Computational
Technology for
High-Throughput Cryo-Electron
Microscopy
Introduction
High resolution electron microscopy (EM) has grown to become
an important new technology within the field of structural
biology. However, in order to achieve high resolution images, the
data set becomes so large, that computing and interpreting data becomes
a major rate-limiting factor. Therefore, this program project was
proposed in order to develop a software that will speed up the process
of computation, thus achieving high resolution images in a reasonable
amount of time.
The program project is a collaboration between multiple organizations,
spanning over fields of structural biology and computing science.
It is funded by the National Health Institute and divided into seven
research projects, involving scientists from the Lawrence Berkeley
National Laboratory, Baylor College of Medicine, Houston Medical
School, and Wadsworth Center in New York.
What is cryo-electron microscopy?
In order to see an object, the wavelength of the
light beam that penetrates the object must be smaller than the object
itself. That is where electron microscopy becomes useful.
Since electron beams have such short wavelengths, it allows scientists
to see both the surface and internal structure of very small things,
which is an advantage in many fields of research.
There are two types of electron microscopes: scanning (SEM) and
transmission (TEM). When using SEM, the sample is coated with a
metal that reflects electrons, which then provides a conducting surface
for the electrons to avoid charging of the sample. The electron
beam is condensed into a small beam scanning over the object. the
image is formed when electrons that bounce off the sample are collected
onto the imaging screen. SEM would produce an image of the
surface of the sample but not the internal structure.
A TEM would produce an image that is a projection of the entire object,
both internal and external structures. Just as its name suggests,
the electron beam of the TEM actually passes through the entire
thickness of the sample. However, since the projection of the
sample in two-dimensional against he view screen, the relations in the
z-axis between the structures are lost. Also, since the electron
beam penetrates and interacts with the sample, the sample needs to be
very thin so as to not absorb the electrons and not become damaged by
the beam.
This is where the "cryo" part comes from. Many biological
molecules need a solvent to remain stable (usually water/salt solution
would do). In order to avoid evaporation of the solvent during
observation under and electron microscope, the sample is treated with
cryogen in order to freeze the solvent in place around the
molecule. Also, cryogen can freeze samples very quickly so that
cubic ice, which readily absorbs the electron beam and thus obscuring
the sample, can not form.
The
TEM actually works in a very similar way as the optical
microscope. At the top of the column, a very high voltage is
supplied, and the electron beam is sent out through the filament, or
electron gun, which is most often a tungsten or lanthanum hexaboride
(LaB6). The needle is superheated until
enough energy is produced to overcome the work function of the metal,
which would cause it to emit electrons. Then the beam passes
through a series of lenses, apertures, and of course, the sample.
The "lenses" are actually magnetic coils used to direct the electron
beam as it comes down. finally, the image is produced on the
viewing screen.
Sounds intriguing? Then explore our websites further to find out what
our program project is really about.
Related
Organizations:
Lawrence
Berkeley National Laboratory
Baylor College of Medicine
Houston Medical School,
University of Texas
Wadsworth
Center, NYS Department of Health
National Institute
of Health
.
