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Electron Spin Resonance Spectroscopy: A Renaissance

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Electron Spin Resonance Electron Spin Resonance Spectroscopy: A Renaissance Spectroscopy: A Renaissance Jack H. Freed Department of Chemistry and Chemical Biology & ACERT Cornell University Ithaca, NY, USA National Center for Research Resources NATIONAL INSTITUTES OF HEALTH NATIONAL INSTITUTES OF HEALTH ACS 235 ACS 235 th th National Meeting National Meeting Physical Chemistry Awards Symposium New Orleans, LA April 8, 2008
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Page 1: Electron Spin Resonance Spectroscopy: A Renaissance

Electron Spin Resonance Electron Spin Resonance Spectroscopy: A RenaissanceSpectroscopy: A Renaissance

Jack H. FreedDepartment of Chemistry and Chemical Biology & ACERT

Cornell UniversityIthaca, NY, USA

National Center for Research ResourcesNATIONAL INSTITUTES OF HEALTHNATIONAL INSTITUTES OF HEALTH

ACS 235ACS 235thth National MeetingNational MeetingPhysical Chemistry Awards Symposium

New Orleans, LA April 8, 2008

Page 2: Electron Spin Resonance Spectroscopy: A Renaissance

A RENAISSANCEA RENAISSANCEIn the 1960’s both ESR & NMR were of comparable interest to physical

chemists. During the 1970’s & 1980’s NMR assumed its great prominence in

chemistry, biology & physics that continues to this day.In the last decade or so, new developments have led to a revitalization

of ESR which parallels the earlier developments in NMR.

KEY DEVELOPMENTS & THEIR APPLICATIONS INCLUDE:1. Very-High-Field & Frequency ESR: Quasi-Optical Methods.2. Improved Modeling of Dynamic ESR Spectra: Stochastic Liouville

Equation.3. Two-Dimensional Fourier-Transform ESR: Intense Nano-second

cm.- & mm.-wave coherent pulses.4. Pulsed Dipolar ESR Spectroscopy & Protein Structure5. ESR Microscopy

Page 3: Electron Spin Resonance Spectroscopy: A Renaissance

Introduction: What is special about ESR, in particular spin-label ESR? (e.g. compared to NMR)

1. ESR is much more sensitive per spin (than NMR).

2. In time domain experiments ESR’s time-scale is nanoseconds (NMR’s is milliseconds).

3. The spin-label spectrum is simple, & can focus on a limited number of spins.

4. ESR spectra change dramatically as the tumbling motion of the probe slows, thereby providing great sensitivity to local “fluidity”.

In NMR nearly complete averaging occurs, so only residualrotational effects are observed by T1 & T2.

5. Multi-frequency ESR permits one to take “fast-snapshots” using very high-frequencies & “slow-snapshots” using lower frequencies to help unravel the complex dynamics of bio-systems.

6. Pulsed ESR methods enable one to distinguish homogeneous broadening reporting on dynamics vs. inhomogeneous broadening reporting on local structure.

Molecular Dynamics by ESRMolecular Dynamics by ESR

Page 4: Electron Spin Resonance Spectroscopy: A Renaissance

Motional Narrowing Regime

ESR Spectra in a FluidESR Spectra in a Fluid

Slow Motional Regime

Rigid Limit

PDT/Toluene at 250GHz

Page 5: Electron Spin Resonance Spectroscopy: A Renaissance

Rotational Tumbling Time:

τR = 1.7 × 10-9 sec

MultiMulti--Frequency ESR SimulationFrequency ESR Simulation

...will look slow at higher frequencies

A motional process that looks fast at lower frequencies

For complex dynamics

of proteins

The slow overall & collective motions will show up best at lower frequencies

Whereas

The fast motions will show up best at higher frequencies

Page 6: Electron Spin Resonance Spectroscopy: A Renaissance

ESR Spectra of ESR Spectra of aqueous solutions of aqueous solutions of T4 Lysozyme spinT4 Lysozyme spin--labeled at mutant site labeled at mutant site 131131 at different frequencies at different frequencies &&

temperaturestemperatures **

* Vs ~ 0.2μL

131

72

69

44

Page 7: Electron Spin Resonance Spectroscopy: A Renaissance

Sensitivity to Anisotropic Motional Dynamics: High FrequencySensitivity to Anisotropic Motional Dynamics: High Frequency

