First step taken to image ultra-fast movements in chemical reactions

A team of international researchers have fired ultra-fast shots of light at oxygen, nitrogen and carbon monoxide molecules as part of a development aimed at mapping the astonishingly quick movements of atoms within molecules, as well as the charges that surround them.

The ultra-short laser that spans only a few hundred attoseconds -- an attosecond is equivalent to one quintillionth of a second -- was fired in a sample of molecules and could pave the way towards imaging the movement of atoms and their electrons as they undergo a chemical reaction -- one of the holy grails of chemistry research.

This latest study has been published today, 16 March, as part of a special issue on attosecond science, in IOP Publishing's Journal of Physics B: Atomic, Molecular and Optical Physics to mark the 10th anniversary of the first ever attosecond laser pulse.

Previous research has been able to probe the structure of molecules using a variety of techniques; however, the inherent challenge is to perform these experiments in systems where changes are rapidly occurring on very small time scales.

The researchers used two lasers in their experiments: the first held the molecule in place whilst the second was fired at it. The second laser operated in the extreme ultra-violet region of the electromagnetic spectrum as this is one of only two regions -- x-ray being the other -- where the laws of physics allow laser pulses to be produced on an attosecond timescale.

Once the target molecule was in place, short pulses of the laser were fired at in an attempt to dislodge an electron. This process, known as photoionization, allows atoms and molecules to be imaged in unprecedented detail as the ejected electrons carry crucial information about where it came from.

In this experiment, the samples, which existed as a gas, were stable, meaning no reactions were taking place; however, the major goal of the research team is to monitor the electrical and molecular changes, in real-time, that occur as atoms undergo a chemical reaction.

They intend to do this by triggering a reaction with the laser, breaking a chemical bond that holds molecules together, and then using the described technique to image the changes that occur in the molecule as they happen.

Lead author of the study Dr Arnaud Rouzée from the Max-Born-Institute said: "We show that the photoelectron spectra recorded for a small molecule, such as oxygen, nitrogen and carbon monoxide contains a wealth of information about electron orbitals and the underlying molecular structure.

"This is a proof-of-principle experiment that electrons ejected within the molecule can be used to monitor ultrafast electronic and atomic motion."

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The above story is reprinted from materials provided by Institute of Physics (IOP), via AlphaGalileo.

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Journal Reference:

Arnaud Rouzée, Freek Kelkensberg, Wing Kiu Siu, Georg Gademann, Robert R Lucchese, Marc J J Vrakking. Photoelectron kinetic and angular distributions for the ionization of aligned molecules using a HHG source. Journal of Physics B: Atomic, Molecular and Optical Physics, 2012; 45 (7): 074016 DOI: 10.1088/0953-4075/45/7/074016

Controlling movements with light

German researchers at the Ruhr-Universitaet have succeeded in controlling the activity of certain nerve cells using light, thus influencing the movements of mice. By changing special receptors in nerve cells of the cerebellum such that they can be activated and deactivated by light, the researchers have shown that the signaling pathways, which are activated by the receptors play a crucial role in controlling movement.

Unlike conventional methods, with the so-called optogenetics, the researchers are able to target one cell type. "We are now going to use this method to find out exactly what goes wrong in the nerve cells in movement disorders such as ataxias", said Prof. Dr. Stefan Herlitze (RUB Department for Biology and Biotechnology). The results are reported in the Journal of Biological Chemistry.

The Bochum team examined a specific signalling pathway that is controlled by a so-called G-protein-coupled receptor. This pathway is important for the modulation of activity in complex neuronal networks. Disturbances of the function can, for example, have an effect on emotional and motor behaviours. "We know that the activity pattern of the Purkinje cells in the cerebellum is crucial for the coordination of movements", Herlitze explained. "It is unclear, however, what contribution is made by the individual receptors." In conventional studies, researchers use drugs that inhibit or stimulate specific proteins in nerve cells to investigate the contribution of these proteins to the activity of the cells. However, Herlitze's team was interested in a protein (G-protein-coupled receptor) which occurs in various cell types. Had the researchers administered a drug, they would not only have deactivated the receptor in the Purkinje cells, but in all cell types in which it occurs. The drug method therefore makes it impossible to observe the contribution of the receptor in the Purkinje cells in isolation.

Optogenetics: replacing drugs with light

To avoid this problem, Herlitze's team replaced the drugs with proteins that are activated by light. Using genetic methods, the researchers integrated rhodopsin, the light-sensitive protein of the eye, into the Purkinje cells of mice. They also implanted a laser probe in the cerebellum, with which they illuminated the rhodopsin. The light-activated rhodopsin then activated the G-protein-coupled receptor in the Purkinje cells, while the same receptors in other cell types remained inactive. The RUB Department of General Zoology and Neurobiology has been instrumental in establishing this method worldwide.

Investigated receptor is crucial for movement control

The researchers found that activation of the G-protein-coupled receptor changed the activity pattern of the Purkinje cells. Herlitze's team had to expose the rhodopsin to light for several seconds to achieve these effects. A twenty to thirty percent reduction in cell activity was sufficient to induce visible motor deficits in the behaviour of the mice, such as impaired balance or coordination problems. "We were able to demonstrate for the first time that the modulation of a specific G-protein-coupled receptor in the Purkinje cells is of crucial importance for the control and coordination of movement", summed up Herlitze.

More information: Gutierrez, D.V., Mark, M.D., Masseck, O., Maejima, T., Kuckelsberg, D., Hyde, R.A., Krause, M., Kruse, W., Herlitze, S. Optogenetic control of motor coordination by Gi/o protein-coupled vertebrate rhodopsin in cerebellar Purkinje cells. J. Biol. Chem., doi: 10.1074/jbc.M111.25367 (2011)

Provided by Ruhr-University Bochum