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Imaging biological structures at molecular levels

BioPhotonics
Apr 2011
Compiled by BioPhotonics staff

UPPSALA, Sweden – Imaging individual biological molecules using even the most powerful microscopes has given scientists grief, but with the aid of extremely intensive and ultrashort x-ray pulses from the first free-electron laser, scientists now can capture images of viruses, membrane structures and single-cell organisms.

Paving the way for studies of biological structures at molecular levels, an international research team from Uppsala University and the Swedish University of Agricultural Sciences has captured an image of an intact virus and a membrane structure from a photo-synthetic bacterium with the free-electron laser. The group’s findings were published in two articles in the journal Nature, Feb. 3, 2011 (doi: 10.1038/nature09748 and doi: 10.1038/nature09750).

For years, biologists have dreamed of capturing images of viruses, single-cell organisms and bacteria without having to section, freeze or mark them with metals, a technique necessary for electron microscopy. The new findings now make it possible.

The free-electron laser in the hard x-ray area – the Linac Coherent Light Source – has a light intensity that can cut through steel. A single pulse contains as much energy as all the sunlight hitting the Earth focused on a square millimeter. Its light pulses are extremely short, ~50 to 70 fs, which means that it can replicate an image of a micrometer-size virus before it is heated up and destroyed.

X-ray diffraction, an instrument in identifying biological structures, requires crystallized samples of sufficient size, which means that many particles must be packed into crystals. For single particles, the x-ray dose must be increased so much that the particle is destroyed. However, a few years ago it was suggested that extremely short pulses from a free electron laser could create an image of the particle before it was damaged. The method currently is being tested on biological materials.

Studies have been conducted to test the method on Mimivirus, the world’s largest known virus. Larger than some single-cell organisms and the only virus that can be infected by a virus of its own, its size and special surface structure could not be studied by conventional imaging methods. The team also has proved that x-ray pulses can be used to study the structure of vitally important membrane proteins, most notably the protein complex that captures sunlight and converts it to energy in photosynthesizing organisms.

With the new methodology, scientists can – for the first time – begin to piece the “blank patches” in structural biology together to study at the level of the atom.



GLOSSARY
hard x-ray
A type of x-ray that is capable of deep penetration; its wavelength is about 10-8 cm.
x-ray diffraction
The bending of x-rays by the regular layers of molecules in a crystal acting like a very small diffraction grating. The diffraction pattern so obtained and recorded on film provides a means for analyzing the crystal structure.  
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