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Werner Kühlbrandt
Max Planck Institute of Biophysics
Electron microscopy in molecular cell biology I
Electron optics and image formation
bio
logy
chem
istry
• Galaxy 106 light years 1022 m • Solar system light minutes 1010 m• Planet 20,000 km 107 m• Man 1.7 m 100 m• Fly 3 mm 10-3 m• Cell 50 µm 10-5 m• Bacterium 1 µm 10-6 m• Virus 100 nm 10-7 m• Protein 10 nm 10-8 m• Sugar molecule 1 nm 10-9 m• Atom 0.1 nm = 1Å 10-10 m• Electron 0.1 pm 10-13 m
Objects of interest
• Galaxy 106 light years 1022 m • Solar system light minutes 1010 m• Planet 20,000 km 107 m• Man 1.7 m 100 m• Fly 3 mm 10-3 m• Cell 50 µm 10-5 m• Bacterium 1 µm 10-6 m• Virus 100 nm 10-7 m• Protein 10 nm 10-8 m• Sugar molecule 1 nm 10-9 m• Atom 0.1 nm = 1Å 10-10 m• Electron 0.1 pm 10-13 m
Which objects can we see?R
esol
utio
n lim
it w
ith v
isib
le li
ght
0.1nm=1Å 1nm 10nm 100nm 1µm 10µm 100µm 1mm 10mm
AFM
x-ray crystallography
electron microscopy
light microscopy
organisms
cellsproteins
small molecules
atomsviruses
bacteria
Structural methods in molecular cell biology
NMR
Part 1:
Electron optics
Microscopy with electrons
• Electrons are energy-rich elementary particles
• Electrons have much shorter wavelength than x-rays (~ 3 pm = 3 x 10-12 m for 100kV electrons)
• Resolution not limited by wavelength
• but by comparatively poor quality of magnetic lenses and radiation damage
• High vacuum is essential - no live specimens!
Electron microscopes300 kV 120 kV
The transmission electron microscope
electron gun
projector lenses
objective lens
condenser
viewing screen
projection chamber
specimen
camera
vacuum pump
electron source
specimen
objective lens
The transmission electron microscope
focus plane
diffraction plane
image plane
electron source
specimen
objective lens
The transmission electron microscope
focus plane
diffraction plane
image plane
Electron sources Thermionic emitters Tungsten filament: emits electrons at ~ 3000 °C (melts at 3380 °C) Large source size, poor coherence of electron beam
Lanthanum hexaboride (LaB6) crystal: single crystal, emits electrons at ~ 2000 °C Electrons are emitted from crystal tip -> smaller source size, better coherence Energy spread ΔE ~ 1 - 2 eV, depending on temperature
Field emitters Oriented tungsten single crystal, tip radius ~ 100 nm Small source size -> high coherence of electron beam, high brilliance Requires very high vacuum (10 -11 Torr) Electrons extracted from crystal by electric field applied to tip (extraction voltage)
Schottky emitter: Zr plated, heated to ~ 1200 °C to assist electron emission and prevent contamination, energy spread ΔE ~0.5 eV
Cold field emitter: not heated, very low energy spread, best temporal coherence, but contaminates easily (frequent ‘flashing’ with oxygen gas)
LaB6 vs FEG
FEG
LaB6
Field emission tip
LaB6 filament field emission tip
incoherent electron beam coherent electron beam
Coherence
temporal coherence: all waves have the same wavelength (they are monochromatic)
spatial coherence: all waves are emitted from the same point source (as in a laser)
According to the dualism of elementary particles in quantum mechanics, electrons can be considered as particles or waves. Both models are valid and used in electron optics.
Electrons can be regarded as particles to describe scattering.
The wave model is more useful to describe diffraction, interference, phase contrast and image formation.
Electrons: particles or waves?
Rest mass m0 = 9.1091x10−31
kgCharge Q = −e = −1.602x10−19
CKinetic energy E = eU,1eV = 1.602x10
−19Nm
Rest energy E0 = m0c 2= 511keV = 0.511MeV
Velocity of li ght c = 2.9979x108ms −1
Planck’s constant h = 6.6256x10−34Nms
Non-relativistic (E << E0) Relativistic (E ~ E0)
Mass m = m 0 m = m0 1− v 2 / c2( )−1 / 2
Energy E = eU =12
m0v2
mc 2= m0c2 + eU = E 0 + E
m = m0 1 + E / E0( )
Velocity v = 2E / m0( )1 / 2
v = c [1−1
1+ E / E0( )2
1 / 2
Momentum p = m 0v = 2m 0E( )1/ 2
p = mv = 2m0E 1+ E /2E0 ( )[ ]1 / 2
=1c
2EE0 + E2
( )1 / 2
deBrogli e wavelength λ =hp = h 2m0E( )
−1 / 2
λ = h 2m0 E 1+ E / 2E0( )[ ]− 1 / 2
=hc 2EE0 + E2
( )−1 / 2
Electron parameters
]
Electrons in an electrostatic (Coulomb) field
The force produced by an electrostatic field E on a particle of (negative) charge e- is
F = -eE
This means that the electron is accelerated towards the anode. The kinetic energy E [eV] of the electron after passing through a voltage U [V] is
E = eU
The wavelength of an electron moving at veolicty v (de Broglie wavelength) is
λ = h/mv where h is Planck’s constant, m the rest mass of the electron. Both the mass and the velocity of the electron are energy dependent. Relativistic treatment is necessary for acceleration voltages > 80 kV.