Z-rotation

X-rotation

Y-rotation

“Powder”170 GHz

Example : complexes of cyclodextrins with spin-labeled fatty acids

gzgy

gx

Page 8: Electron Spin Resonance Spectroscopy: A Renaissance

Schematic Diagrams of QuasiSchematic Diagrams of Quasi--Optical BridgesOptical Bridges

Reflection BridgeReflection Bridge →→

Induction BridgeInduction Bridge →→

*From *From ApplAppl. . MagnMagn. Res (1999). Res (1999)

Page 9: Electron Spin Resonance Spectroscopy: A Renaissance

Stochastic Stochastic LiouvilleLiouville EquationEquation

Assuming the “statistical independence” of the spin evolution & the molecular tumbling we may combine

the spin-density matrix, , and the orientational distribution function, into

a combined spin and orientational distribution function, , obeying:

which is the stochastic Liouville equation (SLE).Note, that we recover the normal density matrix by averaging

over all :

and we recoverby setting the spin(s) S, I = 0.

( )tP ,Ω( )tρ

( )t,Ωρ

( ) [ ] ( )tHit

t,ˆ,ˆ

,ΩΓ−−=

∂Ω∂

Ωρρρ

( )t,ΩρΩ ( ) ( ) ΩΩ= t,t ρρ

( )tP ,Ω

Page 10: Electron Spin Resonance Spectroscopy: A Renaissance

The SRLS Model: A The SRLS Model: A MesoscopicMesoscopic ViewViewe.g. A Spin-Labeled Protein

NO

cR ⊥

cR ||

0⊥R

0||R

Restricted Internal Motion

G-tensor frame

gyygXX

gZZ0

||R

Ф

βMG

MOMD Model: and0||R cR ⊥ 0

Page 11: Electron Spin Resonance Spectroscopy: A Renaissance

0.20.25

0.30.35

0.40.45

0.50.55

0.60.65

0.7

0 10 20 30 40

Temperature (oC)

Ord

er p

aram

eter

131

72

44

69

0.5

1

1.5

2

2.5

3

3.5

4

0 5 10 15 20 25 30 35 40

Temperature [oC]

Rpe

rp x

10-8

(s-1

)

131

72

44 69

T4 Lysozyme

Spin-labeled at Various Residues

250 GHz

9 GHz

37.5°C

10°C

RC=2.8×107s-1

RC=1.5×107s-1

Multifrequency SRLS:T4 Multifrequency SRLS:T4 lysozymelysozyme

Liang et al. J. Phys. Chem. (2004) with W. Hubbell

Page 12: Electron Spin Resonance Spectroscopy: A Renaissance

Protein Dynamics by ESRProtein Dynamics by ESR

• Can use high-frequency (e.g. 250 GHz) to “freeze-out”overall tumbling motions, (& other slow motions).

• This provides dramatic sensitivity to the faster local motions: local ordering, local diffusion tensor, geometry.

• Multi-frequency approach allows separation of different dynamic modes.

• Site-directed spin labeling is efficient. Can produce about 10 mutants in one week.

• Must account for motions of spin label tether, which however is restricted, & newer spin labels further restrict them.

Page 13: Electron Spin Resonance Spectroscopy: A Renaissance

Molecular Dynamics Simulations: An Atomistic View Molecular Dynamics Simulations: An Atomistic View *

ExperimentalESR spectrum

Spin labeldynamics

Atomistic MDsimulations

timedomain

Protein X-raystructure

CalculatedESR spectrum

* D. Sezer, J. H. Freed & B. Roux

R1 R1 Side-Chaincontaining nitroxide moiety

Page 14: Electron Spin Resonance Spectroscopy: A Renaissance

Fits to MultiFits to Multi--frequency Spectrafrequency Spectra

72R172R1experiment*

131R1131R1

Magnetic field offset Magnetic field offset

Page 15: Electron Spin Resonance Spectroscopy: A Renaissance

72R1 131R1

Comparison of 72R1 and 131R1Comparison of 72R1 and 131R1

Conformations of the five most populated Markov states for 72R1 and 131R1.

Page 16: Electron Spin Resonance Spectroscopy: A Renaissance

Summary: MD and ESRSummary: MD and ESR

1. Exact time-domain integrators were required for the quantal dynamics of the spins and for the classical motions of the protein.

