New Technology Developed To Visualize Molecules in Motion

An innovative X-ray technology has been developed that allows the observation of molecular motion on a time scale never reached before.

The technique was developed by a team of researchers from the Ecole Polytechnique Federale de Lausanne (EPFL; Switzerland) and the Paul Scherrer Institute (PSI; Villigen, Switzerland). Results of the research, led by Prof. Majed Chergui, head of EPFL's laboratory of ultrafast spectroscopy in collaboration with collaborators at PSI, was published online December 11, 2008, in the journal Science.

According to the scientists, this discovery provides a promising approach for the study of chemical and biologic systems. It allows a better determination of the structural evolution of molecules during a chemical reaction. The researchers have applied it to the study of metal-based molecular complexes, which is of high interest for chemical researchers. This could lead to applications in magnetic data storage or solar energy. It also opens new perspectives in biology, because the molecules studied are analogous to the active center in hemoproteins (hemoglobin, myoglobin).

It is possible to follow a cat landing on its feel in real time using a camera with shutter times on the order of tens of milliseconds. To do the same with molecules, 100,000 million times smaller than cats, requires shutter times that are 100,000 million times faster--a few tens of femtoseconds (1 femtosecond is to a second what a second is to 32 million years).

Although there are lasers that permit such shutter speeds, no existing optical techniques can capture the molecular structure. To overcome this limitation, Prof. Chergui's team combined lasers delivering femtosecond pulses of ultraviolet-visible light with a source of femtosecond X-ray pulses, in a technique now known as ultrafast X-ray absorption spectroscopy. "With the extremely short wavelengths of this kind of pulsed radiation, it is possible to observe the molecular structure changes, and thus to obtain precise information about the breaking, the formation, or the transformation of chemical bonds between atoms. And all this, in real time,” explained Prof. Chergui.

To reach this level of accuracy, the researchers required a source of stable and tunable femtosecond X-ray pulses. They found it at the Paul Scherrer Institute, in a collaboration with Dr. Rafael Abela's team. Utilizing the femtosecond X-ray pulses extracted from the Swiss Light Source synchrotron in a technique developed at the PSI, the researchers were able to track in real time a structural change of the molecule in 150 femtoseconds. This method, according to the scientists, should be an excellent modality for analyzing reactions in liquid and disordered environments that characterize many biologic and chemical systems.

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Ecole Polytechnique Federale de Lausanne
Paul Scherrer Institute