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Issue Two 2012 - Issue #16

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Through the water window

Story by James Hutson

View articles in related topics: Optical Physics, Molecular Science, Health & Medical

Revealing the detailed structure of the protein molecules in the membrane of biological cells is key to the development of new antibiotics and medication treatments for cancer and other chronic diseases.

Many important pharmaceuticals are dependent on the activity of proteins in the cell membrane; however, little is known about the structure and function of many important membrane proteins because they cannot be studied at the atomic level using current imaging techniques.

Tabletop solution
Professor Lap Van Dao leads a team of Swinburne researchers on the path to developing an affordable, tabletop imaging solution for researchers to study and define the structure of protein molecules in the membrane of living cells.

Currently, imaging of these molecular protein structures is mostly limited to the small number of multimillion-dollar synchrotrons and $3 billion-dollar free-electron laser accelerators such as Stanford University’s kilometre-long Linac Coherent Light Source (LCLS). At LCLS the X-ray light is so intense that samples need to be imaged as they disintegrate in its brightness.

Extremely high demand means use of these facilities is very competitive, extremely limited and incurs high costs, requiring researchers to work in shifts to maximise their access.

“A tabletop solution wouldn’t replace these hugely expensive facilities, but would provide an affordable, compact, versatile, parallel tool for studying atomic, molecular and biological systems,” says Swinburne’s Emeritus Professor Peter Hannaford, a co-investigator on the ARC Centre of Excellence for Coherent X-ray Science project led by the Swinburne Centre for Atom Optics and Ultrafast Spectroscopy.

Generating high order harmonics
In a major milestone, the Swinburne team recently succeeded in generating bright, intense and laser-like soft X-rays at the four- nanometre (nm) wavelength in the centre’s laboratories, generating very high order (350th) harmonics from an infrared laser beam.

It’s an achievement that has been five years in the making, starting in 2007 with visible light at the 800 nm wavelength and progressively reducing to 30 nm in 2008, 10 nm in 2009 and 4 nm in 2011, while all the time increasing the intensity and reducing the divergence of the soft X-ray beam.

Only by understanding the processes at work has the team been able to generate high-energy photons and control the process to emit extremely intense laser-like beams in the wavelength within the “water window” (between 2.3 nm and 4.4 nm) where the light is not absorbed by water but strongly absorbed by carbon.

But the beam also has to be finely focused to capture the required detail of the proteins’ molecular structure.
Recent upgrades to the laser system should allow even shorter, more intense pulses that will result in better images.

The team is also trialling alternative approaches. Dr Michael Pullen and PhD student Naylyn Gaffney are passing the laser through helium and hydrogen gas sprayed out through a lawn sprinkler-like array of individually controlled gas jets. This should allow the laser to be tuned to emit a narrow bandwidth of X-ray photons.

Computer reconstruction
Lenses absorb too much of this soft X-ray radiation to be of much use in focusing the light. Instead, the team uses a lens-less form of imaging called coherent diffractive imaging (CDI).

This points the soft X-rays at a sample, recording the resulting scattered photon diffraction pattern. From this, co-members of the ARC Centre of Excellence at The University of Melbourne – Associate Professor Harry Quiney, and Drs Ruben Dilanian and Bo Chen – create a computer reconstruction of the sample. It is a little like reconstructing an image of a disco ball when you can only see the light it reflects onto the surrounding surfaces.

Professor Dao says the work so far all points to the feasibility of “water window” imaging technology that is accessible (in size and cost) for a large number of research laboratories, allowing high-resolution imaging of these sub-cellular structures in their live, water-rich cellular environment.

“It won’t replace the large free-electron laser imaging facilities, but my hope is that it will accelerate biological research and the development of new drug treatments.”



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