2. Force field parameters were needed for the side chain R1.

3. A systematic procedure for estimating a Markov chain model of the internal R1 dynamics from its MD trajectories was necessary to deal with the longer time scales needed.

4. The formalism was successfully applied to R1 at solvent-exposed sites in T4 Lysozyme.

Page 17: Electron Spin Resonance Spectroscopy: A Renaissance

• model system: R1 linked to poly-Ala a-helix

• conformational analysis ⇒ stable conformers

⇒ chain dynamics

No Free Parameters

EPR spectra of R1 in α-helix domain• overall protein reorientations

• side chain dynamics ⇐

Liouville superoperator with magnetic tensorspartially averaged by chain dynamics

modified SLE:

linewidth contribution from chaindynamics (Redfield theory)

diffusion operator foroverall protein tumbling

( )DT Ω−12( )DΩΓ

( )DΩL

( ) ( ) ( ) ( ) ( )[ ] ( )tTtit

tDDDDD

D ,,,

ΩρΩΓ+Ω−ΩρΩ−=∂Ωρ∂ −1

2L

Adding Atomistic Perspective to Adding Atomistic Perspective to MesoscopicMesoscopic (SLE) Approach (SLE) Approach **

Assumption: Conformers with low barriers exhibit fast exchange

Conformers with high barriers exhibit no exchange

* F. Tombolato, A. Ferrarini, J.H. Freed

Page 18: Electron Spin Resonance Spectroscopy: A Renaissance

Crystal Gel

FluidMolecular Dynamics Molecular Dynamics Simulation of Simulation of PhosphatidylPhosphatidylCholineCholine (PC) (PC) BilayerBilayer

Taken from: H Heller, M Schaefer, K Schulten,J Phys Chem, 97:8343,1993, Rasmol Image by E Martz

Carbon/Carbon/PalmiticPalmitic, , Water, Water, NitrogenNitrogen, , OleicOleic, , PhosphorusPhosphorus, , OxygenOxygen

Page 19: Electron Spin Resonance Spectroscopy: A Renaissance

ESR on Live CellsESR on Live Cells• Do rafts exist in plasma membranes?

It has been proposed that small rafts of Liquid-Ordered lipids exist in a “sea” of Liquid-Disordered lipids. ESR provides insight.

• How does the “dynamic structure” of cell membranes compare with that of model membranes?

Page 20: Electron Spin Resonance Spectroscopy: A Renaissance

CW-ESR Results from the Plasma Membranes of Four Cell Lines Showing Ordering (So) and Rotational Diffusion Rate (R⊥) as a Function of Spin Label Position on the AcylChain. Two Components are Found in All Cases: a liquid-ordered (Lo) and a liquid-

disordered (Ld). The fraction of the Ld spectral component is shown as P(Ld).Cell Line

◊ = L0

Δ = Ld

◊ = L0

Δ = Ld

RBL CHO COS7 3T3

Page 21: Electron Spin Resonance Spectroscopy: A Renaissance

Comparison of Ordering (So) and Rotational Diffusion Rate (R⊥) between SPM/DOPC/Cholesterol Model Membranes & Results for RBL/2H3 Cells

- - - - - = RBL/2H3 cell membrane ⎯⎯⎯ = model membrane

Lo(high cholesterol)

Lo(moderate cholesterol)

Ld(low cholesterol)

Page 22: Electron Spin Resonance Spectroscopy: A Renaissance

1.More readily and unambiguously distinguishes the spectra from the different components, such as liquid-ordered (Lo) and liquid-disordered (Ld).

2.Enables a more accurate assignment of dynamic (i.e. R⊥) and ordering (i.e. So) parameters to the separate spectral components.

While such studies show the capabilities While such studies show the capabilities of of cwcw--ESR for membrane studies, what ESR for membrane studies, what is needed is an is needed is an improved ESR methodimproved ESR methodthat:that:

Page 23: Electron Spin Resonance Spectroscopy: A Renaissance

TwoTwo--Dimensional SpectroscopyDimensional Spectroscopy *

1976 - Richard Ernst, ETH: NMR: 300 cm (MDA)‡

1986 - Jack Freed, Cornell U.: ESR: 3 cm (MDA) ‡2004 - ESR: 3mm

2000 - Robin Hochstrasser, U Penn: VibrationalSpectra: 6 μm (EDA) ‡

2005 - Graham Fleming, UC Berkeley: Optical Spectra: 0.8 μm (EDA) ‡

* “Spectroscopy at a stretch,” R. M. Hochstrasser, Nature, 434, 570 (2005).

