Health and Medicine Technology

New X-Ray Technique Determines Molecular Structure


A new method that might improve the quality of X-ray images considerably when compared to current methods has been developed by scientists from Deutsches Elektronen-Synchrotron (DESY, Hamburg) and Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU).

The method is called incoherent diffractive imaging (IDI) and it could possibly be used to image individual atoms in molecules and nanocrystals faster with a much higher resolution.

X-rays have been used for more than 100 years to help in determining the structure of molecules in crystallography. The foundation of the method are the principles of superposition and diffraction, to which all waves are subject. Light waves consist of photons that are deflected by atoms in the crystal and overlap, similar to waves generated by obstacles in a stream that flows slowly. If enough of these photons can be measured with a device, the wave- or diffraction pattern that is thus obtained is characteristic and the atomic structure of the crystal can therefore be derived. For this to work, the photons should be scattered coherently. This means that there should be a clear phase relationship between reflected and incident photons. To use the water analogy again, this is similar to water waves that are deflected from the obstacles without turbulence or any vortexes. If the photon scattering is however incoherent, the fixed phase relationship between the scattered photons disappears. This makes it impossible to determine the arrangement of the atoms.

Coherent imaging has some problems

There is however also a problem with coherent diffractive imaging. Anton Classen, a member of the FAU working group Quantum Optics and Quantum Information explains that when X-ray light is used, incoherent scattering dominates in most cases as a result of fluorescence resulting from photon absorption and subsequent emission creating a diffused background. This can’t be used for coherent imaging and decreases the reliability of coherent methods.

Using incoherent radiation

The key to the FAU physicists’ novel imaging technique lies in using this undesirable incoherent radiation. Professor Joachim von Zanthier noted that their method does not record the incoherently scattered X-ray photons over longer periods, but rather uses time-resolved short snapshots. When the snapshots are analyzed on their own, it enables the scientists to obtain the information about the arrangement of the atoms. The key to this method is the fact that that the light diffraction is still coherent within short sequences. This is however only possible with very short X-ray flashes that last no longer than a few femtoseconds. These short bursts have only recently been achieved when free-electron lasers like the Linac Coherent Light Source (LCLS) in California, or the European XFEL in Hamburg are used.

Single molecules can be visualized

The new method uses fluorescent light which produces a much stronger signal than before.  This type of light is also scattered to considerably larger angles, thereby providing spatial information that is much more detailed. Filters can also be used to only measure the light of specific atomic species. This enables the researchers to find the position of an individual atom in a molecule or protein. The resolution obtained is much higher than what can be achieved when compared to coherent imaging using X-ray light of the same wavelength. This new method is expected to take the study of proteins in structural biology and medicine to a new level.