Microcrystal electron diffraction, or MicroED,
is a
CryoEM
Cryogenic electron microscopy (cryo-EM) is a cryomicroscopy technique applied on samples cooled to cryogenic temperatures. For biological specimens, the structure is preserved by embedding in an environment of vitreous ice. An aqueous sample sol ...
method that was developed by the
Gonen laboratory in late 2013 at the
Janelia Research Campus
Janelia Research Campus is a scientific research campus of the Howard Hughes Medical Institute that opened in October 2006. The campus is located in Loudoun County, Virginia, near the town of Ashburn. It is known for its scientific research and m ...
of the
Howard Hughes Medical Institute
The Howard Hughes Medical Institute (HHMI) is an American non-profit medical research organization based in Chevy Chase, Maryland. It was founded in 1953 by Howard Hughes, an American business magnate, investor, record-setting pilot, engineer, fil ...
. MicroED is a form of
electron crystallography
Electron crystallography is a method to determine the arrangement of atoms in solids using a transmission electron microscope (TEM).
Comparison with X-ray crystallography
It can complement X-ray crystallography for studies of very small crystals ...
where thin 3D crystals are used for structure determination by
electron diffraction
Electron diffraction refers to the bending of electron beams around atomic structures. This behaviour, typical for waves, is applicable to electrons due to the wave–particle duality stating that electrons behave as both particles and waves. Si ...
.
The method was developed for
structure determination of proteins from
nanocrystal
A ''nanocrystal'' is a material particle having at least one dimension smaller than 100 nanometres, based on quantum dots
(a nanoparticle) and composed of atoms in either a single- or poly-crystalline arrangement.
The size of nanocrystals dist ...
s that are typically not suitable for X-ray diffraction because of their size. Crystals that are one billionth the size needed for X-ray crystallography can yield high quality data. The samples are frozen hydrated as for all other CryoEM modalities but instead of using the
transmission electron microscope
Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a gr ...
(
TEM Tem or TEM may refer to:
Acronyms
* Threat and error management, an aviation safety management model.
* Telecom Expense Management
* Telecom Equipment Manufacturer
* TEM (currency), local to Volos, Greece
* TEM (nuclear propulsion), a Russian ...
) in imaging mode one uses it in
diffraction
Diffraction is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. The diffracting object or aperture effectively becomes a s ...
mode with an extremely low electron exposure (typically < 0.01 e
−/Å
2/s). The nano crystal is exposed to the diffracting beam and continuously rotated
while diffraction is collected on a fast camera as a movie.
MicroED data is then processed using traditional software for
X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
without the need for specialized software for structure analysis and refinement.
Importantly, both the hardware and software used in a MicroED experiment are standard and broadly available.
Development
The first successful demonstration of MicroED was reported in 2013 by the
Gonen laboratory.
The structure of
lysozyme
Lysozyme (EC 3.2.1.17, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside ...
, a classic test protein in
X-ray crystallography
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
. The Abrahams group independently reported rotation
electron diffraction
Electron diffraction refers to the bending of electron beams around atomic structures. This behaviour, typical for waves, is applicable to electrons due to the wave–particle duality stating that electrons behave as both particles and waves. Si ...
data collection using a
Medipix
Medipix is a family of photon counting and particle tracking pixel detectors developed by an international collaboration, hosted by CERN.
Design
These are hybrid detectors as a semiconductor sensor layer is bonded to a processing electronics lay ...
quantum area detector on
lysozyme
Lysozyme (EC 3.2.1.17, muramidase, ''N''-acetylmuramide glycanhydrolase; systematic name peptidoglycan ''N''-acetylmuramoylhydrolase) is an antimicrobial enzyme produced by animals that forms part of the innate immune system. It is a glycoside ...
crystals but were unable to solve the structure.
Experimental setup
Detailed protocols for setting up the electron microscope and for data collections have been published.
Instrumentation
Microscope
MicroED data is collected using
transmission electron (cryogenic) microscopy. The microscope must be equipped with a selected area aperture to use
selected area diffraction
Selected area (electron) diffraction (abbreviated as SAD or SAED), is a crystallographic experimental technique typically performed using a transmission electron microscope (TEM). It is a specific case of electron diffraction used primarily in ma ...
.