‡ MDA = Magnetic Dipole Allowed; EDA = Electric Dipole Allowed.

Page 24: Electron Spin Resonance Spectroscopy: A Renaissance

Ld Logel

2D2D--ELDORELDOR, A Powerful tool for Studying Membrane , A Powerful tool for Studying Membrane Dynamics Over Wide Temperature Dynamics Over Wide Temperature and Composition Rangesand Composition Ranges

Phases of Two Component System: DPPC/Chol

• The spectra from an end-chain labeled lipid are distinctly different in the three different phases.

• The new DPPC/Chol phase diagram determined by 2D-ELDOR is, in general, consistent with what was previously found.

• The ordering and dynamicsare reliably obtained from the analysis of the 2D-ELDOR spectra.

17 GHz

Page 25: Electron Spin Resonance Spectroscopy: A Renaissance

Initial 2DInitial 2D--ELDOR Studies Show Phase Structure Changes in ELDOR Studies Show Phase Structure Changes in Plasma Plasma Membrane Vesicles (PMV) from RBL Cells upon StimulationMembrane Vesicles (PMV) from RBL Cells upon Stimulation

Unstimulated

Stimulated

T=15oC T=20oC T=30oC

Ld

Ld

Ld

Ld Ld

Ld

Lo

Lo

Lo

Lo Lo

Lo

before after before after before after

• 2-phase coexistence in PMV

• The population of the Lo phase decreases upon stimulation.

• The dynamic structure is revealed

2D-ELDOR provides better understanding of membrane phase structure in PMV.

Page 26: Electron Spin Resonance Spectroscopy: A Renaissance

95 GHz Quasi95 GHz Quasi--Optical Optical HighHigh--Power Pulse SPower Pulse Spectrometerpectrometer

Page 27: Electron Spin Resonance Spectroscopy: A Renaissance

Oriented CSL/DPPC membranes at 17o C Oriented CSL/DPPC membranes at 17Oriented CSL/DPPC membranes at 17oo C C

Bo[T]

cw-95 GHz

95 GHz -2D-ELDOR, Bo||n̂

Page 28: Electron Spin Resonance Spectroscopy: A Renaissance

Pulse Dipolar ESR Spectroscopy Pulse Dipolar ESR Spectroscopy & Protein Structure& Protein Structure

Many biological objects can be studied: soluble and membrane proteins and protein complexes, RNA, DNA, peptides, polymers.

A variety of sample types possible: solutions, liposomes, micelles, bicelles, multi-bilayer vesicles, biological membranes.

A variety of sample morphologies possible: uniform, ordered, heterogeneous, etc.

Broad range of concentrations from micromolar to tens of millimolar is amenable. Only ca. 10 microlitersof sample needed.

Page 29: Electron Spin Resonance Spectroscopy: A Renaissance

Distances yielded by PDS span wide range of 10-80 Å and they are fairly accurate. Therefore, a relatively small number of them is sufficient to reveal structures. A single distance can address important structural and functional details.

Several methods for data analysis greatly simplify the task of extracting average distances and distance distributions.

PDS ESR and Protein StructurePDS ESR and Protein Structure

Page 30: Electron Spin Resonance Spectroscopy: A Renaissance

248

61

301

539

248

61

301

539

Insight II -27.86Å

DQC - 26.8Å, 1.5Å FWHM

T1T2

50 40.8

25.6

18.7 21.5/30.4

87TD 90TD

97 TD

109 TD

28 KCSA

T1T2

50 40.8

25.6

18.7 21.5/30.4

87TD 90TD

97 TD

109 TD

28 KCSA

545a 545b

72a387a

318a

80a508a

15a

318b

496b496a

545a 545b

72a387a

318a

80a508a

15a

318b

496b496a

A1A3A2

B1B2 B3

C3C2C1

C4

A A ““ZooZoo”” of Proteins Studied at ACERTof Proteins Studied at ACERT

Page 31: Electron Spin Resonance Spectroscopy: A Renaissance

DEER and DQC Pulse SequencesDEER and DQC Pulse Sequences

Pump-probe technique irradiates only a fraction of spins with ca. 15-30 ns. pulses. (5-10G).

