Todays Lecture

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Todays Lecture
9) Wed, Oct 6 Interactions of NMR in Solution
and Solids a. Chemical shift b. J-coupling c.
Dipole-dipole interactions d. Quadrupolar
interactions e. Solid state NMR experimental
considerations

• Review articles on ssNMR
• Angewandte Chemie 413096-3129 (2002)
• Accounts of Chemical Research 36858-865 (2003)
• Current Opinion in Structural Biology 12661-669
  (2002)

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Determining the Technique
Timescale
10-12 ? 10-9 ? 10-6 ? 10-3? 100 ? 103 seconds
Degrees of Freedom
3 ? 2 ? 1 ? 0
Magnetic Resonance Technique
Liquid NMR ? HRMAS ? Solid State NMR
3
Chemical shift
Levitt
Electrons surrounding a nucleus create a magnetic
field (shield) which affects the size of the
magnetic field seen at the nucleus
4
Random Coil Carbon and Proton Shifts of Amino
Acids
5
Chemical Shift Referencing

• Chemical Shifts depend oneverything
• Temperature
• Solvent
• Concentration
• Physical State
• Conformation
• Isotope effect

To compare shifts in liquids and solids, one must
have a common referencing scheme.
Can measure accurately
Cannot measure very accurately
Would love to measure.
6
Solvent Effects on TMS
The 13C TMS resonance is sensitive to
solvent. Data shown at 600 MHz, under MAS,
unlocked
Zilm and Morcombe,Chemical Shift Referencing in
MAS Solid-State NMR, J. Mag. Res. 2003, 162,173.
External Reference
7
Chemical shift
Levitt
Electrons surrounding a nucleus create a magnetic
field (shield) which affects the size of the
magnetic field seen at the nucleus
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Molecules with different orientations will
experience different induced fields.
9
Chemical Shift Anisotropy (CSA)
Levitt
In liquid NMR we mainly address chemical shift as
a constant offset. This is because the tumbling
of the molecules causes the time average of the
electronic shielding to be symmetric. In solid
state NMR, the molecules do not tumble
isotropically so the chemical shift depends not
only on the chemical identity of an atom but also
the spatial relationship between the molecule it
is in and the external magnetic field.
Orientation probabilities
Hcs ? lt3cos2q-1gt, B0
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Hcs ? lt3cos2q-1gt, B0
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J- or spin-spin coupling
Levitt
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J-couplings have sign dependence
Levitt
Usually Jjk is positive for spins with the same
sign of g separated by one bond It is usually
negative for spins with opposite signs of g
separated by one bond
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J-couplings in peptides
14
Karplus relation and peptide torsion angle phi
Levitt
15
Dipolar Couplings
Levitt
Individual magnetic moments produce magnetic
fields. These affect the magnetic field seen at
nearby magnetic moments.
16
Dipolar couplings depend on orientation
In an isotropic liquid, these interactions are
time averaged to zero, but in solid state NMR
they have a large contribution to the behavior of
individual spins. In partially oriented systems
(proteins bicelles), they are on the order of
J-couplings.
Levitt
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Dipolar couplings are symmetric
Homonuclei
Heteronuclei
Just like J-couplings there are pairs of states
leading to a Pake pattern in solids and a
splitting for partially oriented systems
(proteins/bicelles)
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Nuclei interact with both electric and magnetic
fields
Levitt
What are the implications?
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Quadrupolar Interaction
Spin ½ nuclei are not quadrupolar
The quadrupolar interaction again leads to a
powder pattern. Even though it is a single spin
interaction, for spin 1 there are two possible
transitions between spin states -1?0 0?1
(remember there are 2I 1 possible states) so
there are two superimposed powder patterns. For
spingt1 this gets a little more involved In
liquids we only see it as relaxation mechanism.
Levitt
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Summary of internal interactions
Chemical shift
J coupling
Dipole-dipole coupling
Quadrupole coupling
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Levitt
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Summary of NMR Interactions
To first order
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Glycine 13C1
Magic Angle Spinning
B0
?rotor
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SSNMR Motional Effects Detected Using CPMAS
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CPMAS of N15 on HAP DpSpSEEKFLRRIGRFG
Hydrated
Lyophilized
pS2
pS2
pS3
pS3
F7
F7
L8
L8
I11
I11
G12
G12
Chemical Shift
Chemical Shift
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Dipolar Recoupling NMR
Dipolar interactions are averaged by MAS but can
be reintroduced with rotor-synchronized RF pulses
Without pulses
With pulses
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Dipolar Recoupling NMR An Example
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Sizes of Interactions Solids
H total H external H internal H ext H
Zeeman H Radio Frequency H IntH Dipole-Dipole
H Chemical Shift H J-coupling Magic Angle
Spinning
500 - 900 MHz
0.001 - 200 kHz
lt200 Hz
0.05 - 40 kHz
20 - 40 kHz
0.2 - 50 kHz
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SSNMR of Proteins
Zilm et alJ. Mag. Res. 2003, 165,162.
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Deuterium NMR of dynamics (lipids)