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Electron crystallography
is a method to determine
protein structures using
electron diffraction. It is
conducted with an
electron microscope, usually
on
proteins (such as
membrane proteins), that
cannot easily form the large
3-dimensional
crystals required for
X-ray crystallography. Rather,
structures are usually determined
from either 2-dimensional crystals
(sheets or
helices),
polyhedrons such as
viral capsids, or dispersed
individual proteins. Electrons can
be used in these situations,
whereas
X-rays cannot, because
electrons interact more strongly
with proteins than X-rays do.
Thus, X-rays will travel through a
thin 2-dimensional crystal without
diffracting significantly, whereas
electrons can be used to form an
image. Conversely, the strong
interaction between electrons and
proteins makes thick (e.g.
3-dimensional) crystals impervious
to electrons, which only penetrate
short distances.
One of the main difficulties in
X-ray crystallography is
determining
phases in the
diffraction pattern. Because
no X-ray
lens exists, X-rays cannot be
used to form an image of the
crystal being diffracted, and
hence phase information is lost.
Fortunately, electron microscopes
contain electron lenses, and phase
information tends to be much more
reliable in electron
crystallography.
Conversely, because X-ray
crystallography uses 3-dimensional
crystals, one is able to
simultaneously gather diffraction
patterns from orders of magnitude
more proteins than what can be
achieved in electron
crystallography, enhancing
signal-to-noise ratios. For this
reason, X-ray crystallography has
been much more successful in
determining large numbers of
protein structures.
A common problem to X-ray
crystallography and electron
crystallography is
radiation damage, by which
proteins are damaged as they are
being imaged, limiting the
resolution that can be obtained.
This is especially troublesome in
the setting of electron
crystallography, where that
radiation damage is focused on far
fewer proteins. One technique used
to limit radiation damage is
electron cryomicroscopy, in
which the samples undergo
cryofixation and imaging takes
place at
liquid nitrogen or even
liquid helium temperatures.
The first electron
crystallographic protein structure
to achieve atomic
resolution was
bacteriorhodopsin, determined
by
Richard Henderson and
coworkers at the
Medical Research Council
Laboratory of Molecular Biology
in
1990. Since then, several
other high-resolution structures
have been determined by electron
crystallography, including the
light-harvesting complex, the
nicotinic acetylcholine receptor,
and the bacterial
flagellum.