Electron beams are monochromatic except for a small thermal energy spread arising from the temperature of the electron emitter.
de Broglie wavelength
acceleration velocity wavelength voltage
100 kV 1.64 x 108 m/s 3.7 pm = 0.037 Å
200 kV 2.5 pm = 0.025 Å
300 kV 2.0 pm = 0.02 Å
Highest possible resolution:
1/d = 2/λ
d = λ/2
θ = 90°
Resolution limit in crystallographyReciprocal space!
2 sin θ = (1/d)/(1/nλ)
Resolution limit
Diffraction (Rayleigh) limit: ~ 0.5 λ
• ~150 nm = 1,500 Å for visible light• 0.5 Å for x-rays• 0.01 Å = 1 pm for 300 kV electrons
electron source
specimen
objective lens
The transmission electron microscope
focus plane
diffraction plane
image plane
E
E-ΔE
E E -ΔEE E -ΔE
hν
Z = 0
Z = t
E-ΔE
Incident beam of energy E
elastic inelastic
Electron-‐specimen interactions
R. Henderson, Quart. Rev. Biophys. 1995
scattering cross section
radiation damage
Scattering of electrons, X-rays and neutrons
~105
10 x 8000 eV
4 x 20 eV
carbon
~105
10 x 8000 eV
4 x 20 eV
Large elastic cross section • ~106 higher than for x-‐rays high signal per scattering event
… but even larger inelastic cross section • σinel/σel ≈ 18/Z high radiation damage, especially for light atoms, e.g. C: 3 electrons scattered inelastically per elastic event
Good ratio of elastic/inelastic scattering • 1000 times better than for 1.5 Å x-‐rays
carbon
Energy deposited per elastic event
Electrons 80-‐500 keV
X-‐rays 1.5 Å
Neutrons 1.8 Å
Ratio inelastic/elastic 3 10 0.08
Energy deposited per inelastic event
20 eV 8000 eV 2000 eV
Effective energy deposited per elastic event
60 eV 80 000 eV 160 eV
Considerations for biological specimens
• Biological specimens consist mainly of light atoms (H, N, C, O), which suffer most from radiation damage
• This makes it essential to minimize the electron dose to about ~25 e/Å2 for single-‐particle cryoEM
• Low electron dose means low image contrast
electron source
specimen
objective lens
The transmission electron microscope
focus plane
diffraction plane
image plane
Electrons in a magnetic field Lorentz force on a charge e- moving with velocity v in a magnetic field B (vectors are bold): F = -e (v x B)
The direction of F is perpendicular on v and B. v, B, and F form a right-handed system (right-hand rule, where v = index finger, B = middle finger and c = thumb)
In a homogenous magnetic field, if
v = 0 then F = 0
v is perpendicular to B: the electron is forced on a circle with a radius depending on the field strength |B|
v is parallel to B: no change of direction
Note that unlike the electric field, the magnetic field does not change the energy of the electron, i. e. the wavelength does not change!
Electromagnetic field
Passing a current through a coil of wire produces a strong magnetic field in the centre of the coil. The field strength depends on the number of windings and the current passing through the coil.
Electromagnetic Lens
Pole pieces of soft ironconcentrate the magnetic field
Electromagnetic Lens
The magnetic field is rotationally symmetric and changes gradually
A light beam is refracted (bent) when the wave enters a medium of a different optical density. The refractive index changes abruptly at the interface.
Refraction in light optics
Path of electron moving with velocity v in magnetic field B of a ring coil, exerting force F
Diagram by Elena Orlova, Birkbeck College London
Magnetic lens
Magnetic lenses are electromagnetic coils with soft iron pole pieces. The strength of the magnetic field depends on the number of windings NI and on the pole piece gap. The field is cylindrically symmetrical about the optical axis. The field strength B(z) along the optical axis is bell-shaped. Upon passing through the lens, the image rotates around the optical axis.
The light microscope is focussed by varying the object distance. The electron microscope is focussed by varying the focal length of the lens f.
