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X-rays are, of course, useful in determining the structure of materials and biomolecules, but are relatively insensitive to chirality. Now, a team of scientists in Japan has shown that circularly polarized X-rays at an appropriate wavelength can distinguish 'left' from 'right' in alpha-quartz. The work could have implications for studies of other inorganic organometallic materials, including industrial catalysts, liquid crystals, biomolecules, and pharmaceutical products. Crystallographer Yoshikazu Tanaka of RIKEN SPring-8 Center in Harima, Japan, and colleagues, Tomoyuki Takeuchi, Stephen Lovesey of the UK's Diamond Light Source, Kevin Knight, Ashish Chainani, Yasutaka Takata, Masaki Oura, Yasunori Senba, Haruhiko Ohashi, and Shik Shin, have now demonstrated that circularly polarized X-rays at an energy tuned to the silicon absorption edge can distinguish the 'left-' and 'right-handed' forms of alpha-quartz (silicon dioxide). In the left and right forms of single-crystal alpha-quartz (SiO2), the tetrahedral of Si-O within the crystal structure spiral in opposite directions. Tanaka was frustrated by the inability of X-ray diffraction to distinguish the two handed forms, but has, through serendipity, discovered a way to separate the two crystal forms of quartz. Apparently, he and his colleagues were simply testing an X-ray diffractometer with a crystal of quartz when they noticed that the intensity of one diffraction peak from the quartz varied with the energy of the X-ray beam. This variation, they explain, occurred particularly when the X-ray energy was close to the atomic absorption edge of silicon, which occurs at 1.85 keV. At this point, Tanaka had the idea of using the same x-ray energy to look for a reflection in alpha-quartz that is normally forbidden by the symmetry of the crystal. Such a reflection could, he thought, be used to distinguish left from right if it were rendered visible. Tanaka's hunch turned out to be right. The reflection was present and exists at the resonant energy of 1.85 keV. More importantly, however, the intensity of this reflection was wholly dependent on whether the quartz crystal examined was in the left- or right-handed form. The fact that the team had been using circularly polarised X-rays as opposed to the more conventional linearly polarized, was a critical factor in the observation of such a large difference between the two crystal forms. The team's novel X-ray technique has a clear and obvious advantage over optical techniques when applied to non-transparent materials such as metals or oxides, explains Tanaka. Quartz is a common mineral and researchers know a lot about its alpha form, However, physicists, materials scientists and others would benefit from understanding how the chirality of this material develops as the crystal grows. Moreover, such insights could reflect on other mineral growth and even unrelated materials. Tanaka says he is now interested in using the same technique to explore the role of chirality in a broad range of materials, including liquid crystals, magnets, and multiferroics. The technique may also have applications in biomedicine and biochemistry where proteins, sugars, and pharmaceuticals, and other molecules of biological and medicine interest usually crystallize into two chiral forms. Lovesey revealed to spectroscopyNOW that Tanaka and Lovesey's colleague Steve Collins plan to run an experiment at the Diamond Light Source on the metallic element tellurium, which has the same crystal structure as low quartz. Related links:
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