the molecule sweeps through in the early stage by about 80 percent compared with moving the rings alone.
The study does not make any azobenzene-based drug or material work better, and the experiment observed the molecule in methanol only. KAIST said the work's value lies in showing the path the molecule actually takes, which researchers can use as a reference when designing light-responsive materials and molecular machines.
Kim Jung-min and Ki Ho-sung, both KAIST graduates now at IBS, were co-first authors. All authors belonged to KAIST's chemistry department and the IBS center when the research was carried out.
Time-resolved X-ray liquidography had been difficult to apply to organic molecules without heavy atoms, whose weak signals are easily drowned out by the solvent. The team said the azobenzene study extends the technique to such molecules in solution.
"This study is an achievement that reveals the path azobenzene takes to change its shape after receiving light," Ihee said in a statement released by KAIST on Thursday. "By advancing methods for observing the motion of fast-reacting organic molecules, we expect it to contribute to understanding the working principles of various light-responsive molecules."
[Reference Information]
Journal/Source: Nature
Title: X-ray liquidography decodes complex motions in azobenzene isomerization
Link/DOI: https://bit.ly/4iZlKv5
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