The lens diagram looks the same for glass and magnetic lenses
grid square with holey carbon film
20 µm x 20 µm
600x
EM grid
2mm x 2mm
hole
2µm x 2µm
6,000x60x 60,000x
FAS in vitreous buffer
0.2 µm x 0.2 µm
Magnification steps
electron source
specimen
objective lens
The transmission electron microscope
focus plane
diffraction plane
image plane
Electron diffraction
diffraction angle
2θ
Bragg’s law: nλ = 2d sinθ
d
Electron diffraction
diffraction angle
2θ
Bragg’s law: nλ = 2d sinθ
d
Elastic scattering by atoms (Rayleigh scattering)
n is an integer: scattered waves are in phase, resulting in constructive interference
Bragg’s law: nλ = 2d sinθ
n is not an integer: scattered waves are out of phase, resulting in destructive interference
θd
path length difference nλ x 2 (sin θ = nλ/d)
Interference
constructive interference destructive interference
superposition
1st wave
2nd wave
Highest possible resolution:
1/d = 2/λ
d = λ/2
θ = 90°
Resolution limit in crystallographyReciprocal space!
2 sin θ = (1/d)/(1/nλ)
Large 2D crystal of LHC-II
2D crystals of LHC-II
Kühlbrandt, Nature 1984
Electron diffraction pattern: amplitudes, no phases
3.2 Å
The image is generated by interference of the direct electron beam with the diffracted beams
Part 2:
Phase contrast and image formation
electron source
specimen
objective lens
diffraction plane
image plane
The transmission electron microscope
focus plane(UNDERfocus, i.e. weaker objective lens)
E
E-ΔE
E E -ΔEE E -ΔE
hν
Z = 0
Z = t
E-ΔE
Incident beam of energy E
elastic inelastic
Electron-‐specimen interactions
Contrast mechanisms
no interaction
phase object
amplitude object
modified phase
electron wave
modified amplitude, unchanged phase
modified phase, unchanged amplitude
Image
amplitude contrast
phase contrast shorter
wavelength within object
The scattered electron wave experiences three phase shifts
by the imaged object this carries the information about the structure and is the one we need to determine
by the inner potential of the specimen this is 90° for all scattered waves
by the spherical aberration of the objective lens this depends on the scattering angle (= resolution) and gives rise to the oscillation of the contrast transfer function (CTF)
(note that this only holds for the weak phase approximation!)
Origin of the phase shift
€
The potential V(x,y,z) experienced by the scattered electron upon passing through a sample of thickness t causes a phase shift
The vacuum wavelength λ of the electron changes to λ` within the sample:
When passing through a sample of thickness dz, the electron experiences a phase shift of:
€
Vt (x,y) = V (x,y,z)dz0
t
∫
€
λ =h2meE
€
" λ =h
2me(E +V (x,y,z))
€
dϕ = 2π (dz$ λ −dzλ)
€
dϕ =πλE
V (x,y,z)dz =σV (x,y,z)dz
ψincident
ψexit
V(x,y,z)
(how many times does λ fit into the thickness dz?)(by how many degrees out of 2π = 360° does this change the phase φ?)
The potential V of the specimen shift modifies the phase of the incident plane wave ψincident to:
àFor thin biological specimens there is a linear relation between the transmitted exit wave function and the projected potential of the sample:
€
ϕ =σ V (x,y,z)dz∫ =σVt (x,y)As we have seen, the phase shift is proportional to the projected potential of the specimen:
€
Ψexit =Ψincidente− iϕ =Ψincidente
−iσVt
€
Ψexit ≈ Ψincident
≡1(background )! " # (1− iϕ) =1− iϕ =1− iσVt
€
Ψexit =Ψincidente− iϕ
≈ Ψincident (1− iϕ) =Ψincident − iΨincidentϕ =Ψincident − iΨscattered
For Vt<<1 and thus φ<<2π as for thin biological specimens:
€
i = eiπ2 = cos(π2 ) + isin(π2 )
ψincident
ψexit
V(x,y,z)
The weak phase approximation
Taylor series
€
f (n )(a)n!n=0
∞
∑ (x − a)n
€
sin(x) ≈ x − x3
3!+x 5
5!−x 7
7!+O(xn )
€
ex ≈1+ x +x 2
2!+x 3
3!+O(xn )
€
e−iϕ ≈1− iϕ +ϕ2
2!− iϕ
3
3!+O(ϕn )
≈1− iϕ if φ << 1 !
taken from Wikipedia...
Any function can be approximated by a finite number of polynomial terms
à
à
à
à
Phase contrastexit wave = scattered wave + unscattered wave
exit wave
imaginary
real
Ψexit = ψunscattered + iψscattered
Vector notation. Vector length shows amplitude (not drawn to scale)
Cosine wave notation (amplitudes not drawn to scale)
Scattered wave is 90° out of phase with unscattered wave.
