Technical Highlight - March 2011
Short description: Two independent studies have now used femtosecond pulses from a hard-X-ray free electron laser to push forward the boundaries of structure determination.
Although X-ray crystallography continues to offer glimpses of molecules at atomic resolution, there are hurdles that have to be overcome when using this powerful technology, an example being to grow crystals of sufficient size. One could use smaller crystals and increase the X-ray dose to still obtain a diffraction signal, but this approach is unfeasible, as damage to the sample comes into play. Using a hard-X-ray free electron laser (the Linac Coherent Light Source, or LCLS), two studies have now tried to break this barrier by using pulses of X-rays in the femtosecond range, that is, by attempting to use a timescale so fast as to outrun many damage processes.
In one study, Chapman, Fromme (PSI MPID) and colleagues used the technique to pursue a membrane complex—a complex type notorious for causing headaches to biologists seeking to elucidate structural information. The authors focused on one of the largest complexes previously solved by X-ray crystallography, Photosystem I. By passing a hydrated stream of nanocrystals (2 nm to 2 μm in size) past the source, the authors essentially took snapshots of individual crystals if they happened to be intercepted by an X-ray pulse (photon energy 1.8 keV). The stream constantly replenishes sample, so that in the end the data consist of a series of snapshots of Photosystem I, suggesting that the technology may one day be taken in the direction of examining time-resolved reaction steps. In addition, greater control over the sample relative to the X-ray pulses might lead to even less sample material being required. The structure was presented at 8.5 Å, but use of X-rays of shorter wavelength is likely to increase this resolution further.
In the second study, Hadju, Abergel and colleagues were able to image single mimivirus particles. Mimivirus is the largest known virus, at about 0.45 μm in diameter, and it is difficult to obtain structural information by even cryo-EM because of its size. Here the authors used aerosolized particles and captured snapshots of individual viruses that randomly intercepted LCLS pulses. As in the Photosystem I study, they observed little sample deterioration during the course of the study. The individual mimivirus reconstructions are similar in size and shape to those observed previously by EM. However, the inside of the viral particle was variable, and further analysis may reveal whether there is indeed structural heterogeneity of components in the viral shell. The resolution in this study was at around 32 nm, with shorter, more intense pulses again likely to push this boundary.
The two studies provide great promise for development to further push forward the limits of structural determination and will initiate further work that will hopefully help to widen the bottleneck that is currently represented by crystal growth.