Irradiates (nearly) all the spins with 3 ns. pulses (30-60G).

tx

Page 32: Electron Spin Resonance Spectroscopy: A Renaissance

250Å

Signal Transduction in Signal Transduction in ChemotaxisChemotaxis

Flagellum

CheA is a homodimerassembled into 9 domains.

A bacterial chemoreceptor relays the signal over a 250Å distance to histidine kinase, CheA, where the phosphorylation cascade starts. CheA is attached to the receptor via the coupling protein, CheW.

Bacteria swim to attractants and away from repellents by switching the sense of flagella rotation. A complex chain of events and multiple proteins and protein complexes are involved into the chemotactic response.

P1/b

P3

P4/b

P5/bP5/a

P4/a

CheACheA modelmodelP1/a

P2/a P2/b

Page 33: Electron Spin Resonance Spectroscopy: A Renaissance

A number of single and double cysteine mutants of CheAΔ289 were engineered for PDS study. CheA Δ289 complexes with labeled or unlabeled CheW in various combinations have been used.

CheW: S15, S80, S72

Mutated Residues

579

568

646

553

579

568

646

553

CheAΔ289

CheW CheAΔ289 is a dimer and binds two CheW. Thus, there are four electron spins.

This complication was overcome by selecting spin-labeling sites such as to make the distances of interest distinct from the rest.

15

7280

SpinSpin--labeling Sites and the Distanceslabeling Sites and the Distances

CheAΔ289: N553, E646, S579, D568

Intra-domain and inter-domain distances, Å.

XXXXXX64628XXXXX57935.532.5XXXX5683234.523.5XXX55339.554.54726XX80

32.546492724.5&30X7243.76154.5 3718.227&29 15

646579568553807215Mutated site

Page 34: Electron Spin Resonance Spectroscopy: A Renaissance

15

80

553

646

579

72

568

?NO• locationCα location

15

80

553

646

579

72

568

?NO• locationCα location

The cartoon illustrates the The cartoon illustrates the ““triangulationtriangulation”” grid grid of PDS constraints obtained to solve binding of PDS constraints obtained to solve binding CheACheA--ΔΔ289289 P5 domainP5 domain (blue)(blue) andand CheWCheW (pink).(pink).

The spheres represent volumes occupied by the nitroxide The spheres represent volumes occupied by the nitroxide groups. The increase in number of constraints (which are groups. The increase in number of constraints (which are fairly accurate distances) reduces the uncertainty in the fairly accurate distances) reduces the uncertainty in the position of the backbone.position of the backbone.

PDS: “Triangulation”

!

Page 35: Electron Spin Resonance Spectroscopy: A Renaissance

Starting with random orientations of the two proteins, the program gives the final

conformation of the P5/CheW complex.

P5

CheW at random location

CheW position from crystal structure

CheW position from crystal structure

Example of Rigid Body Refinement by CNS*Example of Rigid Body Refinement by CNS*

*CNS: Distance geometry software package for structure determination based on constraints from NMR or X-ray Crystallography.

Page 36: Electron Spin Resonance Spectroscopy: A Renaissance

Functional Dynamics of ABC Transporters (DEER)Functional Dynamics of ABC Transporters (DEER)

Conformational Cycle of MsbA

ABC transporters, such as MsbA, transport out of cells: cytotoxic drugs, structurally and chemically dissimilar molecules, against their concentration gradients. Energized by ATP hydrolysis, they act in a few power “strokes”culminating in drug expulsion.

The cartoon depicts flipping cytotoxic lipid (in brown) from the inner leaflet of the internal membrane of Gram-negative bacteria to the outer leaflet.

Page 37: Electron Spin Resonance Spectroscopy: A Renaissance

20 30 40 50 60 70 80

P(

r)

r(Å)

10 20 30 40 50 60

P(r)

r (Å)10 20 30 40 50 60

P(r)

r (Å)

0.0 0.5 1.0 1.5 2.0 2.5

0.0 0.2 0.4 0.6 0.8 1.0

t (μs)

DetergentLiposomes

ApoADP/Vi

Distance Distributions

539 Liposomes

248 Liposomes 61 α-ddm micelles

ΔRav

0.0 0.5 1.0 1.5 2.0t (μs)

61 α-ddm micelles

0.0 0.5 1.0 1.5 2.0

0.0 0.1 0.2 0.3 0.4

t (μs)

Time-domain Ku-band DEER data

248 Liposomes

539

Dipolar Data and Distance Distributions for Dipolar Data and Distance Distributions for MsbAMsbA Reconstituted into Micelles & LiposomesReconstituted into Micelles & Liposomes

L539

S248

R61

Page 38: Electron Spin Resonance Spectroscopy: A Renaissance

Reprocessed XReprocessed X--Ray Data Now Tells the Ray Data Now Tells the Same Story as Pulsed and CW ESRSame Story as Pulsed and CW ESR

Reprocessed MsbA structures are consistent with distances from pulsed ESR and accessibility study by CW-ESR. Nucleotide-bound state of MsbA and SAV1688 are both consistent with pulse ESR.

