Biology by X-ray: ultra-short pulses, less damage

Ezine

Published: Feb 15, 2011
Author: David Bradley
Channels: X-ray Spectrometry

thumbnail image: Biology by X-ray: ultra-short pulses, less damage

Pulsing news for structural biologists

Researchers have discovered that ultra-short X-ray pulses can produce exquisite measurements at the molecular level of biological objects by grabbing a "snapshot" just before the sample succumbs to radiation damage.

Two papers in Nature this month reveal the first hard X-ray free-electron laser, the Linac Coherent Light Source, at the Department of Energy's SLAC National Accelerator Laboratory, might change the way biologists study life. In one paper, an international team of researchers describe a shortcut for determining protein structures. They use nanocrystals of the target molecule and so circumvent one of the biggest problems facing structural biologists - how to make large crystals of the proteins of interest. There are currently tens of thousands of proteins awaiting such a shortcut. This technique could accelerate research in the field considerably, with particular gains to be made in structural studies of membrane proteins, which are notoriously difficult to crystallise.

The other paper describes work by Henry Chapman of the Center for Free-Electron Laser Science at the German national laboratory DESY and Janos Hajdu of Sweden's Uppsala University, and some 80 colleagues from 21 institutions. They performed experiments at LCLS just two months after it opened and have now revealed the data to demonstrate just how powerful this source could be. They obtained the first single-shot images of intact viruses, paving the way for snapshots and movies of molecules, viruses and live microbes in action. Their approach uses pulses so short that data can be obtained before the radiation destroys the sample.

Hard, yet somehow gentle

"The LCLS beam is a billion times brighter than previous X-ray sources, and so intense it can cut through steel," Chapman says. "Yet these incredible X-ray bursts are used with surgical, microscopic precision and exquisite control, and this is opening whole new realms of scientific possibilities," including the ability to observe atoms moving and chemical bonds forming and breaking in real time.

The team sprayed their virus particles, or nanocrystals, into the path of the X-ray beam and strobed them with a very short, femtosecond, laser pulse that gathers all the information needed to make an image before the sample is destroyed. This technique was proposed by Hajdu almost a decade prior to these pioneering experiments but researchers at Arizona State University, Lawrence Livermore National Laboratory, SLAC and Uppsala have spent years developing the necessary specialist equipment for injecting samples into the beam, while the team at Germany's Max Planck Advanced Study Group brought in a 10-tonne, $7 million instrument called CAMP to record every single photon of data with a fast, ultra-sensitive X-ray camera for later analysis.

Tests at DESY and Lawrence Berkeley National Laboratory demonstrated proof of principle at lower X-ray energies. But the researchers needed to know whether or not it would still be possible to beat the radiation damage at higher energies, apparently you can. The physics still holds. The protein structure used as the first target was Photosystem I, the solar conversion unit in photosynthesis, which is itself a membrane protein. Membrane proteins elsewhere in biology are important drug targets for a wide range of diseases. Indeed, almost two-thirds of pharmaceutical drugs on the market are targeted at membrane proteins, yet biomedical scientists know the X-ray structures of a mere six of the estimated 30,000 membrane proteins in the human body; because of the crystallisation bottleneck.

Proteins sprayed through a bottleneck

To squeeze through this bottleneck the team sprayed millions of nanocrystals of Photosystem I across the X-ray beam, laser pulses are scattered shedding the data into the detectors but as each nanocrystal is vaporised another is almost instantaneously replacing it so that millions of snapshots can be obtained from which 10,000 or so are selected to allow the team to obtain a good match for the known molecular structure of Photosystem I, thus allowing the researchers to effectively calibrate the technique for proteins of unknown structure. The same approach could also be used with mimivirus viral particles, although fewer successful images are collected even with millions of particles passing through.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

Researchers have discovered that ultra-short X-ray pulses can produce exquisite measurements at the molecular level of biological objects by grabbing a

A three-dimensional rendering of X-ray data obtained from over 15,000 single nanocrystal diffraction snapshots recorded at the Linac Coherent Light Source, the world's first hard X-ray free-electron laser.