Credit: © 2010 AAAS

One-dimensional molecules often form ordered domains, a property exploited by the wide range of liquid-crystal display devices currently available, such as wide-screen TVs and mobile phone displays. The phenomenon is also important in biology, as shown by the bundling and orientation of 1D biomolecules and even filamentous viruses. These molecules and their assemblies are usually relatively large and are thus studied by small-angle X-ray scattering (SAXS). Very powerful X-ray beams, however, can irreversibly damage some molecules and break up these assemblies.

In contrast, Sam Stupp and colleagues from Northwestern University have discovered1 that X-rays can in fact trigger crystallization in filaments formed by a short peptide grafted to a 16-carbon chain. The filaments are around 100 Å in diameter, with their length estimated to be on the scale of tens of micrometres. Stupp and co-workers took 50 consecutive SAXS profiles of a solution at relatively low concentration (1 wt%) by exposing the same spot to X-rays for four seconds, and found diffraction peaks — indicative of crystalline order — appeared over time. At higher concentrations (5 wt%) the order appeared spontaneously.

The ordering was shown not to be driven by heat, and was lost within 40 minutes of the X-ray beam being removed. Unexpectedly, the gap between filaments in the hexagonal crystalline domain was very large, up to 320 Å, compared with those for biomolecule filaments, which are between 30 and 55 Å. Stupp and colleagues propose that the assembly mechanism is caused by the X-rays' reversible ionization of carboxylic acid groups.