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NMR%20Spectroscopy

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5. Chemical Shift Anisotropy Relaxation. 6. Spin Rotation. Relaxation ... size of the chemical shift anisotropy - symmetry at the nuclear site. 6- Spin rotation ... – PowerPoint PPT presentation

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Title: NMR%20Spectroscopy


1
NMR Spectroscopy
  • Relaxation Time
  • Phenomenon Application

2
Relaxation- Return to Equilibrium
t
t
z axis
x,y plane
Longitudinal
Transverse
0
0
1
1
t
t
2
2
E-t/T2
1-e-t/T1
8
8
Transverse always faster!
3
Relaxation
magnetization vector's trajectory The initial
vector, Mo, evolves under the effects of T1 T2
relaxation and from the influence of an applied
rf-field. Here, the magnetization vector M(t)
precesses about an effective field axis at a
frequency determined by its offset. It's ends up
at a "steady state" position as depicted in the
lower plot of x- and y- magnetizations.
http//gamma.magnet.fsu.edu/info/tour/bloch/index.
html
4
Relaxation
The T2 relaxation causes the horizontal (xy)
magnetisation to decay. T1 relaxation
re-establishes the z-magnetisation. Note that T1
relaxation is often slower than T2 relaxation.
5
Relaxation time Bloch Equation
  • Bloch Equation

6
Relaxation time Bloch equation
7
Spin-lattice Relaxation time (Longitudinal) T1
Relaxation mechanisms 1. Dipole-Dipole
interaction "through space" 2. Electric
Quadrupolar Relaxation 3. Paramagnetic
Relaxation 4. Scalar Relaxation 5. Chemical
Shift Anisotropy Relaxation 6. Spin Rotation
8
Relaxation
  • Spin-lattice relaxation converts the excess
    energy into translational, rotational, and
    vibrational energy of the surrounding atoms and
    molecules (the lattice).
  • Spin-spin relaxation transfers the excess energy
    to other magnetic nuclei in the sample.

9
Longitudinal Relaxation time T1
  • Inversion-Recovery Experiment

180y (or x)
90y
tD
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11
T1 relaxation
12
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13
Spin-spin relaxation (Transverse) T2
  • T2 represents the lifetime of the signal in the
    transverse plane (XY plane)
  • T2 is the relaxation time that is responsible for
    the line width.
  • line width at half-height1/T2

14
Spin-spin relaxation (Transverse) T2
  • Two factors contribute to the decay of transverse
    magnetization.
  • molecular interactions
  • ( lead to a pure pure T2 molecular effect)
  • variations in Bo
  • ( lead to an inhomogeneous T2 effect)

15
Spin-spin relaxation (Transverse) T2
180y (or x)
90y
tD
tD
  • signal width at half-height (line-width ) (pi
    T2)-1

16
Spin-spin relaxation (Transverse) T2
17
Spin-Echo Experiment
18
Spin-Echo experiment
19
MXY MXYo e-t/T2
20
Carr-Purcell-Meiboom-Gill sequence
21
T1 and T2
  • In non-viscous liquids, usually T2 T1.
  • But some process like scalar coupling with
    quadrupolar nuclei, chemical exchange,
    interaction with a paramagnetic center, can
    accelerate the T2 relaxation such that T2 becomes
    shorter than T1.

22
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23
Relaxation and correlation time
For peptides in aqueous solutions the
dipole-dipole spin-lattice and spin-spin
relaxation process are mainly mediated by other
nearby protons
24
Why The Interest In Dynamics?
  • Function requires motion/kinetic energy
  • Entropic contributions to binding events
  • Protein Folding/Unfolding
  • Uncertainty in NMR and crystal structures
  • Effect on NMR experiments- spin relaxation is
    dependent on rate of motions ? know dynamics to
    predict outcomes and design new experiments
  • Quantum mechanics/prediction (masochism)

25
Application
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27
Characterizing Protein Dynamics
Parameters/Timescales
Relaxation
28
NMR Parameters That Report On Dynamics of
Molecules
  • Number of signals per atom multiple signals for
    slow exchange between conformational states
  • Linewidths narrow faster motion, wide
    slower dependent on MW and conformational states
  • Exchange of NH with solvent requires local
    and/or global unfolding events ? slow timescales
  • Heteronuclear relaxation measurements
  • R1 (1/T1) spin-lattice- reports on fast motions
  • R2 (1/T2) spin-spin- reports on fast slow
  • Heteronuclear NOE- reports on fast some slow

29
Linewidth is Dependent on MW
  • Linewidth determined by size of particle
  • Fragments have narrower linewidths

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