Phase contrastexit wave = scattered wave + unscattered wave
à Positive phase contrast A perfect lens in focus gives minimal phase contrast
Since ψscattered = φψunscattered with φ << 1:
ψscattered << ψunscattered
Iimage= |ψexit|2 =| ψunscattered + iψscattered|2 ≈ |ψunscattered|2 = Iincident
exit wave
Ψexit = ψunscattered + iψscattered
Lens aberrations cause an additional phase shift that generates phase contrast
Minimal contrast
Minimal contrast
Maximal contrast
Defocus phase contrast
exit wave = scattered wave + unscattered wave
Phase contrast
Lens aberrations
Spherical aberration Chromatic aberration
Lens with spherical aberration
Image plane
Perfect lensImage plane
f1f2
χ = e.g. 3λ/2
Spherical aberration causes a path length difference χ which depends on the scattering angle
In a perfect lens, the number of wavelengths from object to focal point is the same for each ray, independent of scattering angle (Fermat’s principle)
Defocus generates an additional phase shift
Depending on its phase shift, the scattered wave reinforces or weakens the exit wave
Ψexit = ψunscattered + iψscattered
exit wave = scattered wave + unscattered wave
scattered and unscattered wave are in phase: exit wave gets stronger
scattered and unscattered wave are out of phase by π = 180° (half a wavelength): exit wave gets weaker
scattered wave is shifted by π/2 = 90° (quarter of a wavelength) relative to unscattered wave: exit wave is nearly same as unscattered wave, therefore phase contrast is minimal
The phase shift χ(θ) introduced by the spherical aberration Cs of the objective lens depends on the scattering angle θ and on the amount of defocus Δf.
χ(θ) = (2π/λ) [Δf θ2/2 - Cs θ4/4]
The spatial frequency R (measured in Å -1) is the reciprocal of the distance between any two points of the specimen. For small scattering angles, R ≈ θ / λ. With this approximation, the phase shift expressed as a function of R is
χ(R) = (π/2) [2Δf R2 λ - Cs R4 λ3] (Scherzer formula)
At Scherzer focus, the terms containing Δf and Cs nearly add up to 1, and a positive phase shift of χ ≈ +π/2 applies to a wide resolution range.
Optimal (Scherzer) focus
Defocus and spherical aberration have similar but opposite effects
exact focus
Scherzer focus
high underfocus
Contrast transfer in
(Erikson and Klug, 1971)
The phase Contrast Transfer Function B(θ) of the electron microscope is defined as
B(θ) = -2 sin χ(θ)
For any particular defocus Δf, the CTF indicates the range of scattering angles θ in which the object is imaged with positive (or negative) phase contrast.
For scattering angles where CTF = 0, there is no phase contrast.
As spherical aberration Cs is constant, Δf can be adjusted to produce maximum phase contrast of ±π/2 for a particular range of scattering angles.
The contrast transfer function (CTF)
Fourier transforms of amorphous carbon film images
Contrast Transfer Function, CTF
underfocus underfocus
FT FT
CTF as a function of defocus
Movie by Henning Stahlberg, UC Davis
Phase contrast at high resolution is reduced (damped) by incoherence
Spatial and temporal incoherence of the electron beam damp the CTF due to lens aberrations:
Edamping(k) = Espatial(Z,Cs,α,k) • Etemporal(Cc,ΔE,k) • Especimen• Edetector• ….
CS is the spherical aberration coefficient, CC is the coefficient for chromatical aberration
spatial frequency [1/nm]
Z=100 nm, CS= 3 mm, CC = 3 mm Z=100 nm, CS= 0 mm, CC = 3 mm Z=100 nm, CS= 0 mm, CC = 0 mm
Chromatic aberration (Cc)
corrector
Max Haider
electron source
specimen
objective lens
diffraction plane
image plane
The transmission electron microscope
focus plane
Janet Vonck, Deryck Mills
Film image
CCD vs. direct detector
300 kV electrons
conventional CCD detector
Direct electron detector
Richard Henderson, Greg McMullan (MRC-LMB Cambridge, UK)
Scintillator
Fiber optics
CCD chip
Peltier coolerSeparate vacuum
-35°C
50 µm• CMOS active pixel sensor• 5 - 15 µm pixels• radiation-hard• back-thinned• rapid readout (17 - 400 Hz)
DQE of electron detectors (300 kV)
McMullan, Faruqi and Henderson http://arxiv.org/pdf/1406.1389.pdf
DQE = SNR2o/SNR2i
counting mode,
CCD
Greg McMullan, MRC LMB Cambridge
Matteo Allegretti, Janet Vonck, Deryck Mills
Fast readout overcomes drift
Frame displacement in Å
3.36 Å
Science, 28 March 2014Science, 28 March 2014