Todays Lecture

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Todays Lecture
11) Wed, Oct 25 Driving interactions between
spins a. Heteronuclear interactions in
solution b. Heteronuclear interactions in
solids c. Improving sensitivity d. Improving
resolution
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What do we mean by heteronuclear interactions?
Interactions between two atoms with different
nuclear properties
1H?15N 1H?13C 15N?13C 13C?14N 1H?2H
spin ½ interactions
spin ½ interacting with spin 1
These interactions follow the same spin physics
as homonuclear interactions. HOWEVER there is
one important difference..
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Each nucleus has a different g
This leads to Different polarizations Different
frequencies Also, their different electronic
properties lead to different chemical shift ranges
How can we exploit this?
Each nucleus can be treated differently by RF
pulses We can detect nuclei without directly
observing them
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Why Heteronuclear NMR?

• Overlapping 1H resonances can be resolved using
  frequencies of attached heteronuclei
• Heteronuclei create efficient pathways to
  correlate atoms along the backbone and
  side-chains of proteins and nucleic acids
• Heteronuclei provide information on dynamics
  through their relaxation properties
• Selective labeling of different molecules allows
  mixtures to be easily studied (e.g. ligand
  binding)
• Most NMR of proteins or nucleic acids requires
  isotopically labeled samples. Proteins are
  usually produced by over-expressing them in
  bacteria with 15N-labeled, 13C-labeled,
  15N13C-labeled, or 15N13C2H-labeled nutrients.
  Specific labeling strategies have been developed
  (2-13C-glycerol, 1,3-13C-glycerol), but uniform
  isotopic enrichment (13C-glucose, 15NH4Cl) is
  most common.

RELATIVE S/N
15N pulse, acquire 1 1H pulse, transfer to
15N, acquire 10 1H pulse?15N?1H, acquire 300
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Typical pathway 1H pulse? low g nucleus 1 ? low
g nucleus 2?..?1H observe
Modern day twist polarize an electron (via
EPR) or Xe (optically) then transfer to NMR
active nuclei The first technique is used in
DNP-NMR The second is used in imaging when there
isnt a lot of water (lungs)
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Typical solution NMR experiment
Pulse spacings are optimized to allow maximum
polarization transfer via J-couplings
This is here so the proton peaks arent split by
1H-13C J-couplings
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Typical solids NMR experiment
Unfortunately the proton resonances are too broad
to resolve, so are not directly observed
This matches the nutation frequencies so they
transfer polarization through dipole-dipole
interactions
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Back to 2D NMR
15 ml NNdimethylacetamide in 700 ml d-chloroform
at 29 C
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2D NMR The Details
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2D NMR Can we improve resolution?
COSYExperiment 1
What is the difference between these two pulse
sequences? How will the 2D spectrum change?
COSYExperiment 2
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Basic 2D spectrum
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2D COSY Diagonal and Cross Peaks
Cross Peaks are Antiphase
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Diagonal Peaks are Dispersive
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2D COSY Multiple spins
This gets complicated! Alternative pulse
sequences will be introduced on Friday