Faculty of Physics, Hanoi University of Science


X rays Measure Lone Molecules

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"Researchers measured an atomic distance in a molecule without using a crystal, which is usually required. Instead they used a beam of isolated molecules.

A classic way to measure the atomic structure of a molecule is to scatter x rays from a crystal, but crystals are sometimes impossible to make. Now researchers reporting in Physical Review Letters have used x-ray scattering to measure the distance between two atoms in a molecule by using a large number of aligned, identical molecules in a molecular beam. Although the distance was already better known from other techniques, the experiment shows the feasibility of using extremely bright x-ray sources to explore previously inaccessible phenomena in isolated molecules.

Of the many ways of measuring atomic-scale distances, x-ray diffraction has been one of the most precise techniques for a century. And it remains the most flexible and powerful way to determine molecular structure, for example of proteins, molecules that often have thousands of atoms. Usually you need to coax many copies of a molecule to nestle together to form an orderly crystal before you can use x rays to learn the atomic structure. X rays scatter, or “diffract,” strongly from a crystal only in certain special directions, and researchers use the angles of these bright spots to calculate the atomic positions. Large, defect-free crystals are critical for bright diffraction spots, but for many proteins, it is difficult to grow such crystals.

However, even small crystals can work well with superbright x-ray pulses from one of the new free-electron lasers, such as the Linear Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory in California, which began operation in 2009. Researchers have used such sources to analyze tiny crystals with just tens or hundreds of protein molecules. “Experimentally it’s a big step forward, but conceptually it’s still crystallography,” says Jochen Küpper of the Center for Free-Electron Laser Science (CFEL) in Hamburg, Germany. Now he and a large, international team have taken the next step and worked with isolated molecules, rather than a crystal, to measure an atomic distance." ---- http://physics.aps.org/articles/v7/22

Detailed storyhttp://physics.aps.org/articles/v7/22

PRL paperhttp://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.083002