Detectors
A variety of detectors have been used to collected electron diffraction data in MicroED experiments. Detectors utilizing
charge-coupled device (CCD) and
complementary metal–oxide–semiconductor (CMOS) technology have been used. With CMOS detectors, individual electron counts can be interpreted.
Data collection
Still diffraction
The initial proof of concept publication on MicroED used lysozyme crystals.
Up to 90 degrees of data were collected from a single nano crystal, with discrete 1 degree steps between frames. Each diffraction pattern was collected with an ultra-low dose rate of ∼0.01 e
−/Å
2/s. Data from 3 crystals was merged to yield a 2.9Å resolution structure with good refinement statistics, and represented the first time electron diffraction had been used successfully to determine the structure of a dose-sensitive protein from 3D microcrystals in cryogenic conditions.
Continuous stage rotation
Shortly after the proof of principle paper MicroED was improved by applying continuous rotation during the data collection scheme.
Here the crystal is slowly rotated in a single direction while diffraction is recorded on a fast camera as a movie. The methodology is like the rotation method in x-ray crystallography. This led to several improvements in data quality and allowed data processing using standard X-ray crystallographic software.
Benefits of continuous rotation MicroED include a decrease in dynamical scattering and improved sampling of reciprocal space. Continuous-rotation is the standard method of MicroED data collection since 2014.
Data processing
Detailed protocols for MicroED data processing have been published.
When MicroED data is collected using continuous stage rotation, standard
crystallography software can be used.
Differences between MicroED and other electron diffraction methods
Other electron diffraction methods that have been developed for material science of radiation insensitive material like inorganic salts include Automated Diffraction Tomography (ADT)
and Rotation Electron Diffraction (RED
). These methods significantly differ from MicroED: In ADT discrete steps of
goniometer tilt are used to cover reciprocal space in combination with beam precession to fill in the gaps.
ADT uses specialized hardware for precession and scanning transmission electron microscopy for crystal tracking.
RED is done in TEM but the goniometer is coarsely tilted in discrete steps and beam tilting is used to fill in the gaps.
Specialized software is used to process ADT and RED data.
Importantly, ADT and RED were developed and tested on radiation insensitive inorganic materials and salts and have not been demonstrated for use with proteins or radiation sensitive organic material studied in a frozen hydrated state.
Milestones
Method scope
MicroED has been used to determine the structures of large globular proteins, small proteins,
peptides,
membrane proteins,
organic molecules,
and inorganic compounds. In many of these examples hydrogens and charged ions were observed.
Novel structures of α-synuclein of Parkinson's disease
The first novel structures solved by MicroED were published in late 2015.
These structures were of peptide fragments that form the toxic core of
α-synculein, the protein responsible for
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
and lead to insight into the aggregation mechanism toxic aggregates. The structures were solved at 1.4 Å resolution.
Novel protein structure of R2lox
The first novel structure of a protein solved by MicroED was published in 2019.
The protein is the metalloenzyme R2-like ligand-binding oxidase (R2lox) from Sulfolobus acidocaldarius. The structure was solved at 3.0 Å resolution by molecular replacement using a model of 35% sequence identity built from the closest homolog with a know structure. This work demonstrated that MicroED could be used to obtain unknown structure of protein.
Access to MicroED education and services
To learn more about MicroED, one can attend the annua
MicroED Imaging Center Course at UCLAor th
. For more up to date information about upcoming meetings and workshops related to
Cryogenic electron microscopy
Cryogenic electron microscopy (cryo-EM) is a cryomicroscopy technique applied on samples cooled to cryogenic temperatures. For biological specimens, the structure is preserved by embedding in an environment of vitreous ice. An aqueous sample so ...
methods as a whole, please check th
3DEM Meetings and Workshops page
Several universities and companies offer MicroED services, including th
MEDIC – Microcrystal Electron Diffraction Imaging Center at UCLAan
Nanoimaging Services
Several electron microscope systems are capable of recording MicroED data including those developed by JEOL; and Thermo Fisher/FEI.
References
Further reading
**
ttps://www.thermofisher.com/us/en/home/electron-microscopy/life-sciences/microed.html?playlistVideoId=6125661804001 Video interview about the development of MicroED and its applicationsThe Janelia Archives background on MicroEDBackground and publications on MicroED from the Gonen Laboratory
{{Crystallography
Electron microscopy