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New X-ray method for measuring the chirality of biomolecules


An international team of researchers-led by scientists at European XFEL, a new major research facility under construction in Hamburg, Germany-have implemented a method of identifying and measuring spiraling, or circularly polarized, X-ray light, which could open largely unexplored areas of chemistry and other fields of applied science. The procedure was validated at the international research centre Elettra Sincrotrone Trieste in Italy on the free-electron laser (FEL) FERMI, the only facility in the world where intense circularly polarized FEL light is available. The research was published today in the journal Nature Communications.

Circularly polarized X-rays are expected to be an essential tool in various areas of research because they can reveal information about chemical asymmetries, which cause what is known in biochemistry as "handedness". While many reactions and mechanisms associated with handedness are well understood, a deeper understanding of the asymmetries behind those mechanisms could have major impacts on the design of new chemical and pharmaceutical processes, among other applications. Understanding asymmetries could also give insights into magnetic properties, which could lead to innovations in computer data storage.

"These experiments open the door for a new class of investigations which could be essential for a large number of potential applications", says European XFEL leading scientist Michael Meyer, whose group led the research effort. "Our study shows that it is possible to use light with this special character-the circular polarization-as a tool to study asymmetries, both in atoms and in much more complex biomolecules."

At FELs such as FERMI and the future European XFEL, scientists use intense X-ray light flashes to see substances at the atomic level or to observe fast processes. But to explore the full potential of these facilities, scientists seek to take advantage of different properties of the X-rays-including circular polarization-and need to measure those properties as precisely as possible.

Polarization is a commonly observed property of light, used in everyday objects such as sunglasses, windscreens, and TVs, wherein the electromagnetic fields associated with the light are aligned in a certain way. In X-rays that are circularly polarized, those fields spiral like a corkscrew in either a clockwise or anticlockwise direction.

For many experiments, scientists would need to know to what extent the X-rays are circularly polarized and in what direction. These sorts of experiments are varied across many fields, but among the most exciting possibilities is the potential to figure out why so many crucial biomolecules exhibit handedness.

Handedness, also known as chirality, causes some molecules to exist in two mirror-image versions, one "right-handed" and the other "left-handed". One nagging question in biochemistry is why certain molecular building blocks of life are almost exclusively left-handed, as is the case with amino acids, while other molecules, such as sugars, are almost exclusively right-handed.

"There's a question of what the handedness really does in the starting point for life", says Meyer. "Circularly polarized X-ray studies could reveal why any surplus of left- or right-handed biomolecules exists." Similarly, circularly polarized X-ray studies of magnetic materials, wherein scientists try to produce magnetic properties in ever smaller areas, are potentially the starting point of the next generation of ultrahigh capacity hard drives and optical storage.

In some of these experiments, it is difficult to clearly separate circularly polarized X-rays from randomly-oriented unpolarized X-rays-the polarizing filters that would normally separate the two forms of light simply don't exist for X-rays at such high intensities. That's where the new technique comes in handy.

The method-performed by Meyer and his European XFEL team in collaboration with scientists from Elettra Sincrotrone Trieste, DESY, and 13 other institutes-involves exposing helium atoms to the X-ray FEL flashes. The FEL flash knocks one of helium's two electrons out of place, making normally neutral helium suddenly positively charged. The positive helium ion and the outgoing electron together have a polarization that matches that of the FEL flash. When the ion and electron are hit with a pulse from an optical laser with a known high degree of circular polarization and observed through a spectrometer, the properties of the FEL flash can be precisely identified.

"The technique requires precise control of the FEL polarization, and of the synchronization with the optical laser, both of which are a unique prerogative of FERMI, to-date the ideal facility for the demonstration of this technique", says FERMI scientist Carlo Callegari, who is also an author on the study and is coordinator of the Low Density Matter (LDM) beamline where the experiment was performed.

"Just as a naturalist can study the footprints of a wild animal and determine where and how quickly it was moving, we can look at the electromagnetic 'footprint' of the X-rays on the helium and tell about how they are polarized", says Meyer.

Since this technique is relatively simple to perform and does not require much additional equipment, it can be easily transferred to other facilities, including European XFEL. Scientists now have a better chance of using circularly polarized light to approach ever deeper research questions.

"I do not expect that polarization at FELs has to be measured every time an experiment is carried out, but it would allow scientists to check the property of the beam from time to time, especially after changes have been done for example to the optics-the same way musicians have to make sure from time to time that their instrument is properly tuned", Meyer says.

» Original publication

Source: European XFEL GmbH