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Physics

Light web

New X-ray production technique could help to unravel the 3-dimensional structure of proteins

Ultra-fast images laser X-ray group / University of SouthamptonEnergy profile of the X-ray beamUltra-fast images laser X-ray group / University of Southampton

The laser source in the laboratory of chemist Jeremy Frey, at the University of Southampton, in the south of England, is like any other found in research centers in the outside world. But his has a little unusual finality. Built by a research team, in which the physicist Ana Maria de Paula from the state of Minas Gerais, participated, the almost R$ 4 million device is one of the five developed in the world over the last few years to generate a very special type of light: X-rays of ultra-fast pulse, a powerful tool for investigating the spatial structure of molecules such as proteins, essential for the composition and functioning of live organisms.

The equipment installed on top of a U-shaped table, emits a greenish light beam that is filtered and amplified as it passes through a small sapphire and titanium crystal, until it becomes 10 billion times more energetic than a common light bulb. Concentrated in a beam that pulses for quadrillionths of a second (fermoseconds) this energy, corresponding to the production in a second of 100 hydroelectric power stations like Itaipú, excites the molecules of a gas imprisoned in a very fine glass tube, which go on to emit X-rays of ultra-rapid pulse, of a few fermoseconds duration. Just like the laser that generates them, the X-rays in chemist Frey’s laboratory propagate themselves in a unique direction.

Only the equipment mounting, which took up almost three years, was already the motive for the team’s commemoration. Only lacking was to know in detail the properties of the electromagnetic radiation that it generates. In the Southampton case, the researchers already imagined that the X-ray beam formed by the radiation had a wave length of between 15 and 50 nanometers (one millionth of a millimeter). This variation, which in light’s visible spectrum would represent smooth gradations of the same color, corresponds to the so-called soft X-rays. This form of radiation penetrates less than 1 millimeter into a denser material such as a table top, but can manage to pass through molecules diluted in a liquid.

However, before being able to test with these X-rays and unravel the structure of chemical compounds, it was necessary to discover how the different wave lengths that form them are distributed in the electromagnetic beam. In an experiment concluded last year, the Southampton team pointed the X-ray beam at a sheet of aluminum sustained by an extremely fine network nickel threads. On passing through the openings in this web, the light spreads in a very characteristic manner – a phenomenon called diffraction, described almost two centuries ago by the French physicist Augustin Fresnel. From a single diffraction image, captured in thousandths of a second by a special camera, chemist Frey’s team managed to identify what fractions of the X-rays are produced with the greatest intensity and how they distribute themselves in the electromagnetic beam. This was a great advance, since, in general, one can only manage to reconstruct the energy profile of an electromagnetic beam starting from hundreds of images made with a spectrometer, a procedure that can take hours. “Since the 18th century we’ve been able   to calculate how light suffers diffraction”, comments Ana Maria. “The difficulty was in obtaining an image with the registration of the intensity of different wave lengths”, she adds.

The beam’s central region concentrates X-rays of wavelengths from 32 to 37 nanometers, with less intense radiation of 28 to 34 nanometers distributed around, as the researchers discovered and related in the article of the March edition of Nature Physics. “This piece of information is fundamental for discovering how the electromagnetic beam spreads on traversing through a protein and allows for the reconstruction of its 3-dimensional structure”, explains Ana Maria, a professor at the Federal University of  Minas Gerais and a visiting researcher at the University of Southampton.

Three dimensions
In this band of wave length, the radiation goes through the gaps between the atoms that form the protein, giving origin to an image of light and dark patches. Starting from this image, it is possible to reconstruct the molecule’s 3-dimensional structure. With the new equipment the shape of microspheres and polystyrenes have already been reconstructed. “We believe that by the end of the year it’ll be possible to produce the first images of proteins diluted in an aerosol”, says  Ana Maria.

If it really works, this will be a major step in relation to the traditional technique used to identify the spatial structure of proteins, essential information in understanding the role of these molecules in an organism. Since the date that the Austrian biochemist Max Perutz proposed to decipher the first structure of a protein, some 70 years ago, using X-ray diffraction, the techniques has changed little. In general, a high power X-ray beam, generated in the very expensive sources of synchrotron light, is made to pass through the protein crystal, formed by molecules grouped at the same distance one from the other. And the major difficulty is exactly to create this crystal, produced in a process that is still based on trial and error technique. (see Pesquisa FAPESP Nº 113).

As well as allowing for identifying the structure of proteins diluted in water, a situation close to that observed in human beings, the new technique presents other advantages. Produced in ultra-rapid pulses, these X-rays would work like a kind of intermittent light that would allow for filming these molecules in motion, contorting about themselves or combining with others.

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