Page 39: Electron Spin Resonance Spectroscopy: A Renaissance

What is ESR microscopy (ESRM)?What is ESR microscopy (ESRM)?

• ESR Microscopy (ESRM) is an imaging method aimed at obtaining spatially resolved spectroscopic magnetic resonance information from small samples with micron-scale resolution.

• The ESR signal originates from paramagnetic molecules/centers in the sample that may occur naturally, or can be added to the sample (similar to dyes in optics or contrast agents in NMR).

Page 40: Electron Spin Resonance Spectroscopy: A Renaissance

10-100 μm

Why ESR Microscopy ?Why ESR Microscopy ?

Taken from http://genetic-identity.com/Basic_Genetics/basic_genetics.html

Page 41: Electron Spin Resonance Spectroscopy: A Renaissance

ESRM vs. NMR microscopyESRM vs. NMR microscopy

Significant efforts and funding have been invested in the past in the field of NMR microscopy. Recently even a combined NMR-optical microscope was demonstrated. What are the advantages of pursuing the similar, but less mature ESR imaging technology?

•ESR is more sensitive per spin.•ESR resonators have higher Q than NMR micro-coils.•ESR resolution is not limited by diffusion.•ESR is More sensitive to dynamic effects.•Unique probes without “background” proton signal (radicals are added to the sample).•Significantly less expensive magnet technology.•Usually would require the addition of stable radicals (similar to fluorescent dyes or NMR contrast agents).

NMR, 20×20×100 μm Optical, 2×2×25 μm

Goal: Resolution better than [1mm]3in several minutes.

Page 42: Electron Spin Resonance Spectroscopy: A Renaissance

The imaging probeThe imaging probe

Page 43: Electron Spin Resonance Spectroscopy: A Renaissance

Pulse experimental results, 16 GHzPulse experimental results, 16 GHz• 3 LiPc crystals.

• 25 min of acquisition time.

• Resolution of ~3×3×8 μm.

• Image size of 180×180×128 voxels.

• SNR ~550/voxel.

Gy=26 T/m→ 3 μm

MW

Gx=26 T/m→ 3 μm

900 1800 echo signal

0.8 μs

0.7 μs

Gz=1.3 T/m→ 8 μm

Optical image

100 μm

ESR image

Microns

3500

200

100

0

-100

-100 0 100 200

Page 44: Electron Spin Resonance Spectroscopy: A Renaissance

Initial Work on Applications, 16 GHz Initial Work on Applications, 16 GHz Pulsed ProbePulsed Probe

• Drug release: in-vitro observation of slow release of trityl from polymer micro-spheres, and related phenomena.

• Here we observed the T2 weighted image.

Sphere 1

Sphere 2

Air Bubbles

Resonator

ShorterT2, corresponds to “effective”viscosity of ~10 cP inside the sphere.

Page 45: Electron Spin Resonance Spectroscopy: A Renaissance

ACERT STAFFACERT STAFFJaya BhatnagarPëtr BorbatCurt DunnamBoris DzikovskiKeith EarleMingtao GeElka GeorgievaZhichun LiangJozef MoscickiAndrew SmithDmitryi TipikinJoanne TrutkoZiwei Zhang

Previous:Aharon Blank Yun-Wei ChiangWulf HöfbauerSerguei Pachtchenko

COLLABORATORSCOLLABORATORS

Barbara Baird, Cornell Univ.Brian Crane, Cornell Univ.Wayne Hubbell, UCLAHassane Mchaourab, Vanderbilt Univ.Benoit Roux, Univ. of ChicagoDeniz Sezer, Cornell Univ./Univ. of Chicago

www.acert.cornell.edu

Page 46: Electron Spin Resonance Spectroscopy: A Renaissance

The EndThe End


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